Prediction and Synthesis of Dysprosium Hydride Phases at High Pressure.

Crystal structure prediction (CSP) methods recently proposed a series of new rare-earth (RE) hydrides at high pressures with novel crystal structures, unusual stoichiometries, and intriguing features such as high-Tc superconductivity. RE trihydrides (REH3) generally undergo a phase transition from ambient P63/mmc or P3̅c1 to Fm3̅m at high pressure. This cubic REH3 (Fm3̅m) was considered to be a precursor to further synthesize RE polyhydrides such as YH4, YH6, YH9, and CeH9 with higher hydrogen contents at higher pressures. However, the structural stability and equation of state (EOS) of any of the REH3 have not been fully investigated at sufficiently high pressures. This work presents high-pressure X-ray diffraction (XRD) measurements in a laser-heated diamond anvil cell up to 100 GPa and ab initio evolutionary CSP of stable phases of DyH3 up to 220 GPa. Experiments observed the Fm3̅m phase of DyH3 to be stable at pressures from 17 to 100 GPa and temperatures up to ∼2000 K. After complete decompression, the P3̅c1 and Fm3̅m phases of DyH3 recovered under ambient conditions. Our calculations predicted a series of phases for DyH3 at high pressures with the structural phase transition sequence P3̅c1 → Imm2 → Fm3̅m → Pnma → P63/mmc at 11, 35, 135, and 194 GPa, respectively. The predicted P3̅c1 and Fm3̅m phases are consistent with experimental observations. Furthermore, electronic band structure calculations were carried out for the predicted phases of DyH3, including the 4f states, within the DFT+U approach. The inclusion of 4f states shows significant changes in electronic properties, as more Dy d states cross the Fermi level and overlap with H 1s states. The structural phase transition from P3̅c1 to Fm3̅m observed in DyH3 is systematically compared with other REH3 compounds at high pressures. The phase transition pressure in REH3 shows an inverse relation with the ionic radius of RE atoms.

Synthesis of clathrate cerium superhydride CeH9 at 80-100 GPa with atomic hydrogen sublattice.

Hydrogen-rich superhydrides are believed to be very promising high-Tc superconductors. Recent experiments discovered superhydrides at very high pressures, e.g. FeH5 at 130 GPa and LaH10 at 170 GPa. With the motivation of discovering new hydrogen-rich high-Tc superconductors at lowest possible pressure, here we report the prediction and experimental synthesis of cerium superhydride CeH9 at 80-100 GPa in the laser-heated diamond anvil cell coupled with synchrotron X-ray diffraction. Ab initio calculations were carried out to evaluate the detailed chemistry of the Ce-H system and to understand the structure, stability and superconductivity of CeH9. CeH9 crystallizes in a P63/mmc clathrate structure with a very dense 3-dimensional atomic hydrogen sublattice at 100 GPa. These findings shed a significant light on the search for superhydrides in close similarity with atomic hydrogen within a feasible pressure range. Discovery of superhydride CeH9 provides a practical platform to further investigate and understand conventional superconductivity in hydrogen rich superhydrides.

Unraveling the Hidden Martensitic Phase Transition in BaClF and PbClF under High Pressure Using an Ab Initio Evolutionary Approach.

We predict crystal structures of MClF (M = Ba and Pb) compounds by performing an ab initio evolutionary simulation at ambient as well as high pressure. We propose a structural transition sequence in MClF compounds as follows: P4/ nmm → Pmcn → P63/ mmc below 100 GPa. The predicted ambient and intermediate phases are consistent with X-ray and Raman spectroscopic measurements, while the newly proposed high pressure P63/ mmc phase is thermodynamically more favorable than the previously proposed monoclinic ( P21/ m) phase. It is found that the P4/ nmm → Pmcn transition is first order in nature, while the Pmcn → P63/ mmc transition is a martensitic phase transition, which is accompanied by a slight volume change and is of a displacive nature. The austenite and martensite phases coexist in a wide pressure range, especially for PbClF. The martensite phase transition is mainly driven by (1) tilting and transformation of distorted heptahedron to pentahedron environment of MCl6, which leads to negative area compressibility, and (2) cooperative displacive movement of F- ions to form a trigonal bypyramidal (MF5) structure around a metal cation. Overall, the metal cation coordination increases from 9 (MF4Cl5- P4/ nmm) to 10 (MF4Cl6- Pmcn) and, further, to 11 (MF5Cl6- P63/ mmc) under high pressure. The predicted ambient and high pressure phases are mechanically and dynamically stable under the studied pressure range. Electronic structure, bonding, and optical properties are calculated and discussed using new parametrization of Tran Blaha modified Becke Johnson potential. We find nearly isotropic optical properties (except for the ambient phase of PbClF), even though all the predicted ambient and high pressure phases are structurally anisotropic.

Novel magnesium borides and their superconductivity.

With the motivation of searching for new superconductors in the Mg-B system, we performed ab initio evolutionary searches for all the stable compounds in this binary system in the pressure range of 0-200 GPa. We found previously unknown, yet thermodynamically stable, compositions MgB3 and Mg3B10. Experimentally known MgB2 is stable in the entire pressure range 0-200 GPa, while MgB7 and MgB12 are stable at pressures below 90 GPa and 35 GPa, respectively. We predict a reentrant behavior for MgB4, which becomes unstable against decomposition into MgB2 and MgB7 at 4 GPa and then becomes stable above 61 GPa. We find ubiquity of phases with boron sandwich structures analogous to the AlB2-type structure. However, with the exception of MgB2, all other magnesium borides have low electron-phonon coupling constants λ of 0.32-0.39 and are predicted to have Tc below 3 K.

Novel superhard B-C-O phases predicted from first principles.

We explored the B-C-O system at pressures in the range 0-50 GPa by ab initio variable-composition evolutionary simulations in the hope of discovering new stable superhard materials. A new tetragonal thermodynamically stable phase B4CO4, space group I4[combining macron], and two low-enthalpy metastable compounds (B6C2O5, B2CO2) have been discovered. Computed phonons and elastic constants show that these structures are dynamically and mechanically stable both at high pressure and zero pressure. B4CO4 is thermodynamically stable at pressures above 23 GPa, but should remain metastable under ambient conditions. Its computed hardness is about 38-41 GPa, which suggests that B4CO4 is potentially superhard.