[en] Single crystal X-ray diffraction (XRD) is a powerful non-destructive method which allows unambiguously identify crystalline phases, determine a crystal structure (unit cell parameters, a space group, atomic coordinates and atomic occupancies) and, if required, a phase composition. This thesis deals with applications of single-crystal XRD in high pressure and high temperature (HPHT) research using laser-heated diamond anvil cells (DACs). The thesis describes methodological aspects of our single-crystal XRD experiments which involve crystals selection, DACs preparation, maintaining experiments, data processing, and structure solutions and/or refinements. We demonstrate a great potential and novel opportunities provided by high-pressure crystallography in materials- and geo-sciences on the examples of studies of transition metal borides, a metal-doped boron phase, silicates, and oxides, particularly refined crystal structures of Co_5B_1_6, MnB_4, Al-doped β-boron, knorringite, and Fe"3"+-bearing bridgmanite, investigated the high-pressure behaviour of FeB_4, Fe_2B_7, Fe_xB_5_0, and FeOOH. A unique atomic arrangement in the FeB_4 brings it to a class of superhard materials with a nanoindentation hardness of 62(5) GPa. We found that the structure of Fe_xB_5_0 composed of B12 icosahedra has large cavities, so it can contract more effectively than boron polymorphs (α-, β- and γ-boron), also containing chemically bonded B12 icosahedra. The distribution of iron in Al-free, Fe"3"+-bearing Mg-perovskite (bridgmanite) was derived from single-crystal XRD combined with Moessbauer spectroscopy. High-pressure and high-temperature (HPHT) single-crystal XRD was used to search for HPHT polymorphs of Fe_2O_3 and Fe_3O_4 in a megabar pressure range and to uncover the fate of the iron oxide in subducted banded iron formations (BIFs) in the Earth's lower mantle. The authors confirmed that above 29 GPa Fe_3O_4 adopts the crystal structure of CaTi_2O_4 which is stable to at least 70(1) GPa and 2400(100) K. The authors have resolved the over 50-year old controversy regarding the structure of the Fe_2O_3 polymorph stable above ∝50 GPa. Thus, the Fe_2O_3 from subducted BIFs may be a source of an oxygen-rich fluid to the deep Earth's interior with significant amount of oxygen (up to 8 times the amount of oxygen in the modern atmosphere), leading to significant heterogeneity in oxygen fugacity in different parts of the mantle.

[en] The research for wurtzite-type ternary nitride semiconductors containing earth abundant elements with a stoichiometry of 1 : 1 : 2 was focused on metals like Mg or Zn, so far. The vast majority of these Grimm-Sommerfeld analogue compounds crystallize in the β-NaFeO${}_2$ structure, although a second arrangement in space group Pmc2${}_1$ is predicted to be a viable alternative. Despite extensive theoretical and experimental studies, this structure has so far remained undiscovered. Herein, we report on BeGeN${}_2$ in a Pmc2${}_1$ structure, synthesized from Be${}_3$N${}_2$ and Ge${}_3$N${}_4$ using a high-pressure high-temperature approach at 6 GPa and 800 °C. The compound was characterized by powder X-ray diffraction (PXRD), solid state nuclear magnetic resonance (NMR), Raman and energy dispersive X-ray (EDX) spectroscopy, temperature-dependent PXRD, second harmonic generation (SHG) and UV/Vis measurements and in addition also compared to its lighter homologue BeSiN${}_2$ in all mentioned analytic techniques. The synthesis and investigation of both the first beryllium germanium nitride and the first ternary wurtzite-type nitride crystallizing in space group Pmc2${}_1$ open the door to a new field of research on wurtzite-type related structures. (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH)

