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Experimental studies have been carried out on diamond synthesis in Mg-based systems with additions of europium in various charge states, including Eu0, Eu2+, and Eu3+. The experiments were carried out at a pressure of 7.8 GPa and a temperature of 1800 °C. Metallic Eu, Eu-oxalate, Eu-carbonate, Eu2O3, EuCl3, EuF3, and EuF2 were used as additives. The influence of the content of Eu-containing compounds on diamond crystallization and the photoluminescence characteristics of synthesized diamonds was determined. The addition of europium compounds to Mg-based systems in all cases had an integral inhibitory effect on the diamond crystallization process, which manifested in a decrease in the degree of graphite conversion to diamond, a decrease in the diamond growth rate, and the appearance of metastable graphite. It was found that the inhibiting ability of europium compounds decreased in the following sequence: Eu-oxalate > Eu-carbonate > EuCl3 > Eu2O3 > metallic Eu > EuF3 > EuF2. Spectroscopic studies of the synthesized diamonds revealed a new defect center manifesting in photoluminescence as an optical system with a zero-phonon line structure at 502 nm. The center was tentatively assigned to defects involving Eu impurities and the main factors controlling the formation of the 520 nm center were determined.

In this paper, we report on the effect of sulfur additives (as FeS2 in the Fe90Ni10–C system) on the growth processes, morphology, and defect-and-impurity content of diamond crystals at 6 GPa and 1400 °C. It has been established that the sulfide component plays an inhibitory role in diamond crystallization processes. As the concentration of the sulfur additive (XS) in the system increases from 0 to 20 wt%, the degree of graphite-to-diamond transformation decreases from 100% to zero and the solubility of carbon in the melt decreases from 6.6 to 0.4 wt%. Addition of sulfur to the growth system leads to the appearance of metastable graphite and its amount increases with increasing sulfur content. It is found that the concentration of nitrogen impurities in the synthesized diamonds decreases from 50–100 ppm at XS = 0 to 5–10 ppm at XS = 10–15 wt%. Sulfide additives lead to the transformation of flat-faced octahedral diamond crystals to specific crystals with the elements of antiskeletal growth and fragmentation of their vertices. It is suggested that the main reason for the change in the morphology of diamond crystals is the selective adsorption of sulfide clusters on planar defects located mainly near the vertices of the octahedron. Inhibition of the propagation of growth layers on the {111} faces of diamond crystals leads to the formation of faceted macrosteps, the fragmentation of the crystal vertices and the polycentric structure of the faces.

Carbon solubility in a liquid iron alloy containing nitrogen and sulfur has been studied experimentally in a carbon-saturated Fe-C-N-S-B system at pressures of 5.5 and 7.8 GPa, temperatures of 1450 to 1800 °C, and oxygen fugacities from the IW buffer to log fO2 ΔIW-6 (ΔIW is the logarithmic difference between experimental fO2 and that imposed by the coexistence of iron and wüstite). Carbon saturation of Fe-rich melts at 5.5 and 7.8 GPa maintains crystallization of flaky graphite and diamond. Diamond containing 2100–2600 ppm N and 130–150 ppm B crystallizes in equilibrium with BN within the diamond stability field at 7.8 GPa and 1600 to 1800 °C, while graphite forms at other conditions. The solubility of carbon in the C-saturated metal melt free from nitrogen and sulfur is 6.2 wt% C at 7.8 GPa and 1600 °C and decreases markedly with increasing nitrogen. A 1450–1600 °C graphite-saturated iron melt with 6.2–8.8 wt% N can dissolve: 3.6–3.9 and 1.4–2.5 wt% C at 5.5 and 7.8 GPa, respectively. However, the melt equilibrated with boron nitride and containing 1–1.7 wt% sulfur and 500–780 ppm boron dissolves twice less nitrogen while the solubility of carbon remains relatively high (3.8–5.2 wt%). According to our estimates, nitrogen partitions between diamond and the iron melt rich in volatiles at DNDm/Met=0.013−0.024. The pressure increase in the Fe-C-N system affects iron affinity of N and C: it increases in nitrogen but decreases in carbon. The reduction of C solubility in a Fe-rich melt containing nitrogen and sulfur may have had important consequences in the case of imperfect equilibration between the core and the mantle during their separation in the early Earth history. The reduction of C solubility allowed C supersaturation of the liquid iron alloy and crystallization of graphite and diamond. The carbon phases could float in the segregated core liquid and contribute to the carbon budget of the overlying silicate magma ocean. Therefore, the process led to the formation of graphite and diamond, which were the oldest carbon phases in silicate mantle.

Experimental study, dedicated to understanding the effect of S-rich reduced fluids on the diamond-forming processes under subduction settings, was performed using a multi-anvil high-pressure split-sphere apparatus in Fe3C-(Mg,Ca)CO3-S and Fe0-(Mg,Ca)CO3-S systems at the pressure of 6.3 GPa, temperatures in the range of 900–1600 °C and run time of 18–60 h. At the temperatures of 900 and 1000 °C in the carbide-carbonate-sulfur system, extraction of carbon from cohenite through the interaction with S-rich reduced fluid, as well as C0-producing redox reactions of carbonate with carbide were realized. As a result, graphite formation in assemblage with magnesiowustite, cohenite and pyrrhotite (±aragonite) was established. At higher temperatures (≥1100 °C) formation of assemblage of Fe3+-magnesiowustite and graphite was accompanied by generation of fO2-contrasting melts - metal-sulfide with dissolved carbon (Fe-S-C) and sulfide-oxide (Fe-S-O). In the temperature range of 1400–1600 °C spontaneous diamond nucleation was found to occur via redox interactions of carbide or iron with carbonate. It was established, that interactions of Fe-S-C and Fe-S-O melts as well as of Fe-S-C melt and magnesiowustite, were С0-forming processes, accompanied by disproportionation of Fe. These resulted in the crystallization of Fe3+-magnesiowustite+graphite assemblage and growth of diamond. We show that a participation of sulfur in subduction-related elemental carbon-forming processes results in sharp decrease of partial melting temperatures (~300 °C), reducting the reactivity of the Fe-S-C melt relatively to Fe C melt with respect to graphite and diamond crystallization and decrease of diamond growth rate.

The combination of the unique properties of diamond and the prospects for its high-technology applications urges the search for new solvents–catalysts for the synthesis of diamonds with rare and unusual properties. Here we report the synthesis of diamond from melts of 15 rare-earth metals (REM) at 7.8 GPa and 1800–2100 °C. The boundary conditions for diamond crystallization and the optimal parameters for single crystal diamond synthesis are determined. Depending on the REM catalyst, diamond crystallizes in the form of cube–octahedrons, octahedrons and specific crystals bound by tetragon–trioctahedron and trigon–trioctahedron faces. The synthesized diamonds are nitrogen-free and belong to the rare type II, indicating that the rare-earth metals act as both solvent–catalysts and nitrogen getters. It is found that the REM catalysts enable synthesis of diamond doped with group IV elements with formation of impurity–vacancy color centers, promising for the emerging quantum technologies. Our study demonstrates a new field of application of rare-earth metals.

