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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.

An analytical review of the results of experimental studies of diamond crystallization and dissolution in systems with different compositions and mantle P–T parameters is presented. It is shown that the diamond morphology and the set of defect-impurity centers depend to a great extent on the composition of crystallization medium. Specific crystallomorphological and crystallochemical features of diamond that are typical of certain conditions are revealed. The genetic informativeness of some diamond characteristics and possibility of their use as indicators of the composition of crystallization and dissolution media and oxygen fugacity in diamond-origin processes are substantiated based on the experimentally established regularities in the “conditions–properties” system.

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.

EUV and X-ray Optics, Sources, and Instrumentation : [Proceedings] - SPIE, 2021. - ISBN 97815106438649781510643871 - doi:10.1117/12.2589702 EUV and X-ray Optics, Sources, and Instrumentation, Online Only, Czech Republic, 19 Apr 2021 - 24 Apr 2021; Proceedings of SPIE 11776, 117760G (2021). doi:10.1117/12.2589702 special issue: "SPIE Optics + Optoelectronics", In the manuscript we report on characterization of single-crystalline (111) plates prepared from type Ib diamonds with nitrogen content of 100-150 ppm and (100) plates prepared from IIa diamond by means of high-resolution rocking curve imaging (RCI). Contrary to a common opinion about intrinsic poor diffraction quality of type Ib diamonds, RCI showed the presence of nearly defect-free areas of several mm2 in the central part of the (111)-oriented diamond plates. In comparison with the (100)-oriented IIa diamond plates prepared by the same HPHT setup the Ib diamonds possess better diffraction quality. The observed broadening of the rocking curves associates with the cutting and polishing processes, causing strains around the edges of the plates and rare defects. An improvement of preparation technique will thus allow to make single-crystalline diamond plates for Laue and Bragg monochromators and beam splitters from type Ib diamond with areas large enough to be used as wavefront-preserving optical elements at 4th generation synchrotron facilities. Contrary to a common opinion about intrinsically poor diffraction quality of type I diamonds, RCI showed the presence of nearly defect-free areas of several mm2 in the central part of the diamond plates. The observed broadening of the rocking curves results from the cutting and polishing processes, causing strains around the edges of the plates and rare defects. An improvement of preparation technique will thus allow to make single-crystalline diamond plates for Laue and Bragg monochromators and beam splitters from type Ib material with areas large enough to be used as optical elements at 4th generation synchrotron facilities., Published by SPIE, Bellingham, Wash.

We studied the morphology of dissolution and graphitization forms of sectorial natural and synthetic diamonds. Experiments on diamond dissolution (graphitization) were performed in model systems of different composition in a wide range of temperatures and pressures. It was found that, after dissolution, the raised (positive) and etched (negative) regions of the relief are formed along the outcrops of the growth sectors on the round surface of the diamond crystals. The relief patterns associated with the sectorial structure of crystals do not affect in the geometry of dissolution forms, which is mainly determined by external factors such as solvent composition, redox conditions, and process temperature. Based on the relative height of the relief elements at the outcrops of different growth sectors, it is concluded that the rate of dissolution of the growth sectors of synthetic diamonds decreases in the sequence {1 0 0}, {1 1 1}, {3 1 1}, {1 1 0}. In graphitization, the highest graphitization rate is observed in the growth sectors of {1 0 0} faces as well. As a result, deep grooves are formed on the growth sectors {1 0 0} faces. We assume that the dissolution features of various growth sectors are determined by the dependence of the rate of diamond dissolution (graphitization) on the concentration of nitrogen impurities in the growth sectors of various simple forms.

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.

