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Physical review / B 107(1), 014103 (2023). doi:10.1103/PhysRevB.107.014103, A novel high-pressure yttrium allotrope, $hP3$-Y (space group $P6/mmm$), was synthesized in a multi-anvil press at 20 GPa and 2000 K which is recoverable to ambient conditions. Its relative stability and electronic properties were investigated using density functional theory calculations. A $hP3$-Y derivative hydride, $hP3-YHx$, with a variable hydrogen content ($x = 2.8$, 3, 2.4), was synthesized in diamond anvil cells by the direct reaction of yttrium with paraffin oil, hydrogen gas, and ammonia borane upon laser heating to ~3000 K at 51, 45 and 38 GPa, respectively. Room-temperature decompression leads to gradual reduction and eventually the complete loss of hydrogen at ambient conditions. Isostructural $hP3-NdHx$ and $hP3-GdHx$ hydrides were synthesized from Nd and Gd metals and paraffin oil, suggesting that the $hP3$-Y structure type may be common for rare-earth elements. Our results expand the list of allotropes of trivalent lanthanides and their hydrides and suggest that they should be considered in the context of studies of high-pressure behavior and properties of this broad class of materials., Published by Inst., Woodbury, NY

IUCrJ 8(2), 208 - 214 (2021). doi:10.1107/S2052252521000075, At high pressures, autoionization – along with polymerization and metallization – is one of the responses of simple molecular systems to a rise in electron density. Nitro­sonium nitrate (NO$^+$NO$_3$$^{−}$), known for this property, has attracted a large interest in recent decades and was reported to be synthesized at high pressure and high temperature from a variety of nitro­gen–oxygen precursors, such as N$_2$O$_4$, N$_2$O and N$_2$–O$_2$ mixtures. However, its structure has not been determined unambiguously. Here, we present the first structure solution and refinement for nitro­sonium nitrate on the basis of single-crystal X-ray diffraction at 7.0 and 37.0 GPa. The structure model (P2$_1$/m space group) contains the triple-bonded NO$^+$ cation and the NO$_{3}$$^{−}$ sp$^2$-trigonal planar anion. Remarkably, crystal-chemical considerations and accompanying density-functional-theory calculations show that the oxygen atom of the NO$^+$ unit is positively charged – a rare occurrence when in the presence of a less-electronegative element., Published by Chester

High pressure research 42(4), 318 - 335 (2022). doi:10.1080/08957959.2022.2144290, Understanding crystal/melt xenon (Xe) partitioning at depth is key to properly trace planetary processes using Xe isotopes. Partition coefficients measured on experimental samples recovered at room pressure (P) and temperature (T) span 6 orders of magnitude, potentially due to Xe exsolution from crystals upon quenching. We chose two in situ synchrotron X-ray methods to investigate Xe crystal/melt partitioning under high P and T up to 3 GPa and 1050°C using (1) resistive-heated diamond anvil cell with angle-dispersive diffraction and X-ray fluorescence, and (2) a new protocol using large volume press with energy-dispersive diffraction set-up. Results from both methods are consistent, and Xe is found to be compatible at depth, suggesting the continental crust could be a Xe-rich reservoir. This new protocol advances research to probe geological systems at the higher P–T conditions accessible with large volume press while maintaining homogeneous T throughout the sample., Published by Taylor and Francis, London [u.a.]

Physical review / B 105(6), 064110 (2022). doi:10.1103/PhysRevB.105.064110, We use high-pressure resistivity and single crystal x-ray diffraction at ambient and low temperature to investigate the charge ordering phase transition of CaMn$_7$O$_{12}$. We have found that at ambient temperature the Jahn-Teller distortion of the Mn3+O6 octahedra rapidly decreases above 20 GPa, and vanishes at 28 GPa, when two Mn octahedral sites initially occupied by Mn$^{3+}$ and Mn$^{4+}$ become regular and equivalent as the result of a charge delocalization. Such a change correlates with a two orders of magnitude drop in the resistivity and a symmetry increase from the low-pressure rhombohedral R$\bar{3}$ phase to the cubic Im$\bar{3}$ structure, the same as one found at ambient pressure above 440 K. This yields the slope of the charge ordering phase boundary of dT$_c$/dp≈−6 K/GPa. This result is further supported by the lack of a structural phase transition up to the maximum measured pressure of 30 GPa when the experiment is performed at 70 K. The satellite reflections of the structural modulation of the multiferroic phase of CaMn$_7$O$_{12}$ observed at 70 K were found to hold up to 25 GPa with the structure keeping a constant modulation vector k=(000.925) with pressure. The average structure at 70 K does not show other indications of further phase transition., Published by Inst., Woodbury, NY

