Investigation of physical properties of (CaXH3 (X= Ni, Pd and Pt ) compounds used as hydrogen storage material

The structural phase transition properties of perovskite-type hydrides CaXH₃ (X = Ni, Pd, and Pt), which are widely used as hydrogen storage materials in modern technology, were investigated in the pressure range of 0–600 GPa using ab initio calculations based on density functional theory (DFT) as implemented in the SIESTA code. The initial crystal structure of each compound was the cubic phase with the Pm-3m (No. 221) space group. All structures were first optimized at 0 GPa. To examine the structural phase transitions, simulations were performed with pressure increments of 10 GPa. The applied pressures were increased up to 580 GPa for X = Ni, 400 GPa for X = Pd, and 400 GPa for X = Pt. According to the analyses performed using the Kplot program, the cubic structure of CaNiH₃ Pm-3m(No. 221), remains stable without any structural phase transition. The CaPdH₃ compound transforms from its initial cubic phase to a triclinic structure P1(No. 1), at 290 GPa, while CaPtH₃ undergoes a phase transition from cubic Pm-3m(No. 221), to monoclinic (Pm, No. 6) symmetry at 220 GPa. To evaluate the structural stability, energy–volume and enthalpy calculations were also performed. Additionally, the electronic band structures, total density of states (DOS), and partial density of states (PDOS) were calculated for all compounds. The results reveal that all three hydrides exhibit metallic conductivity, indicating their potential as hydrogen storage materials. The theoretical findings are consistent with previously reported experimental and computational studies.