Herein, the filling of single‐walled carbon nanotubes (SWCNTs) with sulfur is performed, and the magnetic properties of the formed nanomaterials are studied. Encapsulation of sulfur species results in the appearance of a specific magnetic ordering in the system due to the formation of nanoscopic grains composed of sulfur and residual catalytic Fe nanoparticles contained in the SWCNTs. The magnetic character of the obtained 1D nanostructures is studied using superconducting quantum interference device (SQUID) magnetometer and a sequential ferromagnetic–antiferromagnetic ordering in the material is revealed. Magnetic and optical properties are strongly dependent on the synthesis protocols. A significant Raman intensity increase related to the encapsulated nanostructures is obtained when filling is performed at high‐pressure high‐temperature conditions. Simultaneously, the magnetic susceptibility gets strongly reduced for high‐pressure filling, which is related to the escape of iron particles from the nanotube interior, and the magnetic properties of the material are governed by a weak ferromagnetic ordering of Fe–S structures remained inside SWCNTs. Sulfur encapsulation provides the new route for controlling the magnetic properties in 1D nanomaterials that pave the way for advanced magneto‐optical applications. Post-print / Final draft

Development of new routes for fabrication of diamond containing color centers with specific optical and magnetic properties is important for its employment in many innovative applications such as the emerging quantum technologies. Here we report a systematic study on the high-pressure synthesis of diamond from various tin-containing systems aimed at establishing growth conditions favorable for the incorporation of Sn atoms in crystallized diamonds with the formation of the tin-vacancy (Sn V) color centers. Synthesis experiments are performed at 6.3–7.5 GPa and 1400–1900 °C using solvent-catalysts of three different types: Fe Sn Al, Sn-(Ti, Al, Zr) and Sn Mg. We established the main features of diamond crystallization from these catalysts and determined photoluminescence characteristics of the synthesized diamonds. It is found that over the range of the P-T conditions and catalyst compositions employed in the study, the Sn Mg C system is a most effective medium for growing Sn-doped single crystal diamond. We show that with the approach developed in the study it is possible to fabricate diamond single crystals with sizes from ∼50 μm to ∼1 mm containing high-quality Sn V color centers with narrow zero-phonon lines.

Crystallization of diamond in the Mg-Ge-C system has been studied at 7.5 GPa and 1800 °C with the catalyst composition ranging from pure Mg to pure Ge. It is found that with increasing Ge content of the solvent-catalyst, the degree of graphite-to-diamond conversion gradually decreases from 92 to 16%, which is a consequence of the low solubility of carbon in the germanium melt. A decrease in the Mg/Ge ratio leads to a change in the diamond growth form from cube to octahedron and a drastic increase in the number of diamond nucleation centers. The spectral properties of the crystallized diamonds are studied by photoluminescence techniques. The correlations between the Ge content of the Mg-Ge-C system and the luminescence characteristics of the diamond crystals caused by the germanium-vacancy, silicon-vacancy, and nitrogen-vacancy optical centers are established. The results obtained in this study demonstrate that by changing the catalyst composition in the Mg-Ge-C system, it is possible to control the design of the diamond crystals and synthesize isometric, pyramidal, prismatic or needle-like crystals containing Ge-V centers.

The dislocation structure of diamond crystals grown in the Mg-Si-C system at pressure of 7.0–7.5 GPa and temperature of 1800 °C was studied by selective etching. We determined the dislocation density and identified individual features of etching for the growth sectors of {111} and {100} faces. On the {111} faces, all etch pits formed at the outcrops of different dislocations were found to be identical. The feature of grown diamond crystals is the presence of numerous large dislocation loops in the {100} growth sectors. The identified patterns of crystals etching and the internal crystal structure features are associated with the presence of a silicon impurity in the grown diamonds.

This article reports a study of the morphology of diamond crystals grown at 7.0 GPa and 1800 °C in the Mg-C system with the addition of silicon in an amount of 0.5 wt%. Step patterns on {111} and {100} faces were studied in a wide range of magnifications using optical microscopy (DIC), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Morphological studies revealed that a reduction in the growth rate and a change in the morphological significance of the {100} and {111} faces were associated with adsorption of a mobile impurity (silicon) leading to poisoning of kinks and (or) steps. This leads to roughing of the faces and formation of macrosteps as well as 2D and 3D nucleation islands. At a silicon concentration of 1.0 wt% or more, immobile impurity particles are apparently formed on macrostep terraces, and growth inhibition occurs according to the Cabrera-Vermilyea model.

We present high-resolution, all-optical thermometry based on ensembles of germanium-vacancy (GeV) color center in diamond and implement this method of thermometry in the fiber-optic format. Due to the unique properties of diamond, an all-optical approach using this method opens a way to produce back-action-free temperature measurements with resolution below 0.1 K in a wide range of temperatures.

Experimental studies modeling the mechanism of diamond crystallization during sulfidation of metal–carbon melts were performed on a multi-anvil high-pressure apparatus BARS at pressure of 6.3 GPa, temperature range of 1270–1470 °C, and run duration of 20 to 35 h. Sandwich experiments with layer filling were carried out using schemes FeS2-[Fe(Fe90Ni10) + C]-FeS2, FeS2-Ol-[Fe90Ni10 + C]-Ol-FeS2 and S-Ol-[Fe90Ni10 + C]-Ol-S. The experiments have revealed that crystallization of diamond and/or graphite occurs during sulfidation of a metal–carbon melt either due to mixing of metal–carbon and sulfide melts or due to influx of sulfur from external sources through a polycrystalline olivine layers. Transport of a sulfur fluid or a metal–sulfide–carbon melt through a silicate matrix occurs through the intergranular space and microcracks as well as by recrystallization of olivine. Sulfidation is accompanied by a decrease in the solubility of carbon and leads to supersaturation that is necessary for crystallization of diamond or graphite. The inhibitory effect of the sulfide component is the cause for changes in the diamond morphology and transition of carbon phase crystallization from diamond to metastable graphite. The synthesized diamond crystals are classified as low-nitrogen or nitrogen-free diamonds. The revealed regularities may be used to explain the genesis of diamonds with central inclusions and nitrogen-free CLIPPIR diamonds.

In this article, we report the unusual growth of diamond crystals produced in Mg–C and Mg–Ge–C systems at high-pressure, high-temperature conditions. We have found that the growth of the habit {100} faces occurs by deposition of a substance (carbon) on two nonequivalent {100} and {111} surfaces. Precipitation of carbon atoms on the (100) plane occurs by elementary layers with a thickness of about 0.1 and 0.2 nm. The change in the density of elementary steps leads to the formation of step bunches that transform into faceted macrostates with an increase in their thickness of more than 400 nm. The maximum inclination angle of macrostep ends corresponds to the {111} faces position. As a result, singular stable {111} microfacets are formed at the ends of the macrosteps, which themselves grow layer by layer. The deposition of carbon on {100} and {111} surfaces of one simple form of diamond crystals leads to the zonal structure of {100} growth sectors.