The morphology of resorbed diamond crystals is a valuable source of information on the composition and ascent rate of kimberlite magmas, as well as on possible redox conditions in protolith. Previously, diamond resorption was thoroughly investigated at P–T–fO2 parameters of the kimberlite magma ascent. In this study, we investigated diamond resorption using unaltered group I kimberlite and model carbonatite at P–T–fO2 parameters that are typical of the peridotite source of kimberlite magmas in the subcontinental lithospheric mantle. An analysis of previous studies made it possible to determine the rate of diamond octahedron transformation into a spherical tetrahexahedron depending on the composition of the carbonate–silicate melt. It was shown that the rate of diamond resorption at 6.3 GPa increases in all the investigated systems as fO2 and temperature rise. There is a steady decrease in the diamond resorption rate as pressure increases from 1 GPa to 6.3 GPa. The morphology comparison of the experimentally produced samples with natural diamonds is indicative of the significant contribution of metasomatic alteration of protolith by the oxidized agent and at the initial stages of kimberlite magma ascent to the resorption of natural 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.

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.

We studied unusual growth macrolayers on the {100} faces of diamond crystals grown in Mg-C and Mg0.9Ge0.1-C systems at 7.0 GPa and in the temperature range of 1800–1900 °C. The layers were shaped as rectangles highly elongated in one of the [110] directions. The layer elongation (length to width ratio) was different, but did not exceed 23. We found that the ends of the macrolayers in most cases were composed of {111} microfacets, and their growth occurred by layers along these microfacets. Using selective etching, we found that the macrolayers were not associated with outcrops of dislocations and any planar defects on the {100} faces. It is supposed that the formation of these highly elongated layers occurs by two-dimensional nucleation that becomes possible at high carbon supersaturation under conditions of kinetically limited diamond growth in Mg-based systems.

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.

The dislocation structure of diamond crystals grown in the Mg-C system at a pressure of 7 GPa and temperatures of 1800–1900 °C is studied by X-ray topography and selective etching. According to the selective etching data, diamond crystals have two types of dislocations whose outputs on the {100} faces of crystal are associated with two types of etch pits. It has been demonstrated that the etch pits with the side wall inclination angle of about 7° are formed at the outcrop points of full edge dislocations, while the etch pits with the inclination angle of about 4° are associated with 45° mixed dislocations. It has been found that the dislocation structure of diamonds grown at 1900 °C is completely determined by the seed crystals structure and the dislocation density is 105 cm− 2. The dislocation density in the diamond crystals grown at 1800 °C increases by two or three orders of magnitude due to nucleation of dislocations at the seed-overgrown layer interface and in the overgrown layer. The high dislocation density leads to the mosaic structure of crystals and misorientation of single blocks, up to 1°. Local ring clusters of edge dislocations were found to be the dominant source of growth layers on the {100} faces of diamond.

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.

Diamond crystallization was experimentally studied in a CO 2 -bearing alkaline carbonate melt with an increased content of nitrogen at pressure of 6.3 GPa and temperature of 1500 °C. The growth rate, morphology, internal structure of overgrown layers, and defect-impurity composition of newly formed diamond were investigated. The type of growth patterns on faces, internal structure, and nitrogen content were found to be controlled by both the crystallographic orientation of the growth surfaces and the structure of the original faces of diamond seed crystals. An overgrown layer has a uniform structure on the {100} plane faces of synthetic diamond and a fibrillar (fibrous) structure on the faceted surfaces of a natural diamond cube. The {111} faces have a polycentric vicinal relief with numerous twin intergrowths and micro twin lamellae. The stable form of diamond growth under experimental conditions is a curved-face hexoctahedron with small cube faces. The nitrogen impurity concentration in overgrown layers varies depending on the growth direction and surface type, from 100 to 1100 ppm.

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...

