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A new gadolinium diphosphate [HGdP2O7 · 2 H2O] · NH3 was synthesized via a soft chemistry route from evaporation of aqueous solution. It crystallizes in the space group Pī with Z = 2 and the follow...

International audience; Na 4 Si 24 is the precursor to Si 24 , a recently discovered allotrope of silicon. With a quasidirect band gap near 1.3 eV, Si 24 has potential to transform silicon-based optoelectronics including solar energy conversion. However, the lack of large, pure crystals has prevented the characterization of intrinsic properties and has delayed deposition-based metastable growth efforts. Here, we report an optimized synthesis methodology for single-crystalline Na 4 Si 24 with crystals approaching the millimeter-size scale with conditions near 9 GPa and 1123 K. Single-crystal diffraction was used to confirm the open-framework structure, and Na atoms remain highly mobile within the framework channels, as determined by electrical conductivity and electron energy loss spectroscopy measurements. An epitaxial relationship between Na 4 Si 24 and diamond cubic silicon (DC-Si), observed through high-resolution transmission electron microscopy, is proposed to facilitate the growth of high-quality Na 4 Si 24 crystals from DC-Si wafers mixed with metallic Na and could provide a viable path forward for scaling efforts of Na 4 Si 24 and Si 24. ■ INTRODUCTION While silicon is a cornerstone of modern technology, the indirect nature of the fundamental band gap places limitations on desired applications. 1−4 However, several other silicon allotropes are known, especially under high-pressure conditions. 5,6 Among these, BC8/R8, Si 136 clathrate and hexagonal diamond (HD)-Si can be stabilized at ambient conditions. 5,7−13 In terms of optoelectronic properties, Si 136 is particularly encouraging as it has a wide direct or nearly direct gap near 1.9 eV, 14−17 though some electronic dipole transitions are forbidden, which detract from its potential photovoltaic merits. 18 Additionally, challenges with large-scale/film growth and sample purity of Si 136 have prevented major development thus far. 12,14,19 Moreover, numerous allotropes with exceptional optical properties are predicted to exist with energy not far above the DC-Si ground state, 4,5,20−22 suggesting potential for novel, metastable silicon allotropes with enhanced optoelectronic properties. Recently, a new open-framework allotrope of silicon, Si 24 , was discovered using a high-pressure precursor synthesis method. 23,24 In the first step of this process, a Na 4 Si 24 precursor was synthesized at high pressure. Na 4 Si 24 possesses the Eu 4 Ga 8 Ge 16 (Cmcm) structure type 25 with four formula units of NaSi 6. This structure type is also known for some alkaline-and rare-earth silicides, MSi 6 (M = Ca, 26 Sr, 27 Ba, 28 Eu, 29 Na 23,24). The structure consists of a clathrate-like sp 3 silicon host lattice comprised of five-, six-, and eight-membered silicon rings that form channels along the crystallographic a-axis. The topology is isostructural with calcium aluminosilicate zeolite (code CAS), 30 and the channels are filled with one-dimensional chains of metal guest atoms. This is unlike other known Na−Si clathrates such as Na 8 Si 46 (sI) and Na 24 Si 136 (sII), where the host Si lattice forms complete polyhedral cages around the Na guest atoms, making Na difficult to remove from the structure. Na removal from from sI is not possible, while complete Na removal from sII is extremely difficult

Silicon is essential for today's electronics because of its ability to show various electronic behaviors that are relevant to numerous fields of cutting-edge applications. It is not a pollutant and, therefore, an ideal candidate to replace the actual materials in photovoltaics, such as compounds based on the arsenic and heavy metals. However, conventional diamond-like Si is an indirect gap semiconductor and cannot absorb solar photons directly. This justifies intensive theoretical and experimental research for the direct-bandgap forms of silicon. Our recent high-pressure studies of the chemical interaction and phase transformations in the Na-Si system, revealed a number of interesting routes to new and known silicon compounds and allotropes. The pressure-temperature range of their formation is suitable for large-volume synthesis and future industrial scaling. The variety of properties observed (e.g. quasi-direct bandgap of open-framework allotrope Si 24 ) allows us to suggest future applications.

International audience; Hexagonal Si allotropes are expected to enhance light absorption in the visible range as compared to common cubic Si with diamond structure. Therefore, synthesis of these materials is crucial for the development of Si-based optoelectronics. In this work, we combine in situ high-pressure high-temperature synthesis and vacuum heating to obtain hexagonal Si. High pressure is one of the most promising routes to stabilize these allotropes. It allows one to obtain large-volume nanostructured ingots by a sequence of direct solid–solid transformations, ensuring high-purity samples for detailed characterization. Thanks to our synthesis approach, we provide the first evidence of a polycrystalline bulk sample of hexagonal Si. Exhaustive structural analysis, combining fine-powder X-ray and electron diffraction, afforded resolution of the crystal structure. We demonstrate that hexagonal Si obtained by high-pressure synthesis correspond to Si-4H polytype (ABCB stacking) in contrast with Si-2H (AB stacking) proposed previously. This result agrees with prior calculations that predicted a higher stability of the 4H form over 2H form. Further physical characterization, combining experimental data and ab initio calculations, have shown a good agreement with the established structure. Strong photoluminescence emission was observed in the visible region for which we foresee optimistic perspectives for the use of this material in Si-based photovoltaics.