Experimental modeling of natural carbide-involving reactions, implicated in the graphite and diamond formation and estimation of the iron carbide stability in the presence of S-bearing fluids, sulfide melts as well as mantle silicates and oxides, was performed using a multi-anvil high-pressure split-sphere apparatus. Experiments were carried out in the carbide-sulfur (Fe 3 C-S), carbide-sulfur-oxide (Fe 3 C-S-SiO 2 -MgO) and carbide-sulfide (Fe 3 C-FeS 2 ) systems, at pressure of 6.3 GPa, temperatures in the range of 900–1600 °C and run time of 18–40 h. During the interaction of cohenite with S-rich reduced fluid or pyrite at 900–1100 °C, extraction of carbon from carbide was realized, resulting in the formation of graphite in assemblage with pyrrhotite and cohenite. At higher temperatures complete reaction of cohenite with newly-formed sulfide melt was found to produce metal-sulfide melt with dissolved carbon (Fe 64 S 27 C 9 (1200 °C)–Fe 54 S 40 C 6 (1500 °C), at.%), which acted as a crystallization medium for graphite (1200–1600 °C) and diamond growth on seeds (1300–1600 °C). Reactions of cohenite and oxides with S-rich reduced fluid resulted in the formation of graphite in assemblage with highly ferrous orthopyroxene and pyrrhotite (900–1100 °C) or in hypersthene formation, as well as graphite crystallization and diamond growth on seeds in the Fe-S-C melt (1200–1600 °C). We show that the main processes of carbide interaction with S-rich fluid or sulfide melt are recrystallization of cohenite (900–1100 °C), extraction of carbon and iron in the sulfide melt, and graphite formation and diamond growth in the metal-sulfide melt with dissolved carbon. Our results evidence that iron carbide can act as carbon source in the processes of natural graphite and diamond formation under reduced mantle conditions. We experimentally demonstrate that cohenite in natural environments can be partially consumed in the reactions with mantle silicates and oxides, and is absolutely unstable in the presence of S-bearing reduced fluid or sulfide melt at temperatures higher than 1100 °C, under lithospheric mantle pressures.

An experimental study, implicated in the revealing of the conditions for the origin for Fe3+-bearing magnesiow&uuml, stite in the lithospheric mantle, was performed using M&ouml, ssbauer spectroscopy of pre-synthesized samples. Experiments were carried out using a multi-anvil high-pressure split-sphere apparatus at 6.3&ndash, 7.5 GPa, in the range of 1100&ndash, 1650 &deg, C in carbonate-metal, carbonate&ndash, oxide-metal, carbonate-oxide, carbide-oxide and carbonate&ndash, metal- sulphur systems. In three experimental series, oxygen fugacity gradient in the samples was created, which enabled the study of the processes of magnesiow&uuml, stite formation under oxidizing and reducing conditions (&Delta, logfO2 (FMQ) values from &minus, 1 to &minus, 5). It was established that Fe3+-bearing magnesiow&uuml, stite can form both in assemblage with oxidized phases, such as carbonate or with reduced ones&mdash, metal, carbides, sulphides, graphite and diamond. According to the M&ouml, ssbauer spectroscopy, the composition of synthesized magnesiow&uuml, stite varied within a range of Fe3+/&Sigma, Fe values from 0 to 0.3, with IV and VI coordination of Fe3+ depending on P, T, fO2, x-parameters. It was established that Fe3+-bearing magnesiow&uuml, stite formation processes under upper mantle P,T-conditions include redox reactions, with magnesiow&uuml, stite being (1) reductant or (2) product of interaction, (3) crystallization processes of magnesiow&uuml, stite from an oxidized melt, where magnesiow&uuml, stite acts as a sink for ferric iron and (4) iron disproportionation.

Diamond crystallization in Mg-R2O3-C systems (R = Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb) was studied at 7.8 GPa and 1800 &deg, C. It was found that rare-earth oxide additives in an amount of 10 wt % did not significantly affect both the degree of graphite-to-diamond conversion and crystal morphology relative to the Mg-C system. The effect of higher amounts of rare-earth oxide additives on diamond crystallization was studied for a Mg-Sm2O3-C system with a Sm2O3 content varied from 0 to 50 wt %. It was established that with an increase in the Sm2O3 content in the growth system, the degree of graphite-to-diamond conversion decreased from 80% at 10% Sm2O3 to 0% at 40% Sm2O3. At high Sm2O3 contents (40 and 50 wt %), instead of diamond, mass crystallization of metastable graphite was established. The observed changes in the degree of the graphite-to-diamond conversion, the changeover of diamond crystallization to the crystallization of metastable graphite, and the changes in diamond crystal morphology with increasing the Sm2O3 content attested the inhibiting effect of rare-earth oxides on diamond crystallization processes in the Mg-Sm-O-C system. The crystallized diamonds were studied by a suite of optical spectroscopy techniques, and the major characteristics of their defect and impurity structures were revealed. For diamond crystals produced with 10 wt % and 20 wt % Sm2O3 additives, a specific photoluminescence signal comprising four groups of lines centered at approximately 580, 620, 670, and 725 nm was detected, which was tentatively assigned to emission characteristic of Sm3+ ions.

The unique properties of diamond make it an appropriate material for various cutting-edge scientific and industrial applications. The progress in the development of new diamond based technologies is inseparably linked to research and development into the synthesis and growth of this material. For instance, developing new routes for the fabrication of diamond containing defects with specific optical, electronic and magnetic properties is crucially important for the emerging quantum technologies. Recently it has been found that diamond synthesis under high-pressure, high-temperature conditions using catalysts based on magnesium demonstrates a number of intriguing characteristics such as an extremely high growth rate, unusual micro- and macro-morphology of crystals, and effective doping of diamond with Si and Ge impurities. In this highlight, we will describe the major features of the growth, morphology, internal structure, and defect-related spectroscopic characteristics of diamonds crystallized using Mg-based catalysts under high-pressure, high-temperature conditions.

An experimental study on diamond crystallization in CO2-rich sodium-carbonate melts has been undertaken at a pressure of 6.3 GPa in the temperature range of 1250–1570 °C and at 7.5 GPa in the temperature range of 1300–1700 °C. Sodium oxalate (Na2C2O4) was used as the starting material, which over the course of the experiment decomposed to form sodium carbonate, carbon dioxide and elemental carbon. The effects of pressure, temperature and dissolved CO2 in the ultra-alkaline carbonate melt on diamond crystallization, morphology, internal structure and defect-and-impurity content of diamond crystals are established. Diamond growth is found to proceed with formation of vicinal structures on the {100} and {111} faces, resulting eventually in the formation of rounded polyhedrons, whose shape is determined by the combination tetragon–trioctahedron, trigon–trioctahedron and cube faces. Spectroscopic studies reveal that the crystallized diamonds are characterized by specific infrared absorption and photoluminescence spectra. The defects responsible for the 1065 cm− 1 band dominating in the IR spectra and the 566 nm optical system dominating in the PL spectra are tentatively assigned to oxygen impurities in diamond.