The etching rate and the morphology of dislocation etch pits on {111} faces of synthetic diamond crystals with nitrogen impurity content varying from 1 ppm to 600 ppm were studied. Etch pits on nitrogen-containing diamond crystals are found to be of triangular shape; their lateral walls are formed by planar faces. Etch pits on nitrogen-gettered diamonds have a shape of a convex triangle and the profile varying from the maximum inclination angle in the pit center to zero at a large distance from it. The rate of diamond etching (oxidation) rises as the content of nitrogen impurity in diamond crystals increases from 1 ppm to 200 ppm and subsequently decreases twice at nitrogen concentration of 600 ppm. These regularities of etching agree well with the existing models of crystal dissolution in the presence of impurities. It is assumed that nitrogen atoms incorporated in the diamond lattice can be regarded on the crystal surface as atoms of a stable inhibiting impurity, which change the rate and character of etching of diamond crystals.

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.

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.

Fusion and solidification of Al and Ag samples, as well as Fe93–Al3–C4, Fe56–Co37–Al3–C4, and Fe57.5–Co38–Al1–Pb0.5–C3 alloys (in wt%), have been investigated at 6.3 GPa. Heater power jumps due to heat consumption and release on metal fusion and solidification, respectively, were used to calibrate the thermal electromotive force of the thermocouple against the melting points (mp) for Ag and Al. Thus, obtained corrections are +100°C (for sample periphery) and +65°C (center) within the 1070–1320°C range. For small samples positioned randomly in the low-gradient zone of a high pressure cell, the corrections should be +80°C and +84°C at the temperatures 1070°C and 1320°C, respectively. The temperature contrast recorded in the low-gradient cell zone gives an error about ±17°C. The method has been applied to identify the mp of the systems, which is especially important for temperature-gradient growth of large type IIa synthetic diamonds.

Diamond nucleation and growth in the magnesium–carbon system were studied at a pressure of 7 GPa and temperatures in the range of 1500–1900 °C. To explore the effects of kinetics in diamond crystallization processes the duration of experiments was varied from 5 min to 20 h. It was established that the induction period preceding diamond nucleation decreased with increasing temperature from about 17.5 h at 1500 °C to almost zero at 1900 °C, while the rate of diamond growth increased by almost three orders of magnitude, from 10 μm h−1 (1500 °C) to 8.5 mm h−1 (1900 °C). The cubic morphology was found to be the stable growth form of diamond over the entire range of temperatures used in the study. Based on the data obtained it was suggested that diamond growth in the Mg–C system took place in the kinetically controlled regime. Spectroscopic characterization revealed that the synthesized diamond crystals contained boron and silicon impurities. A specific continuous absorption, giving rise to the abundant brown coloration of the produced crystals, and a band at about 1480 cm−1 found in the Raman spectra were tentatively assigned to defects involving π-bonded carbon atoms.

Experiments on diamond crystallization in kimberlite melt were performed for 40 h at 6.3 GPa in the temperature range of 1300–1570 °C and at 7.5 GPa in the temperature range of 1450–1570 °C, using a multianvil high-pressure apparatus of split-sphere type. Group I kimberlite from the Udachnaya-East pipe and a synthetic multicomponent mixture modeling the average composition of group II kimberlites were used as starting materials. The experiments have shown that diamond growth on seed crystals in the kimberlite melt in equilibrium with olivine, pyroxene, and garnet starts from 1400 °C at 7.5 GPa and from 1520 °C at 6.3 GPa. Diamond nucleation requires higher temperature and pressure, 1570 °C and 7.5 GPa. The alkali-enriched and silicate-depleted derivates of kimberlite melts ensure the growth and nucleation of diamond at lower P and T values: 1400 °C at 7.5 GPa and 1520 °C at 6.3 GPa. The results obtained evidence that temperature, pressure, and the composition of crystallization medium are the main factors controlling diamond formation processes in the kimberlite melts and their derivates.

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 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.

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...

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...