International audience; Inorganic nanocomposites made of an inorganic matrix containing nanoparticle inclusions provide materials of advanced mechanical, magnetic, electrical properties and multifunctionality. The range of compounds that can be implemented in nanocomposites is still narrow and new preparation methods are required to design such advanced materials. Herein, we describe how the combination of nanocrystal synthesis in molten salts with subsequent heat treatment at a pressure in the GPa range gives access to a new family of boron-based nanocomposites. With the case studies of HfB2/β-HfB2O5 and CaB6/CaB2O4(IV), we demonstrate by X-ray diffraction and through (scanning) transmission electron microscopy the crystallization of borate matrices into rare compounds and unique nanostructured solids, while metal boride nanocrystals remain dispersed in the matrix and maintain small sizes below 30 nm, thus demonstrating a new multidisciplinary approach toward nanoscaled heterostructures.

International audience; The high-pressure and high-temperature formation and stability of recently discovered magnesium carbide Mg2C were studied by in situ X-ray diffraction up to 20 GPa and 1550 K. The insights into the thermodynamics of Mg2C under extreme conditions, and its metastability at 0.1 MPa and 300 K, were provided by ab initio calculations of total energies and phonon density of states as a function of pressure. We illustrate how the compound found occasionally in high-pressure experiments could be systematically predicted and discovered in a time-saving way. Similar theoretical approaches can be useful for prediction of synthesis conditions and recovery of new solids.

Compressibility of boron subphosphide B$_{12}$P$_2$ has been studied under quasi-hydrostatic conditions up to 26 GPa and 2600 K using laser-heated diamond anvil cell and angle-dispersive synchrotron X-ray diffraction. 300-K data fit yields the values of bulk modulus B$_0$ = 192(11) GPa and its first pressure derivative B$_0$′ = 5.5(12). It has been found that at ambient pressure the thermal expansion is quasi-linear up to 1300 K with average volume expansion coefficient $\alpha$ = 17.4(1) × 10$^{−6}$ K$^{−1}$. The whole set of experimental p–V–T data is well described by the Anderson-Grüneisen model with $δ_T$ = 6.

International audience; Phase-pure samples of a metastable allotrope of silicon, Si-III or BC8, were synthesized by direct elemental transformation at 14 GPa and similar to 900 K and also at significantly reduced pressure in the Na-Si system at 9.5 GPa by quenching from high temperatures similar to 1000 K. Pure sintered polycrystalline ingots with dimensions ranging from 0.5 to 2 mm can be easily recovered at ambient conditions. The chemical route also allowed us to decrease the synthetic pressures to as low as 7 GPa, while pressures required for direct phase transition in elemental silicon are significantly higher. In situ control of the synthetic protocol, using synchrotron radiation, allowed us to observe the underlying mechanism of chemical interactions and phase transformations in the Na-Si system. Detailed characterization of Si-III using X-ray diffraction, Raman spectroscopy, Si-29 NMR spectroscopy, and transmission electron microscopy are discussed. These large-volume syntheses at significantly reduced pressures extend the range of possible future bulk characterization methods and applications.

Large-volume, phase-pure synthesis of BC8 silicon ($Ia\overline{3}$, $cI16$) has enabled bulk measurements of optical, electronic, and thermal properties. Unlike previous reports that conclude BC8-Si is semimetallic, we demonstrate that this phase is a direct band gap semiconductor with a very small energy gap and moderate carrier concentration and mobility at room temperature, based on far- and midinfrared optical spectroscopy, temperature-dependent electrical conductivity, Seebeck and heat capacity measurements. Samples exhibit a plasma wavelength near $11\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$, indicating potential for infrared plasmonic applications. Thermal conductivity is reduced by 1--2 orders of magnitude depending on temperature as compared with the diamond cubic (DC-Si) phase. The electronic structure and dielectric properties can be reproduced by first-principles calculations with hybrid functionals after adjusting the level of exact Hartree--Fock (HF) exchange mixing. These results clarify existing limited and controversial experimental data sets and ab initio calculations.

Three different sodium-silicon clathrate compounds–Na8Si46 (sI), Na24Si136 (sII), and a new structure, NaSi6–were obtained for the first time using high-pressure techniques. Experimental and theoretical results unambiguously indicate that Na-intercalated clathrates are only thermodynamically stable under high-pressure conditions. The sI clathrate can be synthesized directly from the elements at pressures from 2 to 6 GPa in the 900–1100 K range. Over the range of conditions studied, sII clathrate only forms as an intermediate compound prior to the crystallization of sI. At higher pressures, we observed the formation of a new intercalated compound, metallic NaSi6, which crystallizes in the orthorhombic Eu4Ga8Ge16 structure. High-pressure crystallization from Na-Si melts provides significant improvements in the electrical properties of bulk clathrate materials (residual resistance ratio RRR = 24 for sI and > 13 for NaSi6), compared to the typical characteristics achieved for single crystals obtained by conve...

