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-T_c 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.