ABS tr A ct Pressless split-sphere apparatus (BARS) equipped with an 8-6 type multi-anvil system is used extensively for large (6 carats) gem quality diamond crystal growth and various mineralogical and geochemical studies at shallow mantle conditions (P ≤ 7.5 GPa). In this study, we extend the pressure range for this apparatus to the conditions of the Earth’s transition zone (P = 20 GPa) using a 6-8 multi-anvil system. A scaled-up Kawai-cell consisting of 47 mm WC cubic anvils with 10 mm truncation and 18 mm MgO pressure medium was employed for the experiments. A 300 mm spherical multi-anvil block compressed by a hydraulic system with 2500 atm capacity was used to compress the Kawai-cell. A pressure of 15.5 GPa was successfully generated at the oil pressure of 1875 atm. The comparison of the BARS apparatus calibrations with those obtained using a uniaxial split-sphere press USSA-5000 suggests that pressure up to 17 and 20 GPa in compressed cell/sample volumes of 1200/60 and 400/18 mm 3 , respectively, can be generated using a combination of the BARS apparatus and the scaled-up Kawai-cell. This cell volume is much larger than that in conventional multi-anvil systems, which does not exceed 80 mm 3 at the same conditions.

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.

The influence of CO 2 and H 2 O on the morphology of diamond dissolution in carbonate melts was studied experimentally at pressure 5.7―7.0 GPa and temperature 1400―1750 °C, using a BARS multi-anvil apparatus. It has been established that diamond dissolution in fluid-free carbonate melts starts with the development of positive trigons on the {111} diamond faces, followed by truncation of crystal edges by trigon-trioctahedral surfaces, and finally by the transformation of diamond into spherical dodecahedroid-like morphology. Diamond dissolution in CO 2 -bearing carbonate melts also begins with the formation of positive trigons on the {111} faces and development trigon-trioctahedron surfaces on the edges. Dissolution form changes from trigon-trioctahedron to dodecahedroid with increasing loss of initial weight. Addition of more than 8 wt% of H 2 O into the carbonate medium changes the orientation of the trigons and the secondary morphology of diamond. At the beginning of the process, negative trigons and ditrigonal (shield-shaped) dissolution layers developed on the {111} faces. Dissolution form of diamond in water-bearing melts is tetrahexahedroid, which is most similar to rounded natural diamonds. The results obtained allow us to regard the morphology of trigons and dissolution forms as an indicator of the composition of the diamond dissolution medium. The experiments suggest that the morphology of diamond during dissolution is controlled by the presence of water in a system. Our data show that the CO 2 /(CO 2 +H 2 O) ratio by weight value was

In this paper, we report on the influence of nitrogen concentration in metal melts on the growth processes, morphology, and defect-and-impurity structure of diamond crystals. In two series of experiments, the concentration of nitrogen in the growth system was varied by adding Fe3N and CaCN2 to the charge; the other parameters and conditions of the growth were constant: FeNiC system, P = 5.5 GPa, T = 1400 °C, and duration of 65 h. It has been found that, with increasing nitrogen concentration (CN) in the metal melt from 0.005 to 0.6 atom %, the growth of single crystal diamond is followed by formation of aggregates of block twinned crystals and then by crystallization of metastable graphite. At the stage of single crystal growth, an increase in CN results in an increase in nitrogen impurity concentration in diamond crystals from about 200 ppm to approximately 1100 ppm, an increase in density of dislocations, twin lamellae, and internal strains, and a change in crystal morphology. Further increases in CN re...

As a result of experiments on diamond crystallization and annealing, diamond crystals with graphite inclusions were produced. Crystallization of diamond was realized in dry and water-containing chloride, carbonate and silicate systems at P = 5.7–7.5 GPa and T = 1400–1800 °C using high-pressure equipment of “split-sphere” type. Annealing of diamond crystals in the field of thermodynamic stability of graphite was carried out at P = 7.0 GPa and T = 2500 °C, as well as at 1500 °C in vacuum (10 − 5 atm). As a result, we produced diamond crystals with graphite inclusions of three genetic types (protogenetic, syngenitic and epigenetic). Initial graphite and newly formed diamond and graphite were studied using optical and scanning electron microscopes, as well as micro-Raman spectroscopy. It was shown experimentally that graphite inclusions in diamond crystals could form not only within the graphite stability field prior or after diamond crystallization, but also within the diamond stability field as a result of joint crystallization of diamond and graphite. The features of graphite inclusions' morphology, Raman spectra, and strain patterns around the inclusions could be indicators of the mechanism of inclusion formation.