Experiments on the synthesis of inclusions-bearing diamond were performed in the SiO 2 –((Mg,Ca)CO 3 –(Fe,Ni)S system at 6.3 GPa and 1650–1750 °C, using a multi-anvil high pressure apparatus of the “split-sphere” type. Diamond synthesis was realized in the “sandwich-type” experiments, where the carbonate–oxide mixture acted as a source of both CO 2 -dominated fluid and carbonate–silicate melt, and Fe,Ni-sulfide played a role of reducing agent. As a result of redox reactions in the carbonate–oxide–sulfide system, diamond was formed in association with graphite and Mg,Fe-silicates, coexisting with CO 2 -rich fluid, carbonate–silicate and sulfide melts. The synthesized diamonds are predominantly colorless or light-yellow monocrystals with octahedral habit (20–200 μm), and polycrystalline aggregates (300–400 μm). Photoluminescence spectroscopy revealed defects related to nickel impurity (S3 optical centers), which are characteristic of many diamonds in nature. The density of diamond crystallization centers over the entire reaction volume was ~3 × 10 2 –10 3 cm − 3 . The overwhelming majority of diamonds synthesized were inclusions-bearing. According to Raman spectroscopy data, diamond trapped a wide variety of inclusions (both mono- and polyphase), including orthopyroxene, olivine, carbonate–silicate melt, sulfide melt, CO 2 -fluid, graphite, and diamond. The Raman spectral pattern of carbonate–silicate melt inclusions have bands characteristic of magnesite and orthopyroxene (± SiO 2 ). The spectra of sulfide melt displayed marcasite and pyrrhotite peaks. We found that compositions of sulfide, silicate and carbonate phases are in good agreement not only with diamond crystallization media in experiments, but with data on natural diamond inclusions of peridotitic and eclogitic parageneses. The proposed methodological approach of diamond synthesis can be used for experimental simulation of the formation of several types of mineral, fluid and melt inclusions, observed in natural diamonds.

Diamond crystallization in the Cu-C system is studied at high pressure high temperature conditions of 7.0 GPa and 1500–1800 °C. The minimum temperature of diamond synthesis by spontaneous nucleation is found to be 1800 °C. Diamond layers of optical quality are grown on the seed crystals via the chemical-potential difference method. From the infrared absorption measurements it is found that the diamond layers grown on the (100) faces of the seed crystals contain nitrogen impurities in the form of A centers with concentration from 700 to 1100 ppm and show strong hydrogen-related absorption peak at 3107 cm − 1 . A number of specific optical centers are found in photoluminescence spectra recorded for diamond layers grown on both (100) and (111) faces of the seed crystals. The newly observed 1.748 eV center is tentatively assigned to defects involving Cu impurities. The 1.787 eV center is suggested to appear preferentially in diamonds with high contents of nitrogen and hydrogen impurities.

In this work, high pressure high temperature (HPHT) diamonds synthesized in the Mg–C system with germanium, silicon and boron additives were studied. In the photoluminescence spectra of the samples doped with Ge, an intense system 602 nm attributed to germanium-vacancy defects was detected. In the electron paramagnetic resonance (EPR) spectra of these samples, a new paramagnetic center with S = 1 was detected along with substitutional nitrogen P1 and silicon-vacancy KUL1 (SiV0) centers. The angular dependence investigation of the new spectrum allowed us to establish its spin Hamiltonian parameters: g|| = 2.0025, g⊥ = 2.0027, D = 80.3 mT, E = 0. The center was determined to have the symmetry axis parallel to 〈111〉. Hyperfine structure (HFS) of one 73Ge atom (I = 9/2) was observed for the new spectrum. The novel paramagnetic center was proposed to be the neutral germanium split-vacancy defect. EPR and luminescence studies of diamonds doped with Si and B revealed a new paramagnetic center that can be associated with the sharp luminescence system 720 nm. An analysis of the angular dependence of the EPR spectrum showed that it had electronic spin S = 1/2 and anisotropic g-factor: g1 = 2.0033, g2 = 2.0004, and g3 = 2.0024. Based on the principal values and directions of the g-tensor the detected center was suggested to have the structure of silicon and boron atoms in the nearest carbon positions.

Here we report on successful synthesis of Ge-doped single crystal diamond at high-pressure high-temperature (HPHT) conditions of 7.0 GPa and 1500–1900 °C from an Mg–Ge–C system. The systematic study of diamond crystallization processes reveals that addition of Ge to the Mg–C system suppresses the intensity of diamond spontaneous nucleation enabling synthesis of relatively large (2–3 mm) diamond crystals via seeded growth. Temperature is found to be a key factor controlling diamond crystallization mechanisms, morphological characteristics, and optical properties of the crystals. It is established that the diamond growth rate depends almost exponentially on the temperature increasing from 3.3 μm/h at 1500 °C to 4.2 mm/h at 1900 °C. Spectroscopic characterization of the synthesized diamonds reveals the abundance of the 2.06 eV Ge–V centers in the photoluminescence spectra confirming successful doping of the diamonds with germanium. The feasibility of the Mg–Ge–C system for growing bulk low-strain diamond dop...

Experimental modeling of the formation of graphite and diamond as a result of carbide&ndash, fluid interactions was performed in the Fe3C&ndash, SiO2&ndash, Al2O3&ndash, (Mg,Ca)CO3 systems at 6.3 and 7.5 GPa and 1100&ndash, 1650 &deg, C. In the experiments with &fnof, O2-gradient (7.5 GPa, 1250&ndash, 1350 &deg, C), graphite + magnesiow&uuml, stite + garnet &plusmn, cohenite assemblage was formed. Graphite was produced through the redox interactions of carbide with carbonate or CO2 (reducing conditions), and redox reactions of magnesiow&uuml, stite and CO2 (oxidizing conditions). At 1450&ndash, C, crystallization of graphite, garnet, magnesiow&uuml, stite and ferrospinel, as well as generation of Fe2+,3+-rich carbonate&ndash, silicate melt occurred. This melt, saturated with carbon, acted as a medium of graphite crystallization and diamond growth on seeds. In the experiments without &fnof, O2-gradient (6.3 GPa), decarbonation reactions with the formation of CO2-fluid and Fe,Mg,Ca-silicates, as well as C0-producing redox reactions of CO2-fluid with cohenite were simultaneously realized. As a result, graphite (&plusmn, diamond growth) was formed in assemblage with Fe2+,Fe3+,Mg-silicates and magnetite (1100&ndash, 1200 &deg, C), or with Fe3+-rich garnet and orthopyroxene (1300&ndash, 1500 &deg, C). It has been established that a potential mechanism for the crystallization of graphite or diamond growth is the oxidation of cohenite by CO2-fluid to FeO and Fe3O4, accompanied by the extraction of carbon from Fe3C and the corresponding reduction of CO2 to C0.

Ca,Mg-sulfates are subduction-related sources of oxidized S-rich fluid under lithospheric mantle P,T-parameters. Experimental study, aimed at the modeling of scenarios of S-rich fluid generation as a result of desulfation and subsequent sulfide formation, was performed using a multi-anvil high-pressure apparatus. Experiments were carried out in the Fe,Ni-olivine&ndash, anhydrite&ndash, C and Fe,Ni-olivine&ndash, Mg-sulfate&ndash, C systems (P = 6.3 GPa, T of 1050 and 1450 &deg, C, t = 23&ndash, 60 h). At 1050 &deg, C, the interaction in the olivine&ndash, C system leads to the formation of olivine + diopside + pyrrhotite assemblage and at 1450 &deg, C leads to the generation of immiscible silicate-oxide and sulfide melts. Desulfation of this system results in the formation of S-rich reduced fluid via the reaction olivine + anhydrite + C &rarr, diopside + S0 + CO2. This fluid is found to be a medium for the recrystallization of olivine, extraction of Fe and Ni, and subsequent crystallization of Fe,Ni-sulfides (i.e., olivine sulfidation). At 1450 &deg, C in the Ca-free system, the generation of carbonate-silicate and Fe,Ni-sulfide melts occurs. Formation of the carbonate component of the melt occurs via the reaction Mg-sulfate + C &rarr, magnesite + S0. It is experimentally shown that the olivine-sulfate interaction can result in mantle sulfide formation and generation of potential mantle metasomatic agents&mdash, S- and CO2-dominated fluids, silicate-oxide melt, or carbonate-silicate melt.