International audience; --The γ B28 phase is a recently established high pressure phase of boron. Its structure consists of icosahedral B12 clusters and B2 dumbbells in a NaCl type arrangement (B2)δ+(B12)δ- and displays a sig nificant charge transfer δ ~ 0.5-0.6. The discovery of this phase proved to be essential for the understand ing and construction of the phase diagram of boron. It was first experimentally obtained as a pure boron allotrope in early 2004 and its structure was discovered in 2006. This paper reviews recent results and con tentious issues related to the equation of state, hardness, putative isostructural phase transformation at ~40 GPa, and debates on the nature of chemical bonding in this phase. Our analysis confirms that (a) calcula tions based on density functional theory give an accurate description of its equation of state, (b) the reported isostructural phase transformation in γ B28 is an artifact, (c) the best estimate of hardness of this phase is 50 GPa, (d) chemical bonding in this phase has a significant degree of ionicity. Apart from pre senting an overview of the previous results within a consistent view grounded in experiment, thermody namics and quantum mechanics, we present new results on Bader charges in γ B28 using different levels of quantum mechanical theory (GGA, exact exchange, and HSE06 hybrid functional), and show that the earlier conclusion about a significant degree of a partial ionicity in this phase is very robust. An additional insight into the nature of the partial ionicity is obtained from a number of boron structures theoretically constructed in this work.

International audience; Kinetics of the wurtzite-to-rock-salt transformation in ZnO has been studied in the 5 7 GPa pressure range at temperatures below the activation of diffusion processes. The detailed analysis of non-isothermal experimental data using the general evolution equation describing the kinetics of direct phase transformations in solids allowed us to study the kinetic particularities of both nucleation and growth of the rock-salt phase in parent wurtzite ZnO. The main rate-limiting processes are thermally activated nucleation (EN = 383 kJ mol 1 at 6.9 GPa) and thermally nonactivated (most probably quasimartensitic) growth (kG = 0.833 min 1 at 6.9 GPa). The high impact of thermal deactivation of nucleation places has been evidenced in the case of slow heating, which indirectly indicates that the rs- ZnO nucleation places are mainly produced by pressure-induced stresses in the parent phase

The 300 K equations of state of graphite-like B–C phases with boron content from 0 to 50 at% have been studied using powder X-ray diffraction with synchrotron radiation up to 35 GPa. The compression axis moduli show the non-monotone dependence on boron content and strongly correlate with lattice parameters. The values of bulk moduli B 0 vary from 17.5 (0.6) GPa (t-BC4) to 30.7 (0.5) GPa (t-BC1.5), while for turbostratic graphite B 0=29.5 (1.0) GPa.

The intrinsic character of the correlation between hardness and thermodynamic properties of solids has been established. The proposed thermodynamic model of hardness allows one to easily estimate hardness and bulk moduli of known or even hypothetical solids from the data on Gibbs energy of atomization of the elements or on the enthalpy at the melting point. The correctness of this approach is illustrated by an example of the recently synthesized superhard diamond-like BC5 and orthorhombic modification of boron, γ-B28. The pressure and/or temperature dependences of hardness were calculated for a number of hard and superhard phases, i.e. diamond, cBN, B6O, B4C, SiC, Al2O3, β-B2O3 and β-rh boron. Excellent agreement between experimental and calculated values is observed for temperature dependences of Vickers and Knoop hardness. In addition, the model predicts that some materials can become harder than diamond at pressures in the megabar range.

Chemical interaction and phase transformations in the B–BN system have been in situ studied by X-ray diffraction with synchrotron radiation at pressures up to 5.3 GPa and temperatures up to 2800 K using multianvil press. New rhombohedral boron subnitride B13N2 has been synthesized by crystallization from the B–BN melt at 5 GPa. The structure of B13N2 belongs to the R-3m space group (a=5.4455(2) A, c=12.2649(9) A) and represents a new structural type. The subnitride is an individual compound and not a solid solution, in contrast to boron carbide. Besides, the formation of two other boron-rich B–N phases denoted as “B6N” and “B50N2” has been observed. Their structures seem to be much more sophisticated and have not been even resolved to present time.

Phase relations and chemical interaction in the B-BN system have been studied in situ using X-ray diffraction with synchrotron radiation at pressures to 5.3 GPa and temperatures to 2700 K. The results obtained have been used for the thermodynamic analysis of this system and for the construction of its phase diagram at 5 GPa. The values of thermodynamic functions of the phases competing in the B-BN system at high pressures and temperatures have been calculated in the framework of phenomenological models, unknown parameters of which have been found from the experimental data. It has been shown that only one thermodynamically stable boron subnitride (rhombohedral B13N2) exists in the system. It melts incongruently and forms an eutectic equilibrium with boron.