In this paper, we report on diamond synthesis in the sulfur―carbon (S―C) system at temperatures in the range of 1550―2000 °C and pressures of 6.3―7.5 GPa. It is established that crystallization of carbon phases (diamond and graphite), the degree of the graphite-to-diamond conversion, and diamond crystal morphology are primarily determined by the pressure―temperature (P―T conditions. It is suggested that the most probable medium of diamond crystallization in the S―C system is the melting of carbon disulfide CS 2 . Fourier transform infrared (FTIR) microspectroscopy revealed that synthesized diamond crystals contained nitrogen impurity in the form of A and C defects with a total nitrogen concentration up to 1500 ppm. For the crystals with high nitrogen content, hydrogen-related centers are typically observed. Addition of water to the S―C system results in a decrease in the diamond synthesis temperature and a change in diamond crystal morphology from cuboctahedron to octahedron.

The effect of high pressure high temperature treatment on the spectroscopic features of boron-doped type IIb synthetic diamonds has been studied. Synthetic HPHT diamond crystals with different concentrations of boron acceptors were annealed at temperatures in the range 1800–2650 °C under a stabilizing pressure of 7–7.5 GPa. Fourier-transform infra-red absorption spectroscopy and cathodoluminescence were applied to study the effect of the annealing. We found that for annealing temperatures up to 2650 °C, the IR absorption features related to neutral boron acceptors did not change in intensity, demonstrating that single substitutional boron was not affected by the treatment. This finding is in good agreement with the recent first-principle calculations of Goss and Briddon (Phys. Rev. B 73 (2006) 085207) predicting very high activation energies for migration of substitutional boron in the diamond lattice. For diamonds with boron concentration of about 1 ppm, it was found that annealing at 2650 °C for 1 h produced an intense CL emission band peaking at 2.85 eV, which is known to be related to dislocations. It was suggested that dislocations in the studied boron-doped diamonds were formed due to plastic deformation occurred during anneals.

Etching of synthetic diamond single crystals in NaNO 3 and KNO 3 melts at 750–800 °C has been carried out to study linear etch figures and related etch pits. Two types of planar defects in {1 1 1} planes in large synthetic diamond crystals have been found from the structure of linear etch figures. They are microtwin lamellae and stacking faults. It is established that linear etch figures, bound by the etch pits with α (1 1 1) =3° are formed at the emergence lines of stacking faults. The linear etch figures, bound by the etch pits with α (1 1 1) =2° or showing no etch pits are formed at the emergence lines of the microtwin lamellae or double stacking faults. Possible types of partial dislocations bounding stacking faults and triangular pits bounding the linear etch figures have been correlated. The analysis has shown that shallow etch pits with inclination angles α (1 1 1) =2° and 3° are formed at the emergence points of partial dislocations and etch pits with α (1 1 1) =4° at the emergence points of screw dislocations.

In this paper, we report results of experiments on the dissolution of octahedral, pseudo-dodecahedral, and cubic natural diamond crystals in water-containing carbonate and silicate systems at high-pressure and high-temperature conditions in the diamond stability field. The dissolution agents used include CaCO3, CaMg(CO3)2, CaMgSi2O6, and kimberlite from the Udachnaya pipe, Yakutia, with addition of distilled water. The obtained diamond dissolution forms were studied using scanning electron microscopy and double-beam interferometry. A quantitative analysis of rounded diamonds was carried out by the photogoniometry method. The experimental data show that diamonds change their morphology from octahedrons, dodecahedrons, and cubes to tetrahexahedroids when dissolved in water-containing systems. Octahedron transforms into tetrahexahedroid when the weight loss is 20–25%; cube, when the loss is >50%; and pseudo-dodecahedron passes into tetrahexahedroid when the weight loss is as low as 10%. Comparison of crystal morphology, surface features, and goniometric data of diamond dissolution forms produced in water-containing systems and of rounded natural diamonds showed their complete identity. It has been established that the morphological variations of rounded natural diamonds depend on the initial habit of the crystals and the degree of their dissolution. With the significant dissolution of the starting crystals the dissolution forms of initial octahedrons, pseudo-dodecahedron, and cubes are similar. The evolution of the diamond crystals morphology is terminated with the formation of tetrahexahedroid with curvature parameters AÂ = 36°07′, ND = 13°15′, and DD = 13°15′. The obtained quantitative data allowed us to construct a scheme for the morphological evolution of natural diamond crystals during their dissolution.