Experimental simulation of the interaction of wustite with a CO 2 -rich fluid and a carbonate-silicate melt was performed using a multianvil high-pressure split-sphere apparatus in the FeO-MgO-CaO-SiO 2 -Al 2 O 3 -CO 2 system at a pressure of 6.3 GPa and temperatures in the range of 1150 °C–1650 °C and with run time of 20 h. At relatively low temperatures, decarbonation reactions occur in the system to form iron-rich garnet (Alm 75 Prp 17 Grs 8 ), magnesiowustite (Mg# ≤ 0.13), and CO 2 -rich fluid. Under these conditions, magnesiowustite was found to be capable of partial reducing CO 2 to C 0 that leads to the formation of Fe 3+ -bearing magnesiowustite, crystallization of magnetite and metastable graphite, and initial growth of diamond seeds. At T ≥ 1450 °C, an iron-rich carbonate-silicate melt (FeO ~ 56 wt.%, SiO 2 ~ 12 wt.%) forms in the system. Interaction between (Fe,Mg)O, SiO 2 , fluid and melt leads to oxidation of magnesiowustite and crystallization of fayalite-magnetite spinel solid solution (1450 °C) as well as to complete dissolution of magnesiowustite in the carbonate-silicate melt (1550 °C–1650 °C). In the presence of both carbonate-silicate melt and CO 2 -rich fluid, dissolution (oxidation) of diamond and metastable graphite was found to occur. The study results demonstrate that under pressures of the lithospheric mantle in the presence of a CO 2 -rich fluid, wustite/magnesiowustite is stable only at relatively low temperatures when it is in the absolute excess relative to CO 2 -rich fluid. In this case, the redox reactions, which produce metastable graphite and diamond with concomitant partial oxidation of wustite to magnetite, occur. Wustite is unstable under high concentrations of a CO 2 -rich fluid as well as in the presence of a carbonate-silicate melt: it is either completely oxidized or dissolves in the melt or fluid phase, leading to the formation of Fe 2 + - and Fe 3 + -enriched carbonate-silicate melts, which are potential metasomatic agents in the lithospheric mantle.

A series of experiments on diamond growth in the Mg45Cu55–C, Mg50Ni50–C, Mg50Cu25Ni25–C, Mg50Ag25Ni25–C and Mg25Cu25Ni50–C systems at a pressure of 6.3 GPa in the temperature range of 1550–1650 °C was conducted. The morphology of diamond crystals and the features of their growth were studied. The {111} and {100} faces, with the cube faces playing the dominant role, were found to be the plane-faced forms of diamond crystal growth. If the magnesium content in the system is 45−50 at%, crystal growth occurs in the regime of inhibition of the {100} faces with the formation of antiskeletal crystals. In case of a constant intensity of the face inhibition up to its complete growing out, the quasi-faces of trapezohedron {311} are formed with striations in the [110] direction. The inhibition of face growth and formation of antiskeletal crystals are related to oxygen inflow from the cell parts and are accompanied by crystallization of magnesium oxide on the diamond surface. A reduction in the magnesium content in the system from 50 to 25 at% or an increase in the temperature leads to the cessation of {100} face inhibition and promotes growth of plane-faced crystals.

Diamond crystallization from the tin–carbon system has been studied at 7 GPa and temperatures ranging from 1600 to 1900 °C with reaction times from 1 to 20 h. Both diamond growth on the seed crystals and diamond spontaneous nucleation were established, providing evidence for the catalytic ability of tin. A distinctive feature of the Sn–C system is the existence of a significant induction period preceding diamond spontaneous nucleation. Temperature and kinetics are found to be the main factors governing diamond crystallization process. The minimum parameters of diamond spontaneous nucleation are determined to be 7 GPa, 1700 °C and 20 h. The stable form of diamond growth is octahedron and it does not depend on temperature. Synthesized diamonds contain high concentrations of nitrogen impurities up to about 1600 ppm.

The phase relations in the K 2 CO 3 –FeCO 3 system were studied in multianvil experiments using graphite capsules at 6 GPa and 900–1400°C. Subsolidus assemblages comprise the stability fields of K 2 CO 3 + K 2 Fe(CO 3 ) 2 and K 2 Fe(CO 3 ) 2 + siderite with the transition boundary at X (K 2 CO 3 ) = 50 mol%. The K 2 CO 3 –K 2 Fe(CO 3 ) 2 and K 2 Fe(CO 3 ) 2 –FeCO 3 eutectics are established at 1100°C and 65 mol% and at ~1150°C and 46 mol% K 2 CO 3 , respectively. Siderite is a subliquidus phase at 1400°C at X (K 2 CO 3 ) 2 CO 3 –MgCO 3 system, which has two eutectics at 1200°C and 74 mol% and at 1250°C and 48 mol% K 2 CO 3 , respectively. The natural siderite used in the present study contained 6 mol% MnCO 3 and 7 mol% MgCO 3 . Although the obtained Fe-bearing carbonate phases exhibit uniform Mn/(Fe + Mn + Mg) ratio, magnesium tends to redistribute into the solid phases K 2 Fe(CO 3 ) 2 or siderite. At 1200°C and X (K 2 CO 3 ) = 50 mol%, the K 2 Fe 0.88 Mn 0.06 Mg 0.06 (CO 3 ) 2 melt coexists with the K 2 Fe 0.78 Mn 0.06 Mg 0.16 (CO 3 ) 2 compound. Assuming continuous solid solution between K 2 Fe(CO 3 ) 2 and K 2 Mg(CO 3 ) 2 , the K 2 Fe(CO 3 ) 2 end-member should melt congruently slightly below 1200°C, which is about 50° lower than the melting point of K 2 Mg(CO 3 ) 2 . The siderite–magnesite system was studied at 6 GPa and 900–1700°C. Complete solid solution is recorded between Fe 0.94 Mn 0.06 CO 3 siderite and magnesite. At X (MgCO 3 ) = 7 mol% and 1600°C, the (Fe 0.90 Mn 0.06 Mg 0.04 )CO 3 partial melt coexists with (Fe 0.86 Mn 0.06 Mg 0.08 )CO 3 siderite, whereas at X (MgCO 3 ) = 26 and 35 mol%, the (Fe 0.71 Mn 0.06 Mg 0.23 )CO 3 partial melt coexists with (Fe 0.51 Mn 0.06 Mg 0.43 )CO 3 siderite. Based on these data, Fe 0.94 Mn 0.06 CO 3 siderite should melt slightly below 1600°C, i.e . 300° lower than magnesite. Development of bubbles in the quenched melt at X (MgCO 3 ) = 7 mol% and 1700°C suggests incongruent melting of siderite according to the reaction: siderite = liquid + CO 2 fluid.