A strong correlation relationship has been established between the structure and specific Gibbs free energy of the substance atomization on the one hand, and the substance hardness and volume compressibility on the other. In the framework of the model proposed hardness is directly proportional to the specific Gibbs free energy per bond in isodesmic crystals. An application of a correction coefficient to the ionic component of chemical bonds allows one to evaluate the hardness of compounds having both the covalent (polar and nonpolar) and ion bonds. In the framework of the suggested approach we have been the first to correctly calculate the temperature dependence of the hardness by the example for diamond and cubic boron nitride.

We have prepared new nanocarbon phases using high‐temperature (1000–1600 K) treatment of C60 at moderate (0.1–1.5 GPa) pressures attainable for large‐volume high‐pressure apparatus. The structure of the phases has been studied by x‐ray diffraction and high resolution transmission electron microscopy (HRTEM). The Raman spectra and electron energy loss spectroscopy (EELS) provide evidence of the sp 2 bonding in the new materials. Mechanical and elastic properties are also presented. Whereas the values of density for the new nanocarbon phases are lower than that of graphite, they are still quite hard materials (H≈10–15 GPa) and have a high (∼90%) elastic recovery and record high values of the hardness‐to‐Young modulus ratio H/E≈0.17–0.22.

Understanding of the covalent clathrate formation is a crucial point for the design of new superhard materials with intrinsic coupling of superhardness and metallic conductivity. It has been found that silicon clathrates have the archetype structures, which can serve an existent model compounds for superhard clathrate frameworks Si–B, Si–C, B–C and C with intercalated atoms (e.g., alkali metals or even halogens) that can assure the metallic properties. Here we report our in situ and ex situ studies of high-pressure formation and stability of clathrates Na₈Si₄₆ (structure I) and Na₂₄₊xSi₁₃₆ (structure II). Experiments have been performed using standard Paris–Edinburgh cells (opposite anvils) up to 6 GPa and 1500 K. We have established that chemical interactions in the Na–Si system and transition between two structures of clathrates occur at temperatures below silicon melting. The strong sensitivity of crystallization products to the sodium concentration has been observed. A tentative diagram of clathrate transformations has been proposed. At least up to ~ 6 GPa, Na₂₄₊xSi₁₃₆ (structure II) is stable at lower temperatures as compared to Na₈Si₄₆ (structure I). Изучен in situ и ex situ процесс образования при высоких давлениях и стабильности клатратов Na₈Si₄₆ (структура I) и Na₂₄₊xSi₁₃₆ (структура II). Эксперименты были проведены в стандартных Париж-Эдинбургских ячейках (opposite anvils) при давлениях и температурах до 6 ГПа и 1500 K соответственно. Установлено, что химическое взаимодействие в системе Na–Si и переходы между двумя структурами клатратов происходят при температурах ниже температуры плавления кремния. Предложено первое приближение диаграммы превращений клатратов. Отмечена большая чувствительность продуктов кристаллизации к концентрации натрия. Na₂₄₊xSi₁₃₆ (структура II) является стабильной при более низких температурах по сравнению с Na₈Si₄₆ (структура I), по крайней мере до ~ 6 ГПа. Вивчено in situ і ex situ процес утворення при високих тисках і стабільності клатратів Na₈Si₄₆ (структура I) і Na₂₄₊xSi₁₃₆ (структура II). Експерименти було проведено в стандартних Париж-Единбурзький комірках (opposite anvils) при тисках і температурах до 6 ГПа і 1500 K відповідно. Встановлено, що хімічна взаємодія в системі Na–Si і переходи між двома структурами клатратов відбуваються при температурах нижче температури плавлення кремнію. Запропоновано перше наближення діаграми перетворень клатратів. Відзначено велику чутливість продуктів кристалізації до концентрації натрію. Na₂₄₊xSi₁₃₆ (структура II) є стабільною при більш низьких температурах у порівнянні з Na₈Si₄₆ (структура I), принаймні до ~ 6 ГПа.

The restricted growth of a solid phase from a fluid has been studied in the framework of the Johnson–Mehl–Avrami model. In the case of three-dimensional growth, the three- and two-dimensional space restrictions due to the particle overlap (intergrowth) have been studied. The resulting equations relate the true and extended conversion degrees, and allow us to describe the overlap in systems with various behaviours of the mean distance between growing particles (whether it changes or not) and various nucleation types (homogeneous, heterogeneous on distant surfaces or in bulk). The equations have been applied for analysis of the experimental kinetic curves.

Solid-state synthesis of boron subnitride, B 6 N, as a result of chemical interaction between boron and boron nitride at 7.5 GPa and 1700 °C has been previously reported by Hubert et al. However, a critical analysis of the results has shown that the evidence for the formation of boron subnitride with B 6 O-like structure is inconclusive. We have studied in situ the interaction between boron and BN at the same p – T conditions using X-ray diffraction with synchrotron radiation. At 7.4 GPa and 1700 °C the formation of a new phase has not been observed. At the same time, HP–HT treatment has resulted in strong and unpredictable preferred orientation of boron crystallites. This leads to the rise of some weak boron reflections that might be erroneously attributed to the appearance of a new phase. To cite this article: V.L. Solozhenko et al., C. R. Chimie 9 (2006) .