Experiments are applied to constrain the composition of primary kimberlitic magmas which were in equilibrium with lithospheric peridotite and could resorb the entrained diamond to form typical dissolution features. The experiments are run on samples of a model carbonatite and a melt of the Udachnaya kimberlite at 6.3 GPa and 1400 °C, and at unbuffered or Re–ReO2-buffered oxygen fugacity (1–2 log units above Ni–NiO). Near-liquidus dry Fe3+-free carbonatitic melt (derived from carbonated harzburgite) is saturated with the Ol–Grt–Opx–Mgs assemblage and is almost inert to diamond. Carbonatitic melts that bear 4.6–6.8 wt% Fe2O3 or 1.5 wt% H2O are in equilibrium only with Mgs ± Ol near the liquidus. Dissolution of diamond by these melts produces surface textures uncommon (corrosion sculptures) or common (negative-oriented trigons, shield-shaped laminae and elongate hillocks) to kimberlitic diamonds. The near-liquidus melt of the Udachnaya kimberlite (Yakutia) with 10–12 wt% H2O is saturated with the Ol–Grt–Cpx assemblage and may result from melting of carbonated garnet-bearing wehrlite. Hydrous kimberlitic melt likewise resorbs diamonds forming typical negative-oriented trigons, shield-shaped laminae and elongate hillocks on their surfaces. Therefore, the melts that could originate in the thermal conditions of subcratonic lithosphere, entrain diamond and dissolve it to produce dissolution features on crystal surfaces, were compositionally close to kimberlite (16–19 wt% SiO2) and rich in H2O. Dry Fe3+-bearing carbonatites with fO2 controlled by the ferric/ferrous equilibrium slightly above the Ni–NiO buffer cannot be diamond carriers.

The interaction of graphite with (Fe,Ni)9S8 and FeS sulfide melts has been studied at P = 6.3–7.5 GPa and T = 1450–2200 °C in experiments with a duration from 2 to 65 h. It has been found that both pentlandite and pyrrhotite melts interacting with graphite show similar behavior. At

Etching of synthetic and natural diamond single crystals has been carried out in KNO 3 or NaNO 3 melts at 600–900 °C. A correlation between the etch pits and dislocations emerging on the {1 1 1} faces has been established by the etching of oppositely matched cleavages, thin plates, natural diamond crystals with trigons on the octahedral faces and repeated etching. The etching results were correlated with X-ray topography data. It has been revealed that pyramidal etch pits on the {1 1 1} diamond faces with the inclination angles of 2–8° are formed at the emergence points of dislocations. In this case, the etch pits with the inclinations of 5°, 6°, 5° and 8° originate at the emergence points of edge dislocations, more shallow pits are probably related to screw and partial dislocations. The etch pits with the inclination of 11–15° are not associated with dislocations and can be related to impurity clusters, microcracks and other surface defects.

