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Among Universe's most consequential events are large impacts generating rapidly-evolving extreme pressures and temperatures. Crystalline and amorphous forms of (Mg, Fe)2SiO4 are abundant and widespread, within planets and in space. The behavior of these minerals is expected to deviate form thermodynamic equilibrium in many of the processes that are critical to the formation and evolution of planets, particularly shock events. To further the understanding of the behavior of the silicate under extreme conditions, we statically compressed a crystal of forsterite up to 160.5 GPa, far beyond the compound's stability field, and probed its long-range ordering with synchrotron microdiffraction. We found that forsterite retains long-range ordering up to the highest pressure reached. Forsterite III, emerging at about 58 GPa, persists in compression to 160.5 GPa and in decompression down to about 13 GPa, for a rare combined occurrence of a metastable phase of nearly 150 GPa. These observations dispute earlier reports of pressure-induced amorphization and are a unique testimony of the resilience of the crystalline state in quasi hydrostatic compression. We confirm that highly disordered forsterite can be obtained from the decompression of forsterite III as suggested from the substantial loss of long-range ordering observed at 7 GPa after further decompression. Such kinetic pathway may explain how synthetic olivine glass have been obtained in shock experiments and could be a mechanism of generation of amorphous forsterite in cosmic dust. The 120 GPa Hugoniot discontinuity finds no correspondence in our data, marking a departure from the parallelism between static "cold compression" and dynamic compression., Comment: 11 pages, 6 figures

The pressure-induced polymorphism of binary octect compounds has long been considered a settled problem although the possible atomic disordering of some phases remains a puzzling observation. Taking GaP as a case study, we conclude, through x-ray microdiffraction and first-principles calculations, that its high-pressure phase II (previously reported as being disordered) adopts in fact an ordered base-centered monoclinic structure previously unknown in this class of compounds. The formation of layered patterns with variable degrees of interlayer dimerization, as observed in GaP, marks a paradigm shift of our understanding of ordering in octect high-pressure phases which calls for a more extensive re-examination. A rich polymorphism with fine tuning of chemical and physical properties can be envisioned.

Metallization of quantum spin liquid (QSL) materials has long been considered as a potential route to achieve unconventional superconductivity. Here we report our endeavor in this direction by pressurizing a three-dimensional QSL candidate, LiYbSe$_2$, with a previously unreported pyrochlore structure. High-pressure X-ray diffraction and Raman studies up to 50 GPa reveal no appreciable changes of structural symmetry or distortion in this pressure range. This compound is so insulating that its resistance decreases below 10$^5$ ${\Omega}$ only at pressures above 25 GPa in the corresponding temperature range accompanying the gradual reduction of band gap upon compression. Interestingly, an insulator-to-metal transition takes place in LiYbSe$_2$ at about 68 GPa and the metallic behavior remains up to 123.5 GPa, the highest pressure reached in the present study. A possible sign of magnetic or other phase transition was observed in LiYbSe$_2$. The insulator-to-metal transition in LiYbSe$_2$ under high pressure makes it an ideal system to study the pressure effects on QSL candidates of spin-1/2 Yb$^{3+}$ system in different lattice patterns.

Gold is one of the most inert metals, forming very few compounds, some with rather interesting properties, and only two of them currently known to be superconducting under certain conditions (AuTe$_2$ and SrAuSi$_3$). Compounds of another noble element, Ag, are also relatively rare, and very few of them are superconducting. Finding new superconducting materials containing gold (and silver) is a challenge - especially having in mind that the best high-$T_c$ superconductors at normal conditions are based upon their rather close ''relative'', Cu. Here we report combined X-ray diffraction, Raman, and resistivity measurements, as well as first-principles calculations, to explore the effect of hydrostatic pressure on the properties of the sylvanite mineral, AuAgTe$_4$. Our experimental results, supported by density functional theory, reveal a structural phase transition at $\sim$5 GPa from a monoclinic $P2/c$ to $P2/m$ phase, resulting in almost identical coordinations of Au and Ag ions, with rather uniform interatomic distances. Further, resistivity measurements show the onset of superconductivity at $\sim$1.5 GPa in the $P2/c$ phase, followed by a linear increase of $T_c$ up to the phase transition, with a maximum in the $P2/m$ phase, and a gradual decrease afterwards. Our calculations indicate phonon-mediated superconductivity, with the electron-phonon coupling coming predominantly from the low-energy phonon modes. Thus, along with the discovery of a new superconducting compound of gold/silver, our results advance understanding of the mechanism of the superconductivity in Au-containing compounds, which may pave the way to the discovery of novel ones.

The tetragonal intermetallic compound EuAl$_{4}$ hosts an exciting variety of low temperature phases. In addition to a charge density wave below 140 K, four ordered magnetic phases are observed below 15.4 K. Recently, a skyrmion phase was proposed based on Hall effect measurements under a $c$-axis magnetic field. We present a detailed investigation of the phase transitions in EuAl$_{4}$ under $c$-axis magnetic field. Our dilatometry, heat capacity, DC magnetometry, AC magnetic susceptibility, and resonant ultrasound spectroscopy measurements reveal three magnetic phase transitions not previously reported. We discuss what our results reveal about the character of the magnetic phases. Our first key result is a detailed $H \parallel [001]$ magnetic phase diagram mapping the seven phases we observe. Second, we identify a new high-field phase, phase VII, which directly corresponds to the region were skyrmions have been suggested. Our results provide guidance for future studies exploring the complex magnetic interactions and spin structures in EuAl$_{4}$., Comment: 20 pages, 15 figures

X-ray diffraction indicates that the structure of the recently discovered room temperature carbonaceous sulfur hydride (C-S-H) superconductor is derived from previously established van der Waals compounds found in the H$_2$S-H$_2$ and CH$_4$-H$_2$ systems. Crystals of the superconducting phase were produced by a photochemical synthesis technique leading to the superconducting critical temperature $T_c$ of 288 K at 267 GPa. Single-crystal x-ray diffraction patterns measured from 124 to 178 GPa, within the pressure range of the superconducting phase, give an orthorhombic structure derived from the Al$_2$Cu-type determined for (H$_2$S)$_2$H$_2$ and (CH$_4$)$_2$H$_2$ that differs from those predicted and observed for the S-H system to these pressures. The formation and stability of the C-S-H compound can be understood in terms of the close similarity in effective volumes of the H$_2$S and CH$_4$ components over a broad range of pressures. The relative amounts of carbon and sulfur in the structure is not determined, and denser carbon-bearing S-H structures may form at higher pressures. The results are consistent with hole-doping enhancement of $T_c$ by carbon proposed for the room-temperature superconductivity in this system., Comment: 8 pages, 6 figures

Using combined experimental and computational approaches we show that at 43 GPa and 1300 K gallium phosphide adopts the super-Cmcm structure, here indicated with its Pearson notation oS24. First-principles enthalpy calculations demonstrate that this oS24 structure is more thermodynamically stable above about 20 GPa than other proposed polymorphs, such as cubic ZB and SC16 or the ubiquitous orthorhombic Cmcm phase (oS8). It is found that oS24-structured GaP exhibits short P-P bonds, according to our high-resolution structural analysis. Such phosphorous dimerization in GaP, observed for the first time and confirmed by ab initio calculations, sheds light on the nature of the super-Cmcm structure and provides critical new insights into the high-pressure polymorphism of octet semiconductors. Bond directionality and anisotropy, in addition to 5-fold phosphorous coordination, explain the relatively low symmetry of this high-pressure structure.