Physical review materials 6(2), 023402 (2022). doi:10.1103/PhysRevMaterials.6.023402, Charged molecular species, such as [N$_2$]$^{x–}$, [O$_2$]$^{x–}$, [C$_2$]$^{x–}$, and [S$_2$]$^{x–}$, follow the paradigm of carrying integer values of electrons. Here, the Na$_3$(N$_2$)$_4$, Ca$_3$(N$_2$)$_4$, Sr$_3$(N$_2$)$_4$, and Ba(N$_2$)$_3$ compounds were produced and characterized, Published by APS, College Park, MD

High pressure research 41(3), 290 - 305 (2021). doi:10.1080/08957959.2021.1970154, The orthorhombic phase of Si-doped Fe carbide is synthesized at high-pressures and temperatures using laser-heated diamond anvil cell (LHDAC), followed by its characterization using X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Raman spectroscopy. Room-temperature high-pressure XRD measurements are carried out up to about 104 GPa for the determination of the equation of state parameters. No evidence of structural transition is observed. Pressure evolution of isothermal bulk modulus shows elastic stiffening around 28 GPa followed by softening around 78 GPa, which are possibly related to magnetic transitions driven by pressure-induced anisotropic strain in the unit cell. Extrapolation of the density profile of our study to the inner core conditions agrees very well with PREM data with an uncertainty of about 3���4%. Our estimated bulk modulus value at core pressures seems to be 8-9% less than that of PREM data and is best matched in comparison to other reported values., Published by Taylor and Francis, London [u.a.]

Physical review letters 127(13), 135501 (2021). doi:10.1103/PhysRevLett.127.135501, Changes in the bonding of carbon under high pressure leads to unusual crystal chemistry and can dramatically alter the properties of transition metal carbides. In this work, the new orthorhombic polymorph of yttrium carbide, $�����Y_4C_5$ was synthesized from yttrium and paraffin oil in a laser-heated diamond anvil cell at $���50$ GPa. The structure of $�����Y_4C_5$ was solved and refined using in situ synchrotron single-crystal x-ray diffraction. It includes two carbon groups: $[C_2]$ dimers and nonlinear $[C_3]$ trimers. Crystal chemical analysis and density functional theory calculations revealed unusually high noninteger charges ($[C_2]^{5.2���}$ and $[C_3]^{6.8���}$) and unique bond orders ($, Published by APS, College Park, Md.

Physical review letters 126(17), 175501 (2021). doi:10.1103/PhysRevLett.126.175501, High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN$_{4}$. A triclinic phase of beryllium tetranitride tr−BeN$_{4}$ was synthesized from elements at ∼85 GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN$_{4}$ layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated π systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN$_{4}$ layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN$_{4}$ layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions., Published by APS, College Park, Md.

Physical review / B 102(14), 140101 (2020). doi:10.1103/PhysRevB.102.140101, We report two reversible pressure-induced isosymmetric phase transitions in $α−Ag_{2}S$ that are accompanied by two compressive anomalies at 7.5 and 16 GPa, respectively. The first transition arises from a sudden and drastic puckering of the wrinkled Ag-S layers, which leads to an anomalous structural softening at high pressure and gives rise to the ultrahigh compressive ductility in $α−Ag_{2}S$. The second transition stems from a pressure-driven electronic state crossover from a conventional semiconductor to a topological metal. The band-crossing points near the Fermi energy form a nodal-line structure due to the preservation of the time-reversal and space-inversion symmetries under pressure. Our findings not only reveal the underlying mechanism responsible for the ultrahigh ductility in this class of inorganic semiconductors, but also provide a distinctive member to the growing family of topological metals and semimetals., Published by Inst., Woodbury, NY

Physical review / B covering condensed matter and materials physics 101(3), 035112 (2020). doi:10.1103/PhysRevB.101.035112, Published by Inst., Woodbury, NY

Physical review / B covering condensed matter and materials physics 102(6), 064105 (2020). doi:10.1103/PhysRevB.102.064105, X-ray synchrotron diffraction (XRD) measurements of single-crystal and powder molybdenum disulfide MoS$_2$ are performed at pressures (P) up to 78 GPa and temperatures (T) of 20 to 298 K in diamond-anvil cells. The results on single crystals demonstrate a sharp pressure induced isosymmetric phase transition of 2H$_c$ to 2H$_a$ modification at 23 GPa at 40 and 300 K. The structure of the high-pressure 2H$_a$ phase previously inferred theoretically and from powder XRD data is confirmed by our single-crystal XRD data solution, which also definitively determines the atomic potions as a function of pressure. No additional periodicity (commensurate or incommensurate) or distortion is found in the whole $P-T$ range of this study. These results suggest that a previously proposed hypothetic charge-density-wave phase does not host pressure induced superconductivity experimentally found above 90 GPa., Published by Inst., Woodbury, NY