[en] Owing to its outstanding elastic properties, the nitride spinel γ‐Si${}_3$N${}_4$ is of considered interest for materials scientists and chemists. DFT calculations suggest that Si${}_3$N${}_4$‐analog beryllium phosphorus nitride BeP${}_2$N${}_4$ adopts the spinel structure at elevated pressures as well and shows outstanding elastic properties. Herein, we investigate phenakite‐type BeP${}_2$N${}_4$ by single‐crystal synchrotron X‐ray diffraction and report the phase transition into the spinel‐type phase at 47 GPa and 1800 K in a laser‐heated diamond anvil cell. The structure of spinel‐type BeP${}_2$N${}_4$ was refined from pressure‐dependent in situ synchrotron powder X‐ray diffraction measurements down to ambient pressure, which proves spinel‐type BeP${}_2$N${}_4$ a quenchable and metastable phase at ambient conditions. Its isothermal bulk modulus was determined to 325(8) GPa from equation of state, which indicates that spinel‐type BeP${}_2$N${}_4$ is an ultraincompressible material. (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

[en] We report the results of single-crystal X-ray diffraction and Raman spectroscopy studies of scandium oxide, Sc_2O_3, at ambient temperature under high pressure up to 55 and 28 GPa, respectively. Both X-ray diffraction and Raman studies indicated a phase transition from the cubic bixbyite phase (so-called C-Res phase) to a monoclinic C2/m phase (so-called B-Res phase) at pressures around 25–28 GPa. The transition was accompanied by a significant volumetric drop by ∼6.7%. In addition, the Raman spectroscopy detected a minor crossover around 10–12 GPa, which manifested in the appearance of new and disappearance of some Raman modes, as well as in softening of one Raman mode. We found the bulk modulus values of the both C-Res and B-Res phases as B_0 = 198.2(3) and 171.2(1) GPa (for fixed B′ = 4), respectively. Thus, the denser high-pressure lattice of Sc_2O_3 is much softer than the original lattice. We discuss possible mechanisms that might be responsible for the pronounced elastic softening in the monoclinic high-pressure phase in this “simple” oxide with an ultra-wide band gap

[en] Ir_1-xRe_x (0.15≤x≤0.40) phases prepared under high-pressure, high-temperature conditions from nanopowders of iridium and rhenium were characterized using powder X-ray diffraction and scanning electron microscopy. Structural characteristics of the phases obtained were identical with the corresponding parameters for the solid solutions prepared by means of melting and thermal decomposition of the precursors. The data obtained make it possible to improve solid-state solubility limits in the binary Ir-Re phase diagram. As revealed, the maximum solid solubility of Ir in Re is 68at.%, and that of Re in Ir is 20at.%. (orig.)

[en] Highlights: • Sc addition to bcc-Al2CoCrFeNi high entropy alloy results in precipitation of W-phase. • W-phase dissolves in the matrix with pressure. • Addition of Sc and high-pressure treatment improve mechanical properties of HEA. The exceptional performance of some High Entropy Alloys (HEAs) under extreme conditions holds out the possibility of new and exciting materials for engineers to exploit in future applications. In this work, instead of focusing solely on the effects of high temperature on HEAs, the effects of combined high temperature and high pressure were observed. Phase transformations occurring in a pristine HEA, the as-cast bcc–Al₂CoCrFeNi, are heavily influenced by temperature, pressure, and by scandium additions. As-cast bcc–Al₂CoCrFeNi and fcc–Al_0.3CoCrFeNi HEAs are structurally stable below 60 GPa and do not undergo phase transitions. Addition of scandium to bcc–Al₂CoCrFeNi results in the precipitation of hexagonal AlScM intermetallic (W-phase), which dissolves in the matrix after high-pressure high-temperature treatment. Addition of scandium and high-pressure sintering improve hardness and thermal stability of well-investigated fcc- and bcc- HEAs. The dissolution of the intermetallic in the main phase at high pressure suggests a new strategy in the design and optimization of HEAs.