Quasi-hydrostatic compression of aluminum carbide, Al 4 C 3 has been studied to 6 GPa at room temperature using energy-dispersive X-ray powder diffraction with synchrotron radiation. A fit of the experimental p – V data to the Birch equation of state yields the values of the bulk modulus, B 0 , of 130(5) GPa and the first pressure derivative of the bulk modulus, B ′ 0 , of 4.6(9). The compression is found to be anisotropic, with the a -axis being more compressible than the c -axis.

The pressure-volume-temperature equations of state have been constructed by combining experimental data and semiempirical estimations for a number of compounds recently synthesized under extreme pressure-temperature conditions. The solids with various bonding types were considered: covalent hard and superhard boron-rich and diamond-like compounds (e.g. B$_{6}$O, B$_{13}$N$_{2}$, BP, c-BC$_{5}$, and nano-cBN), ionic semiconductors (e.g. Mg$_{2}$C and Mg$_{2}$C$_{3}$),as well as intercalation compounds (e.g. clathrates Na$_{4}$Si$_{24}$and Na$_{24+x}$Si$_{136}$), and simple substances (e.g. boron allotropes γ-B$_{28}$ and t′-B$_{52}$, and open-framework silicon allotrope o-Si$_{24}$ with quasi-direct bandgap). We also showed how the reliable p-V-T equations of state may be constructed using different types of data available.

International audience; ​Silicon is ubiquitous in contemporary technology. The most stable form of ​silicon at ambient conditions takes on the structure of diamond (cF8, d-Si) and is an indirect bandgap semiconductor, which prevents it from being considered as a next-generation platform for semiconductor technologies1, 2, 3, 4. Here, we report the formation of a new orthorhombic allotrope of ​silicon, Si24, using a novel two-step synthesis methodology. First, a Na4Si24 precursor was synthesized at high pressure5; second, sodium was removed from the precursor by a thermal ‘degassing’ process. The Cmcm structure of Si24, which has 24 Si atoms per unit cell (oC24), contains open channels along the crystallographic a-axis that are formed from six- and eight-membered sp3 silicon rings. This new allotrope possesses a quasidirect bandgap near 1.3 eV. Our combined experimental/theoretical study expands the known allotropy for element fourteen and the unique high-pressure precursor synthesis methodology demonstrates the potential for new materials with desirable properties.

X-ray powder diffraction with synchrotron radiation was used for the first time to study in situ diamond crystallization from the Fe−Ni−C melt at pressures up to 6 GPa and temperatures up to 1700 K. At 5.2 GPa over the whole temperature range of diamond crystallization (1510−1605 K), the melt is in equilibrium with both diamond and fcc Fe−Ni−C solid solution (γ-phase); that is, the L = C + γ monovariant eutectic reaction takes place. From the non-isothermal kinetic data, it follows that diamond crystallization is controlled by carbon diffusion in the melt. Kinetic data are best fitted by the model that assumes a constant nucleation rate and a three-dimensional growth of nuclei, with an apparent activation energy of 148(64) kJ mol-1.

Silicon is the second abundant element, after oxygen, in the earth crust. It is essential for today's electronics because of its ability to show various electronic behaviors that allow covering the numerous fields of cutting-edge applications. Moreover, silicon is not a pollutant and, therefore, is an ideal candidate to replace the actual materials in photovoltaics, like compounds based on the arsenic and heavy metals. It has not replaced them so far because Si is an indirect gap semiconductor and cannot absorb directly the solar photons without thermal agitations of crystal lattice (phonons). This puts it apart from the next-generation applications (light diode, high-performance transistor). That justifies the attempts to create silicon materials with direct gap that can absorb and emit light. Our recent high-pressure studies of the chemical interaction and phase transformations in the Na-Si system, revealed a number of interesting routes to new and known silicon compounds and allotropes. The pressure-temperature range of their formation is suitable for large-volume synthesis and future industrial scaling. The variety of properties observed (e.g. quasi-direct bandgap of open-framework allotrope Si24) allows us to suggest future industrial applications.

The new title polymorph of Mg2C3 is prepared from the elements under high pressure/high temperature conditions (9—20 GPa, 1700—2000 K) and characterized by synchrotron powder XRD, mass spectrometry, solid state NMR and Raman spectroscopy, and DFT calculations.