The new Na4Ca(CO3)3, Na2Ca3(CO3)4 and Na2Ca4(CO3)5 compounds were synthesized in the system Na2CO3–CaCO3 in multianvil experiments at 6 GPa and characterized by Raman spectroscopy. In addition, the Na2Ca3(CO3)4 compound was studied using in situ energy dispersive and single-crystal X-ray diffraction. Single bands in the CO32− symmetric stretching region ( v1 ) and out-of-plane bending region ( v2 ) in the Na4Ca(CO3)3 Raman spectrum suggest a single crystallographically distinct carbonate group in the structure. In contrast, the spectra of Na2Ca3(CO3)4 and Na2Ca4(CO3)5 show two and three bands, respectively, in both the symmetric stretching region ( v1 ) and out-of-plane bending region ( v2 ), suggesting more than one crystallographically distinct carbonate group in the unit cell. Raman activity in the forbidden v2 mode and multiple bands are observed in the in-plane bending region ( v4 ) for the three compounds, proving the reduction of site symmetry of the CO32− ions with the loss of the threefold rotation axis ( D 3h → D 2h or C s). Such a decrease in symmetry suggests distortion of the group itself, but may be attained by rearrangements of the coordinated metal cations as in the aragonite-group carbonates. At 6.5 GPa and 1000°C, the structure of Na2Ca3(CO3)4 was found to be orthorhombic or monoclinic with a β angle close to 90° and the lattice parameters: a = 7.3357(6) A, b = 8.0377(9) A, and c = 31.5322 (32) A, with V = 929.59(14) A3. No structural changes were observed during pressure decrease down to 1 GPa, while a discontinuous increase in unit-cell parameters and volume was observed upon decompression from 1 GPa at room temperature. This indicates a pressure-induced phase transition to a structurally related ambient-pressure phase. The abnormally long c -parameter and proximity of the β-angle to 90° of Na2Ca3(CO3)4 at ambient conditions suggest that, in the monoclinic system, the metric symmetry is higher than the Laue symmetry, which is a common sign for merohedral twinning.

We report the results of an experimental study on diamond crystallization in the Sb–C system under HPHT conditions. The experiments were run at pressures of 6.3 and 7.5 GPa in the temperature range of 1500–1800 °C for 1 to 40 h. Diamond crystallization via spontaneous nucleation and growth on the seed crystals has been established. It is shown that the kinetic factor plays a crucial role in diamond crystallization in the Sb–C system. For run duration of 40 h the minimal P–T parameters of diamond growth on the seed crystals are found to be 6.3 GPa and 1500 °C, while the minimal parameters for spontaneous nucleation are 7.5 GPa and 1600 °C.

A solid-state system combining a stable spin degree of freedom with an efficient optical interface is highly desirable as an element for integrated quantum optical and quantum information systems. We demonstrate a bright color center in diamond with excellent optical properties and controllable electronic spin states. Specifically, we carry out detailed optical spectroscopy of a Germanium Vacancy (GeV) color center demonstrating optical spectral stability. Using an external magnetic field to lift the electronic spin degeneracy, we explore the spin degree of freedom as a controllable qubit. Spin polarization is achieved using optical pumping, and a spin relaxation time in excess of 20 $\mu$s is demonstrated. Optically detected magnetic resonance (ODMR) is observed in the presence of a resonant microwave field. ODMR is used as a probe to measure the Autler-Townes effect in a microwave-optical double resonance experiment. Superposition spin states were prepared using coherent population trapping, and a pure dephasing time of about 19 ns was observed. Prospects for realizing coherent quantum registers based on optically controlled GeV centers are discussed., Comment: 6 pages, 4 figures

The effect of rare-earth metals on diamond crystallization processes in the Mg-REM-C system (REM = Sc, Y, La, Pr, Nd, Dy, and Yb) was studied at 7.8 GPa and 1800 °C. It was found that the rare-earth metals added in an amount of 10 wt% had different effects on diamond crystallization. Sc, La, Pr, and Dy, only slightly affected the intensity of diamond synthesis as compared to the undoped Mg-C system. Yb, Y, and Nd significantly reduced the degree of conversion of graphite to diamond. The effect of higher contents of rare-earth metals on diamond crystallization was studied for the Mg-Pr-C system with a Pr content varied from 0 to 80 wt%. It was found that with increasing Pr content the degree of the graphite-to-diamond conversion decreased and at 80 wt% Pr diamond synthesis was terminated and mass crystallization of metastable graphite took place. The morphology of diamonds synthesized in the Mg-REM-C system was determined by the {1 0 0} faces. The surface microrelief of the faces was connected with crystal defectiveness and characterized by specific features, depending on the system composition and the degree of graphite-to-diamond conversion. From the spectroscopic characterization it was found that similarly to their action on diamond synthesis, different REM additives produced different effects on photoluminescence characteristics of the synthesized diamonds. Those rare-earth metals which only slightly affected diamond crystallization, showed no significant effects on the PL spectra relative to the undoped Mg-C system. Diamonds produced with Y, Nd and Yb additives showed a number of new photoluminescence features, which could be related to defects inhibiting diamond crystal growth.

The phase relations in the Na 2 CO 3 -(Fe 0.87 Mn 0.06 Mg 0.07 )CO 3 system have been studied in Kawai-type multi-anvil experiments using graphite capsules at 6.0 GPa and 900–1400 °C. Subsolidus assemblages comprise the stability fields of Na 2 CO 3 + Na 2 Fe(CO 3 ) 2 and Na 2 Fe(CO 3 ) 2 + siderite with the transition boundary at X (Na 2 CO 3 ) = 50 mol%. Intermediate Na 2 Fe(CO 3 ) 2 compound has rhombohedral R 3 eitelite structure with cell parameters a = 4.9712(16), c = 16.569(4) A, V = 354.61(22). The Na 2 CO 3 -Na 2 Fe(CO 3 ) 2 eutectic is established at 1000 °C and 66 mol% Na 2 CO 3 . Na 2 Fe(CO 3 ) 2 disappears between 1000 and 1100 °C via incongruent melting to siderite and a liquid containing about 55 mol% Na 2 CO 3 . Siderite remains a subliquidus phase at 1400 °C at X (Na 2 CO 3 ) ≤ 30 mol%. The ternary Na 2 CO 3 -FeCO 3 -MgCO 3 system can be built up from the corresponding binary systems: two systems with intermediate Na 2 (Mg,Fe)(CO 3 ) 2 phase, which melts congruently at the Mg-rich side and incongruently at the Fe-rich side, and the (Mg,Fe)CO 3 system with complete solid solution. The phase relations suggest that the maximum contribution of FeCO 3 component into the lowering solidus temperatures of Na-bearing carbonated mantle domains could not exceed several tens of degrees Celsius.