Physical review / B covering condensed matter and materials physics 100(21), 214104 (2019). doi:10.1103/PhysRevB.100.214104, Germanium oxide is a prototype network-forming oxide with pressure-induced structural changes similar to those found in crystals and amorphous silicate oxides at high pressure. Studying density and coordination changes in amorphous GeO2 allows for insight into structural changes in silicate oxides at very high pressure, with implications for the properties of planetary magmas. Here, we report the density of germanium oxide glass up to 133 GPa using the x-ray absorption technique, with very good agreement with previous experimental data at pressure below 40 GPa and recent calculation up to 140 GPa. Our data highlight four distinct compressibility domains, corresponding to changes of the local structure of GeO2. Above 80 GPa, our density data show a compressibility and bulk modulus similar to the counterpart crystal phase, and we propose that a compact distorted sixfold coordination, similar to the structural motif of the pyritelike crystalline GeO2 polymorph, is likely to be stable in that pressure range. Our density data point to a smooth continuous evolution of the average coordination for pressure above 20 GPa with persistent sixfold coordination, without sharp density or density slope discontinuities. These observations are in very good agreement with theoretical calculations and spectroscopic measurements, and our results indicate that glasses and melts may behave similarly to their high-pressure solid counterparts with comparable densities, compressibility, and possibly average coordination., Published by Inst., Woodbury, NY

Physical review / B 98(13), 134435 (2018). doi:10.1103/PhysRevB.98.134435, The structural and magnetic properties of the $\mathrm{Ca_{3}Co_{2}O_{6}}$ spin-chain compound have been studied by means ofneutron and x-ray powder diffraction at pressures up to 6.8 and 32 GPa, respectively. A suppression of the initialspin-density wave state $(T_{N} = 25 K)$ and stabilization of the collinear commensurate antiferromagnetic (AFM)state at high pressures $(T_{NC} = 26 K at P = 2.1$ GPa) were observed. The pressure behavior of the competingintra- and interchain magnetic interactions was analyzed on the basis of obtained structural data and their role inthe formation of the magnetic phase diagram is discussed. The pressure behavior of the Néel temperature of thecommensurate AFM phase was evaluated within the mean field theory approach and a good agreement with theexperimental value $dT_{NC} /dP = 0.65 K/GPa$ was obtained., Published by APS, Woodbury, NY

We present a high-pressure study of Mg$_{1–x}$Fe$_{x}$O (ferropericlase, Fp) single crystals 0.04(1)$\leqslant x \leqslant$ 0.33(1) with a focus on ferrous iron spin-state crossover and the material behavior preceding it. Using Vegard's law and highly accurate high-pressure single-crystal experimental data, we extract the lattice parameter $a^{HS}_{FeO}$ of the “FeO high spin lattice contribution” in the MgO-FeO solid solution. We find that ferropericlases with a wide range of compositions share the same critical parameter $a_C$ (defined as the minimum $a^{HS}_{FeO}$ after which spin crossover starts). Furthermore, we discuss the effect of composition on spin crossover in ferrous iron in ferropericlase in the limits of low and moderate concentrations of Fe$^{2+}$.

IUCrJ 4(5), 671 - 677(2017). doi:10.1107/S2052252517010612, Due to their high technological and geological relevance, silicates are one of the most studied classes of inorganic compounds. Under ambient conditions, the silicon in silicates is almost exclusively coordinated by four oxygen atoms, while high-pressure treatment normally results in an increase in the coordination from four- to sixfold. Reported here is a high-pressure single-crystal X-ray diffraction study of danburite, CaB$_2$Si$_2$O$_8$, the first compound showing a step-wise transition of Si coordination from tetrahedral to octahedral through a trigonal bipyramid. Along the compression, the Si$_2$O$_7$ groups of danburite first transform into chains of vertice-sharing SiO$_5$ trigonal bipyramids (danburite-II) and later into chains of edge-sharing SiO$_6$ octahedra (danburite-III). It is suggested that the unusual formation of an SiO$_5$ configuration is a consequence of filling up the pentacoordinated voids in the distorted hexagonal close packing of danburite-II., Published by Chester

Synchrotron X-ray diffraction studies of the structural characteristics of Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$ langasite were performed at high hydrostatic pressures (up to 60 GPa) created in a diamond anvil cell. The Mössbauer absorption spectra of $^{57}$Fe nuclei and optical absorption spectra were recorded at pressures up to 10 and 54 GPa, respectively. The cascade of structural phase transitions at pressures of about ~ 3, ~ 20, and ~ 41 GPa was discovered. The first transition corresponds to the displacements of light oxygen atoms and associated with an increase in the local symmetry of oxygen 3f tetrahedra surrounding iron ions. The volume drop at the first transition is about 1.5%. The second transition at ~ 20 GPa is accompanied by a pronounced change in the lattice parameters a and c, while the unit cell volume undergoes a stepwise drop by 7%. Significant decrease in the parameter c at the structural transition should result in a strong increase of the exchange coupling constants between iron ions in neighboring ab planes, which may lead to a giant increase in the Néel temperature in this crystal at pressures above ~ 20 GPa. At the third transition at about 38–44 GPa, only a small anomaly in the $V\text{-}P$ relation is observed, but this anomaly is accompanied by substantial changes in the optical gap and in the quadruple splitting parameter.