[en] A nitrogen-rich compound, ReN₈.xN₂, was synthesized by a direct reaction between rhenium and nitrogen at high pressure and high temperature in a laser-heated diamond anvil cell. Single-crystal X-ray diffraction revealed that the crystal structure, which is based on the ReN₈ framework, has rectangular-shaped channels that accommodate nitrogen molecules. Thus, despite a very high synthesis pressure, exceeding 100 GPa, ReN₈.xN₂ is an inclusion compound. The amount of trapped nitrogen (x) depends on the synthesis conditions. The polydiazenediyl chains [-N=N-]_∞ that constitute the framework have not been previously observed in any compound. Ab initio calculations on ReN₈.xN₂ provide strong support for the experimental results and conclusions. (copyright 2018 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] A crystal structure of aluminum doped β-rhombohedral boron was studied by single-crystal X-ray diffraction at 80 K. The crystals were synthesized using high-pressure high temperature technique at 3 GPa and 2100 K. The structure is based on three-dimensional framework made of B₁₂ icosahedra with voids occupied by the B₂₈–B–B₂₈ units, it has the R-3m space group with a=10.9014(3), c=23.7225(7) Å lattice dimensions in hexagonal setting. Aluminum atoms are located in A1 and D special positions of the β-B structure with occupancies of 82.7(6)% and 11.3(4)%, respectively. Additional boron atoms are located near the D-site. Their possible distribution is discussed. Finally we have found two appropriate structural models whose refinement suggests two possible chemical compositions, AlB_44.8(5) and AlB_37.8(5), which are in a good agreement with the chemical analysis data obtained from EDX. The crystal structure of AlB_44.8(5) is described in detail. - Graphical abstract: The atomic distribution near the B(15) atom (non-labeled atom in the center of the picture) shown along the c axis. Anisotropic displacement ellipses for Al(2) (D-site) and B(15) are shown with 50 % probability level. The mirror plane with Miller indices (1 1 0) and related to it (−1 2 0) and (−2 1 0) generated by the 3-fold rotation-inversion axis parallel to the c axis splits the position of B(16) over two sites. Highlights: ► The crystal structure of the AlB_44.8(5) has been refined. ► Aluminum atoms partially fill certain types of voids (the A1- and D-sites). ► We have got two possible models of atomic distribution near the D-site.

[en] Highlights: • A first boron-rich cobalt boride, Co₅B₁₆, was synthesized under HPHT conditions. • Co₅B₁₆ has a unique orthorhombic structure (space group Pmma). • It is based on a rigid network of boron atoms similarly to metal tetraborides. • The material has high hardness of 30.1 ± 2GPa. • It is paramagnetic, with a weak temperature dependence of magnetic susceptibility. - Abstract: A first cobalt boride with the Co:B ratio below 1:1, Co₅B₁₆, was synthesized under high-pressure high-temperature conditions. It has a unique orthorhombic structure (space group Pmma, a = 19.1736(12), b = 2.9329(1), and c = 5.4886(2) Å, R₁ (all data) = 0.037). The material is hard, paramagnetic, with a weak temperature dependence of magnetic susceptibility

[en] The synthesis of polynitrogen compounds is of great importance due to their potential as high-energy-density materials (HEDM), but because of the intrinsic instability of these compounds, their synthesis and stabilization is a fundamental challenge. Polymeric nitrogen units which may be stabilized in compounds with metals at high pressure are now restricted to non-branched chains with an average N-N bond order of 1.25, limiting their HEDM performances. Herein, we demonstrate the synthesis of a novel polynitrogen compound TaN${}_5$ via a direct reaction between tantalum and nitrogen in a diamond anvil cell at circa 100 GPa. TaN${}_5$ is the first example of a material containing branched all-single-bonded nitrogen chains [N${}_5$${}^{5-<!--\; ???\; -->}$]${}_{\mathrm{\infty <!--\; ???\; -->}}$. Apart from that we discover two novel Ta-N compounds: TaN${}_4$ with finite N${}_4$${}^{4-<!--\; ???\; -->}$ chains and the incommensurately modulated compound TaN${}_{2-<!--\; ???\; -->x}$, which is recoverable at ambient conditions. (© 2021 Wiley-VCH GmbH)