Phase relations in the system K2CO3-CaCO3 have been studied in the compositional range, X (K2CO3), from 100 to 10 mol%, at 6.0 GPa and 900–1450 °C. At 900–950 °C, the system has three intermediate compounds: K6Ca2(CO3)5, K2Ca(CO3)2, and K2Ca3(CO3)4. The K2Ca(CO3)2 compound decomposes to the K6Ca2(CO3)5 + K2Ca3(CO3)4 assembly above 950 °C. The K6Ca2(CO3)5 and K2Ca3(CO3)4 compounds melt congruently slightly above 1200 and 1300 °C, respectively. The eutectics were established at 64 and 44 mol% near 1200 °C and at 23 mol% near 1300 °C. K2CO3 remains as a liquidus phase at 1300 °C and 75 mol% and melts at 1425 ± 20 °C. Aragonite remains as a liquidus phase at 1300 °C and 20 mol% and at 1400 °C and 10 mol%. CaCO3 solubility in K2CO3 and K2CO3 solubility in aragonite are below the detection limit (

The subsolidus and melting phase relations in the CaCO3-siderite system have been studied in multi-anvil experiments using graphite capsules at pressure of 6 GPa and temperatures of 900–1700 °C. At low temperatures, the presence of ankerite splits the system into two partial binaries: siderite + ankerite at 900 °C and ankerite + aragonite up to 1000 °C. Extrapolated solvus curves intersect near 50 mol% just below 900 °C. At 1100 and 1200 °C, the components appear to form single-phase solid solutions with space group symmetry R 3 c , while CaCO3 maintains aragonite structure up to 1600 °C and 6 GPa. The FeCO3 solubility in aragonite does not exceed 1.0 and 3.5 mol% at 900–1000 and 1600 °C, respectively. An increase of FeCO3 content above the solubility limit at T > 1000 °C, leads to composition-induced phase transition in CaCO3 from aragonite, Pmcn , to calcite, R 3 c , structure, i.e., the presence of FeCO3 widens the calcite stability field down to the P-T conditions of sub-cratonic mantle. The siderite-CaCO3 diagram resembles a minimum type of solid solutions. The melting loop for the FeCO3-CaCO3 join extends from 1580 °C (FeCO3) to 1670 °C (CaCO3) through a liquidus minimum near 1280 ± 20 °C and 56 ± 3 mol% CaCO3. At X (Ca) = 0–30 mol%, 6 GPa and 1500–1700 °C, siderite melts and dissolves incongruently according to the reaction: siderite = liquid + fluid. The apparent temperature and X (Ca) range of siderite incongruent dissolution would be determined by the solubility of molecular CO2 in (Fe,Ca)CO3 melt. The compositions of carbonate crystals and melts from the experiments in the low-alkali carbonated eclogite (Hammouda 2003; Yaxley and Brey 2004) and peridotite (Dasgupta and Hirschmann 2007; Brey et al. 2008) systems are broadly consistent with the topology of the melting loop in the CaCO3-MgCO3-FeCO3 system at 6 GPa pressure: a Ca-rich dolomite-ankerite melt coexists with Mg-Fe-calcite in eclogites at CaO/MgO > 1 and Mg-dolomite melt coexists with magnesite in peridotites at CaO/MgO < 1. However, in fact, the compositions of near solidus peridotite-derived melts and carbonates are more magnesian than predicted from the (Ca,Mg,Fe)CO3 phase relations.

Diamond crystals synthesized in the MgSiC system at high pressure high temperature (HPHT) conditions have been studied by EPR and photoluminescence. In the EPR spectra of the samples, two centers with S = 1 and S = 1/2 were detected along with substitutional nitrogen P1. Investigation of the angular dependence allowed us to establish spin Hamiltonian parameters of these centers, which turned out to be silicon containing centers KUL1 and KUL8, neutral and negatively charged silicon atom in a split-vacancy configuration, respectively. In the photoluminescence spectra of the diamond crystals there was an intense line at 737 nm corresponding to the (Si-V)− defect. X-ray irradiation, as well as UV excitation, led to an increase in relative intensities of P1 and KUL1 EPR signals, at the same time relative intensity of KUL8 decreased. Heat treatment at 773 K led to the recovery of the initial EPR spectrum. Our results confirm the hypothesis that KUL8 center is negatively charged state of Si-V defect.

The real structure of the {111} sectors of large single synthetic diamond crystals with different nitrogen contents has been studied by the selective etching method: (1) nitrogen-gettered diamonds with nitrogen impurity content at a level of 1–2 ppm, (2) diamond crystals grown without additives with nitrogen impurity of 180–220 ppm and (3) diamonds nitrogen alloyed during growth up to the concentrations of 550–600 ppm. All diamond crystals have been grown by the temperature gradient method using the high pressure apparatus of “split-sphere” type (BARS) at T =1350 °C, P =5.7 GPa and (100) seed crystal. It is found that the density of dislocations increases from 85 cm −2 to 4.0×10 3 cm −2 and the total length of planar defects increases from 30 to 300 μm/cm 2 as the nitrogen content in the crystals increases up to 600 ppm. It is shown that partial dislocations are the dominant dislocations (60–90%) in all the crystals studied. A proportion of perfect dislocations increases as the nitrogen content in crystals, and correspondingly, the dislocation density increase. Possible causes for the increase of the extended defects density in the synthetic diamond crystals with the nitrogen impurity concentration rise are discussed.

Subduction tectonics imposes an important role in the evolution of the interior of the Earth and its global carbon cycle; however, the mechanism of the mantle-slab interaction remains unclear. Here, we demonstrate the results of high-pressure redox-gradient experiments on the interactions between Mg-Ca-carbonate and metallic iron, modeling the processes at the mantle-slab boundary; thereby, we present mechanisms of diamond formation both ahead of and behind the redox front. It is determined that, at oxidized conditions, a low-temperature Ca-rich carbonate melt is generated. This melt acts as both the carbon source and crystallization medium for diamond, whereas at reduced conditions, diamond crystallizes only from the Fe-C melt. The redox mechanism revealed in this study is used to explain the contrasting heterogeneity of natural diamonds, as seen in the composition of inclusions, carbon isotopic composition, and nitrogen impurity content.

In this paper, we report on the diamond growth and morphology evolution of diamond crystals of cube-octahedron series under the effect of impurity adsorption. In the first series of experiments it is established that in the Ni0.7Fe0.3–C system without impurity additives diamonds of predominantly cubic, cuboctahedral and octahedral habit crystallize at 1300, 1335, and 1370 °C, respectively. In the second series of experiments, the effect of H2O additive, in amount 0.3 wt %, on diamond growth and morphology was studied at different temperatures. It is found that the impurity adsorption effect leads to growth inhibition followed by complete blocking of growth of the {100} faces. At these conditions, the {111} faces continue to grow but reduce in area due to the inhibition of the ends of the {111} growth layers. As a result, a rich morphological diversity of antiskeletal diamond crystals was produced, whose growth and design are determined by the impurity adsorption effect and crystallization temperature. The...