The p-V-T equation of state of superhard boron suboxide B6O has been measured up to 6 GPa and 2700 K using multianvil technique and synchrotron X-ray diffraction. To fit the experimental data, the theoretical p-V-T equation of state has been derived in approximation of the constant value of the Anderson-Gruneisen parameter δ T . Bulk modulus B 0 =181 GPa and its first pressure derivative B 0′ = 6 at 300 K; two parameters describing thermal expansion at 0.1 MPa, i.e., a = 1.4 × 10−5 K−1 and b = 5 × 10−9 K−2 as well as δ T = 6 have been included in the model. The good agreement between fitted and experimental isobars has been achieved to the absolute volume changes up to 5% as compared to the volume at standard conditions, V 0. The fitted thermal expansion at 0.1 MPa well consistent with the experimental data as well as with ambient-pressure heat capacity c p , bulk modulus B 0 and δ T describing its evolution with volume and temperature have been discussed. The fitted value of Gruneisen parameter γ = 0.85 in agreement with previous empiric estimations for B6O and experimental values for other boron-rich solids has been reported.

Quasi-hydrostatic compression of rhombohedral boron subnitride has been studied up to 30 GPa at room temperature using angle-dispersive X-ray diffraction with synchrotron radiation. The α -rhombohedral boron type R 3 m structure of B13N2 is stable in the whole pressure range under study. A fit of the experimental p – V data to the Birch equation of state yields the values of the bulk modulus, B 0 , of 200(15) GPa with fixed first pressure derivative of the bulk modulus, B 0 ′ , of 4.0. The compression is found to be slightly anisotropic with higher compressibility along c -axis.

International audience; A new monoclinic variation of Mg2C3 was synthesized from the elements under high-pressure (HP), high-temperature (HT) conditions. Formation of the new compound, which can be recovered to ambient conditions, was observed in situ using X-ray diffraction with synchrotron radiation. The structural solution was achieved by utilizing accurate theoretical results obtained from ab initio evolutionary structure prediction algorithm USPEX. Like the previously known orthorhombic Pnnm structure (α-Mg2C3), the new monoclinic C2/m structure (β-Mg2C3) contains linear C3 4− chains that are isoelectronic with CO2. Unlike α-Mg2C3, which contains alternating layers of C3 4− chains oriented in opposite directions, all C3 4− chains within β- Mg2C3 are nearly aligned along the crystallographic c-axis. Hydrolysis of β- Mg2C3 yields C3H4, as detected by mass spectrometry, while Raman and NMR measurements show clear CC stretching near 1200 cm−1 and 13C resonances confirming the presence of the rare allylenide anion

International audience; The 300 K equation of state of cubic (zinc blende) boron phosphide BP has been studied by in situ single crystal X ray diffraction with synchrotron radiation up to 55 GPa. The measurements have been performed under quasi hydrostatic conditions using a Ne pressure medium in a diamond anvil cell. A fit of the experimental p V data to the Vinet equation of state yields the bulk modulus B0 of 179(1) GPa with its pressure derivative of 3.3(1). These values are in a good agreement with previous elastic measurements, as well as with semiempirical estimations.

Quasi-hydrostatic compression of aluminum silicon carbide, α-Al 4 SiC 4 , has been studied up to 6 GPa at room temperature using energy-dispersive X-ray powder diffraction with synchrotron radiation. A fit of the experimental p – V data to the Birch equation of state yields the values of the bulk modulus, B 0 , of 182(10) GPa with fixed first pressure derivative of the bulk modulus, B ′ 0 , of 4.0. The compression is found to be anisotropic with higher compressibility along c -axis.

International audience; Solid-state phase transformations and melting of high-purity crystalline boron have been in situ and ex situ studied at pressures to 20 GPa in the 1500-2500 K temperature range where diffusion processes become fast and lead to formation of thermodynamically stable phases. The equilibriumphase diagram of boron has been constructed based on thermodynamic analysis of experimental and literature data. The high-temperature part of the diagram contains p-T domains of thermodynamic stability of rhombohedral b-B106, orthorhombic c-B28, pseudo-cubic (tetragonal) t'-B52, and liquid boron (L). The positions of two triple points have been experimentally estimated, i.e. b-t'-L at , 8.0 GPa and , 2490 K; and b-c-t' at , 9.6 GPa and , 2230 K. Finally, the proposed phase diagram explains all thermodynamic aspects of boron allotropy and significantly improves our understanding of the fifth element.

Journal of physics / Conference Series 950(4), 042023 (2017). doi:10.1088/1742-6596/950/4/042023, The pressure-volume-temperature equations of state have been constructed by combining experimental data and semiempirical estimations for a number of compounds recently synthesized under extreme pressure-temperature conditions. The solids with various bonding types were considered: covalent hard and superhard boron-rich and diamond-like compounds (B$_6$O, B$_{13}$N$_2$, BP, cBC$_5$, and nano-cBN), ionic semiconductors (Mg$_2$C and Mg$_2$C$_3$), as well as intercalation compounds (clathrates Na$_4$Si$_{24}$ and Na$_{24+x}$S$_{136}$), and simple substances (γ-B$_{28}$ and t'-B$_{52}$ boron allotropes, Na and Mg metals, and open-framework silicon allotrope o-Si$_{24}$). We also showed how the reliable p-V-T equations of state may be constructed using different types of available data., Published by IOP Publ., Bristol

During past years, a number of reports have been published on synthesis of tetragonal allotrope of boron, t-B52 phase. However, no unambiguous characterization of the crystal structure has been performed to the present time, while remarkable variation of the a/c lattice-parameter ratio raises strong doubts about its uniqueness. Here the Rietveld refinement of the crystal structure of the high pressure-high temperature boron phase synthesized by a direct solid-state transformation of rhombohedral β-B106 at 20 GPa and 2500 K has been reported for the first time. Although this boron allotrope belongs to the t-B52 type, its structure can be considered as pseudo-cubic with the a/c ratio of $\sqrt 2 $ .