Crystallization of diamond in the phosphorus-graphite system has been studied at 7 GPa and 1750 °C in a series of experiments with duration from 5 to 20 h. Spontaneous diamonds crystallized through both film growth (FG) and temperature gradient growth (TGG) processes as well as diamond layers grown on seed crystals were obtained. Morphology of the crystallized diamonds was studied by scanning electron microscopy and goniometry. For TGG diamonds faces of tetrahexahedron and trapezohedron indexed as {310} and {911}, respectively, which are the new growth forms of synthetic diamond, were established. By FTIR measurement it was shown that bluish coloration characteristic of crystallized diamonds arises from a continuum of absorption, which is similar to the phosphorus photo-ionization continuum seen in P-doped CVD diamond films.

The capability of iron nitride, Fe3N for converting graphite to diamond was explored at P=7 GPa and T=1550–1850 °C in experiments with a duration of 20 h. It was established that depending on the synthesis temperature the iron nitride melt provides conditions for crystallisation of diamond and/or graphite, with the minimal temperature for spontaneous diamond nucleation being approximately 1700 °C. Based on the results obtained it was argued that the iron nitride acts as the solvent-catalyst for diamond formation. The crystallised diamonds were found to contain nitrogen in concentration up to approximately 3300 ppm, which depending on the synthesis temperature was present in either the A form or both A and C forms. Absorption peaks caused by hydrogen-related defects were observed in IR spectra of all diamonds examined. For the 3107 cm−1 line a tendency to increase in intensity with increasing the nitrogen content was found. The well-known blue band-A, N3, H3 and 2.156 eV systems as well as a band with zero-phonon energy at 1.787 eV were observed in cathodoluminescence.

Crystallization of diamond and graphite from the carbon component of magnesite, upon its decarbonation in reactions with coesite and enstatite at pressures of 6-7 GPa and temperatures of 1350-1800 °C has been accomplishedexperimentally. In a series of experiments, diamond was obtained in association with enstatite, coesite, and magnesite, as well as with forsterite, enstatite, and magnesite. Octahedral diamond crystals with sizes up to 450 pm were studied by FTIR spectroscopy and were found to contain nitrogen and hydrogen, which are known as the most abundant impurities in natural type Ia diamonds. We found that growth of diamond on the cubic faces of seed crystals proceeds with formation of a cellular surface structure, which is similar to natural fibrous diamonds. The isotopic composition of synthesized diamonds (δ 1 3 C = -1.27‰) was determined to be close to that of the initial magnesite (δ 1 3 C = -0.2‰).

We have experimentally studied the formation of diamonds in alkaline carbonate–carbon and carbonate–fluid–carbon systems at 5.7–7.0 GPa and 1150–1700 °C, using a split-sphere multi-anvil apparatus (BARS). The starting carbonate and fluid-generating materials were placed into Pt and Au ampoules. The main specific feature of the studied systems is a long period of induction, which precedes the nucleation and growth of diamonds. The period of induction considerably increases with decreasing P and T , but decreases when adding a C–O–H fluid to the system. In the range of P and T corresponding to the formation of diamonds in nature, this period lasts for tens of hours. The reactivity of the studied systems with respect to the diamond nucleation and growth decreases in this sequence: Na 2 CO 3 –H 2 C 2 O 4 ·2H 2 O–C>K 2 CO 3 –H 2 C 2 O 4 ·2H 2 O–C>>Na 2 CO 3 –C>K 2 CO 3 –C. The diamond morphology is independent of P and T , and is mainly governed by the composition of the crystallization medium. The stable growth form is a cubo-octahedron in the Na 2 CO 3 melt, and an octahedron in the K 2 CO 3 melt. Regardless of the composition of the carbonate melt, only octahedral diamond crystals formed in the presence of the C–O–H fluid. The growth rates of diamond varied in the range from 1.7 μm/h at 1420 °C to 0.1–0.01 μm/h at 1150 °C, and were used to estimate, for the first time, the possible duration of the crystallization of natural diamonds. From the analysis of the experimental results and the petrological evidence for the formation of diamonds in nature, we suggest that fluid-bearing alkaline carbonate melts are, most likely, the medium for the nucleation and growth of diamonds in the Earth's upper mantle.