Phase relations in the Na2CO3-MgCO3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900–1400 °C temperatures. Sub-solidus assemblages are represented by Na2CO3+Na2Mg(CO3)2 and Na2Mg(CO3)2+MgCO3, with the transition boundary near 50 mol% MgCO3 in the system. The Na2CO3-Na2Mg(CO3)2 eutectic is established at 1200 °C and 29 mol% MgCO3. Melting of Na2CO3 occurs between 1350 and 1400 °C. We propose that Na2Mg(CO3)2 disappears between 1200 and 1250 °C via congruent melting. Magnesite remains as a liquidus phase above 1300 °C. Measurable amounts of Mg in Na2CO3 suggest an existence of MgCO3 solid-solutions in Na2CO3 at given experimental conditions. The maximum MgCO3 solubility in Na-carbonate of about 9 mol% was established at 1100 and 1200 °C. The Na2CO3 and Na2Mg(CO3)2 compounds have been studied using in situ X-ray coupled with a DIA-type multi-anvil apparatus. The studies showed that eitelite is a stable polymorph of Na2Mg(CO3)2 at least up to 6.6 GPa and 1000 °C. In contrast, natrite, γ-Na2CO3, is not stable at high pressure and is replaced by β-Na2CO3. The latter was found to be stable at pressures up to 11.7 GPa at 27 °C and up to 15.2 GPa at 1200 °C and temperatures at least up to 800 °C at 2.5 GPa and up to 1000 °C at 6.4 GPa. The X-ray and Raman study of recovered samples showed that, under ambient conditions, β-Na2CO3 transforms back to γ-Na2CO3. Eitelite [Na2Mg(CO3)2] would be an important mineral controlling insipient melting in subducting slab and upwelling mantle. At 6 GPa, melting of the Na2Mg(CO3)2+MgCO3 assemblage can be initiated, either by heating to 1300 °C under “dry” conditions or at 900–1100 °C under hydrous conditions. Thus, the Na2Mg(CO3)2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the Na- and Mg-rich carbonatite melts similar to those found as inclusions in olivines from kimberlites and the deepest known mantle rock samples—sheared peridotite xenoliths (190–230 km depth).

Phase relations in the system Na2CO3-CaCO3 have been studied in the compositional range, X (Na2CO3), from 100 to 10 mol%, at 6.0 GPa and 900–1400 °C. Below 1100 °C, the system has three intermediate compounds: Na4Ca(CO3)3, Na2Ca3(CO3)4, and Na2Ca4(CO3)5. The Na4Ca(CO3)3 and Na2Ca3(CO3)4 compounds melt congruently slightly above 1200 and 1300 °C, respectively. The eutectics were established at 70 and 52 mol% near 1200 °C and at 21 mol% near 1300 °C. The Na2Ca4(CO3)5 compound decomposes to the Na2Ca3(CO3)4 + aragonite assembly at 1100 °C. Maximum solid solution of CaCO3 in Na2CO3 is 6–8 mol% at 1100–1300 °C. Melting of Na2CO3 occurs between 1350 and 1400 °C. Na solubility in aragonite does not exceed the detection limit (

Phase relations in the K 2 CO 3 -MgCO 3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900–1450 °C temperatures. Subsolidus assemblies comprise the fields K 2 CO 3 +K 2 Mg(CO 3 ) 2 and K 2 Mg(CO 3 ) 2 +MgCO 3 with the transition boundary near 50 mol% MgCO 3 in the system. The K 2 CO 3 -K 2 Mg(CO 3 ) 2 eutectic is established at 1200 °C and 25 mol% MgCO 3 . Melting of K 2 CO 3 occurs between 1400 and 1450 °C. We propose that K 2 Mg(CO 3 ) 2 disappears between 1200 and 1300 °C via congruent melting. Magnesite is observed as a subliquidus phase to temperatures in excess of 1300 °C. At 6 GPa, melting of the K 2 Mg(CO 3 ) 2 +MgCO 3 assemblage can be initiated either by heating to 1300 °C under “dry” conditions or by adding a certain amount of water at 900–1000 °C. Thus, the K 2 Mg(CO 3 ) 2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the K- and Mg-rich carbonatite melts similar to those found as microinclusions in “fibrous” diamonds. The K 2 Mg(CO 3 ) 2 compound was studied using in situ X-ray coupled with a DIA-type multi-anvil apparatus. At 6.5 GPa and 1000 °C, the structure of K 2 Mg(CO 3 ) 2 was found to be orthorhombic with lattice parameters a = 8.8898(7), b = 7.8673(7), and c = 5.0528(5), V = 353.39(4). No structure change was observed during pressure decrease down to 1 GPa. However, recovered K 2 Mg(CO 3 ) 2 exhibited a trigonal R 3 m structure previously established at ambient conditions.

In this paper, we report on the influence of the H2O additive in the metal melt on the growth processes, morphology, and defect-and-impurity structure of diamond crystals. As a source of water, a mixture of Mg(OH)2 and SiO2 was used, which reacted under the experimental conditions following the reaction 2Mg(OH)2 + SiO2 = Mg2SiO4 + 2H2O. The main experiments were performed in the Ni0.7Fe0.3–C system at a pressure of 6 GPa, a temperature of 1370 °C, a duration of 15 h, and a H2O concentration (CH2O) ranging from 0 to 0.5 wt %. It is found that, as the H2O content increases from 0 to 0.2 wt %, the diamond morphology changes from bulky octahedra to {111} platelike crystals and then to {111} dendrites. Further increases in CH2O in the range of 0.22–0.4 wt % result in crystallization of bulky rhombic dodecahedra and then {110} dendrites. At a CH2O higher than approximately 0.43 wt %, nucleation and growth of diamond are completely terminated and only graphite crystallizes. The systematic changes in diamond grow...

Diamond crystallization in phosphorus–carbon systems was studied at high pressures and high temperatures of 6.3 and 7.5 GPa and 1400–1850 °C. To allow for the crystallization kinetics, the run times were set as long as 40–60 h. As a result, the range of the pressure–temperature (P–T) conditions of diamond nucleation and growth on seeds was considerably extended and the fields of diamond spontaneous nucleation, diamond growth on seeds, and formation of metastable graphite were determined. It is experimentally established that the main parameters governing diamond crystallization processes are pressure, temperature, and run duration. The occurrence of Ca,Mg-phosphate, insoluble to phosphorus melt, is found to affect significantly the growth and morphology of diamond. The phosphate melt adsorbs selectively on the tetrahexahedral {310} faces giving rise to peculiar ⟨321⟩ growth steps. Crystallized diamonds were characterized by Fourier transform infrared (FTIR) absorption and Raman spectroscopy. It is found t...

Octahedral diamond crystals grown by the temperature gradient method at 1550 °C using a BARS apparatus have been studied. Dislocations and planar defects in diamond crystals have been found and characterized by selective etching and X-ray diffraction topography. It is found that the diamond crystals contained not more than four bunches of extended defects. Large planar defects and narrow bunches of straight 〈1 1 1〉 dislocations extend from the seed crystal. 〈1 1 1〉 dislocations initiate stacking faults and partial dislocations in the 〈1 1 2〉 direction. These defects also give rise to 〈2 2 1〉 dislocations. Partial dislocations are dominant. Screw and then edge and mixed dislocations appear as the densities of linear and planar defects increase in the bunch. Combined cathodo- and photoluminescence topographic, X-ray topographic and selective etching studies of {1 1 1} faces showed, that single 〈1 1 1〉 dislocations are the sources of large low-elevation hillocks, which appeared during crystal growth. It is concluded that diamond crystal growth at the specified average rates of 39–45 μm/h is a phenomenon involving the simultaneous participation of dislocations and two-dimensional nucleation as sources of growth steps. The studies have shown that dislocation-free regions in the octahedral diamond crystals weighing 3 carats occupy about 58 mm 3 , and some crystals have completely dislocation-free {1 1 1} growth sectors.