Compressibility of high-purity nanostructured cBN has been studied under quasi-hydrostatic conditions at 300 K up to 35 GPa using diamond anvil cell and angle-dispersive synchrotron powder X-ray diffraction. It has been found that the data fit to the Vinet equation of state yields the values of the bulk modulus B₀ of 375(4) GPa with its first pressure derivative B₀´ of 2.3(3), thus, the nanometer grain size (≈ 20 nm) results in a decrease of the bulk modulus by ≈ 9‰. Сжимаемость высокочистого наноструктурированного cBN была изучена в квазигидростатических условиях при 300 K до 35 GPa в алмазных наковальнях с помощью угловой дисперсионной рентгеновской дифракции синхротронного излучения. Описание полученных данных уравнением состояния Винэ дает значение модуля сжимаемости B₀ = 375(4) ГПа и его первой производной по давлению B₀' = 2.3(3). Наноразмер зерна (~ 20 нм) приводит к уменьшению модуля сжимаемости на ~ 9 %. Стисливість високочистого наноструктурованого cBN була вивчена в квазігідростатичних умовах при 300 K до 35 ГПa в алмазних наковальнях за допомогою кутової дисперсійної рентгенівської дифракції синхротронного випромінювання. Опис одержаних даних рівнянням стану Віне дає значення модуля стисливості B₀ = 375(4) ГПа і його першої похідної по тиску B₀' = 2.3(3). Нанорозмір зерна (~ 20 нм) приводить до зменшення модуля стисливості на ~ 9 %.

International audience; Nanomaterials in the form of zero-, one-, and two-dimensional nanostructures make a high-impact background for both science and technology. At the same time, the synthesis of bulk nanostructured materials remains the least-explored but challenging domain that allows combination of desired physical, chemical, and mechanical properties and gives rise to nanoelectronics, nanomechanics, bandgap engineering, etc. The common methods of soft chemistry allow nanoparticles to be obtained whose direct sintering unavoidably leads to grain growth and loss of nanostructures. Extreme pressure is a parameter of choice to suppress the self-diffusion responsible for high-temperature recrystallization. Here we report the synthesis of high-purity bulk nanostructured cubic boron nitride (cBN) with a unique combination of properties (ultrahardness, high thermal and chemical stability, etc.) by direct phase transformation of graphite-like BN with an "ideal random layer" structure[1] at 20 GPa and 1770 K.

Novel superhard phases are expected to be found among various high-pressure polymorphs of light element compounds. Besides diamond-like phases, the icosahedral boron-rich solids are of particular interest because they could combine high hardness with advanced electronic and phonon transport properties, lightness, high thermal and chemical stability. Here we review some recent results on high-pressure synthesis of novel boron-rich solids.

International audience; In the present study the analysis of available data on structural, thermodynamic and mechani cal properties of B6O has been performed. Although the compound is known for half a century and has been extensively studied, many properties of this boron rich solid remain unknown or doubtful. A semi empirical analysis of our experimental and literature data has allowed us to choose the best values of main thermodynamic and mechanical characteristics among previously reported data, to predict the thermo elastic equation of state of B6O, and dependence of its hardness on non stoichiometry and temperature

Here we report the systematic study of solid-state phase transformations between boron polymorphs: {\alpha}-B12, {\beta}-B106, {\gamma}-B28, T-B52 and amorphous boron (am-B). It is evident that the Ostwald rule of stages plays an important role during phase transformations not only of amorphous boron, but also of crystalline forms. We have observed the crystallization of tetragonal boron T-B52 from amorphous phase of high purity (99.99%), which, however, cannot be easily distinguished from B50C2 boron compound. Many factors influence the transformations of amorphous phase, and it is possible to observe not only well known am-B \rightarrow {\alpha}-B12 and am-B \rightarrow {\beta}-B106 transformations, but also am-B \rightarrow T-B52, never reported so far. At ~14 GPa the crystallization order becomes {\beta}-B106 \rightarrow {\alpha}-B12 \rightarrow {\gamma}-B28, while at ~11 GPa the intermediate crystallization of T-B52 still was observed. This unambiguously indicates that {\alpha}-B12 is more thermodynamically stable than {\beta}-B106 at high pressures, and renders possible to transform, at least partially, common {\beta}-phase of high purity into {\alpha}-B12 at very high pressures and moderate temperatures (below 1600 K), i.e. outside the domain of its stability.

