Pressure-temperature stable conditions of the hydride-ion conductive phase in BaH₂ and SrH₂

The high-pressure (HP)/high-temperature (HT) phase diagram of barium hydride (BaH₂) was investigated using X-ray diffraction measurements and diamond-anvil cells to clarify the relationships between some phases, including the hydride-ion (H^-) conductive HT phase that adopts the Ni₂In-type structure. The results revealed that the HT phase and the HP phase were the same in the phase diagram and constitute an HP/HT phase. Furthermore, it was found that the Ni₂In-type structure is stable over a wide range up to approximately 50 GPa and 500°C. The coexistence of two phases during the phase transition may be responsible for the previously reported anomalous pressure dependency of ionic conductivity in the HP phase. Additionally, the HP/HT phase diagram of strontium hydride (SrH₂), which exhibits a similar HP structural sequence, was also investigated. It was found that the Ni₂In-type structure of SrH₂ is stable at pressures above 7 GPa within the temperature range of this work (~500 °C). These wide stable HP/HT regions imply that the structural properties of hydride-ion conductive BaH₂ and SrH₂ can be investigated under the wide conditions in the future. Furthermore, the pressure dependence of the lattice parameters and volume was obtained, and the bulk moduli of BaH₂ and SrH₂ in the Ni₂In-type structure are determined to be 31.5(15) GPa and 42.4(7) GPa, respectively. The pressure dependence of the crystallographic parameters obtained in this study will be useful for understanding the origin of the properties from the crystallographic viewpoint in future work.