In the present work, we have proposed a method that allows one to easily estimate the hardness and bulk modulus of known or hypothetical solid phases from the data on Gibbs energy of atomization of the elements and corresponding covalent radii. It has been shown that hardness and bulk moduli of compounds strongly correlate with their thermodynamic and structural properties. The proposed method may be used for a large number of compounds with various types of chemical bonding and structures; moreover, the temperature dependence of hardness may be calculated, which has been performed for diamond and cubic boron nitride. The correctness of this approach has been shown for the recently synthesized superhard diamond-like BC5. It has been predicted that the hypothetical forms of B2O3, diamond-like boron, BC x and CO x , which could be synthesized at high pressures and temperatures, should have extreme hardness.

International audience; Novel superhard phases are expected to be found among various high-pressure polymorphs of light element compounds. Besides diamond-like phases, the icosahedral boron-rich solids are of particular interest because they could combine high hardness with advanced electronic and phonon transport properties, lightness, high thermal and chemical stability. Here we review some recent results on high-pressure synthesis of novel boron-rich solids.

The basic known and hypothetic one- and two-element phases of the B-C-N-O system (both superhard phases having diamond and boron structures and precursors to synthesize them) are described. The attention has been given to the structure, basic mechanical properties, and methods to identify and characterize the materials. For some phases that have been recently described in the literature the synthesis conditions at high pressures and temperatures are indicated., Comment: Review on superhard B-C-N-O phases

The chemical interaction and phase relations in the B-BN system have been in situ studied at 5 GPa and temperatures up to 2800 K using X-ray diffraction with synchrotron radiation. The thermodynamic analysis of the B-BN system based on experimental data allowed us to construct equilibrium and metastable phase diagrams of the system at 5 GPa. The only thermodynamically stable boron subnitride, B(13)N(2), melts incongruently at 2600 K and forms eutectic equilibrium with boron at 2300 K and 4 at. % of nitrogen.

A number of successful theoretical models of hardness have been developed recently. A thermodynamic model of hardness, which supposes the intrinsic character of correlation between hardness and thermodynamic properties of solids, allows one to predict hardness of known or even hypothetical solids from the data on Gibbs energy of atomization of the elements, which implicitly determine the energy density per chemical bonding. The only structural data needed is the coordination number of the atoms in a lattice. Using this approach, the hardness of known and hypothetical polymorphs of pure boron and a number of boron-rich solids has been calculated. The thermodynamic interpretation of the bonding energy allows one to predict the hardness as a function of thermodynamic parameters. In particular, the excellent agreement between experimental and calculated values has been observed not only for the room-temperature values of the Vickers hardness of stoichiometric compounds, but also for its temperature and concentration dependencies. В останній час було запропоновано ряд вдалих теоретичних моделей твердості. Термодинамічна модель твердості, яка ґрунтується на кореляції між твердістю і термодинамічними властивостями твердих тіл, дає можливість спрогнозувати твердість відомих або навіть гіпотетичних твердих тіл, виходячи з даних по енергії Гіббса атомізації елементів, які опосередковано визначають енергетичні характеристики хімічного зв’язку. При цьому єдиною необхідною структурною характеристикою є координаційне число атомів у решітці. У рамках даного підходу була розрахована твердість відомих і гіпотетичних модифікацій елементарного бору и ряду сполук на його основі. Термодинамічна інтерпретація енергетичних характеристик хімічного зв’язку дає можливість розрахувати твердість фаз як функцію їх термодинамічних параметрів. Зокрема, хороше співвідношення між експериментальними та розрахунковими значеннями твердості за Віккерсом спостерігали не тільки для стехіометричних сполучень при кімнатній температурі, але і для температурної та концентраційної залежностей твердості. В последнее время был предложен ряд удачных теоретических моделей твердости. Термодинамическая модель твердости, основанная на корреляции между твердостью и термодинамическими свойствами твердых тел, дает возможность спрогнозировать твердость известных или даже гипотетических твердых тел, исходя из данных по энергии Гиббса атомизации элементов, которые косвенно определяют энергетические характеристики химических связей. При этом единственной необходимой структурной характеристикой является координационное число атомов в решетке. В рамках данного подхода была рассчитана твердость известных и гипотетических модификаций элементарного бора и ряда соединений на его основе. Термодинамическая интерпретация энергетических характеристик химических связей позволяет рассчитать твердость фаз как функцию их термодинамических параметров. В частности, хорошее соответствие между экспериментальными и расчетными значениями твердости по Виккерсу наблюдали не только для стехиометрических соединений при комнатной температуре, но и для температурной и концентрационной зависимостей твердости.

International audience; The structure of graphite like BCx phases (x = 1, 1.5, 3, 4, 32) has been studied using conven tional X ray diffraction. The results have been obtained, which unambiguously point to turbostratic (one dimensionally disordered) structure of all phases under study. The crystal lattice parameters, sizes of coherent scattering domains, and microstrain values have been defined, which have allowed us to find a correlation between the structure and stoichiometry of the phases synthesized at the same temperature.