Understanding the superior stability and enhanced physical performance of tetragonal XH2 (X = Y, Zr) hydrides over their cubic counterparts via ab-initio computational investigation

Metal hydrides such as YH₂ and ZrH₂ are highly versatile materials with a wide range of applications in advanced technologies due to their unique combination of properties, including high neutron moderation efficiency, excellent thermal stability, corrosion resistance, high hydrogen density, and radiation resistance. In this study, we employ first-principles DFT calculations to comprehensively investigate the structural, phonon dynamical, mechanical, elastic, electronic, thermodynamic, and optoelectronic properties of tetragonal XH₂ (X = Y, Zr). While the cubic phases of XH₂ (X = Y, Zr) are found to be mechanically unstable, their tetragonal counterparts are confirmed to be structurally, mechanically, thermodynamically, and vibrationally stable. The calculated lattice parameters show excellent agreement with available published computational and experimental data. Both compounds exhibit metallic behavior with a bandgap of 0 eV and zero net magnetization. The elastically anisotropic XH₂ hydrides display a hardness trend of ZrH₂ > YH₂. Tetragonal ZrH₂ has a higher melting temperature of 1328 K compared to YH₂, which has a measured melting temperature of 1123 K. Thermodynamic analysis reveals that ZrH₂ has a higher Debye temperature, lower T*entropy, and more stable free energy compared to YH₂. ZrH₂ also offers higher volumetric hydrogen storage capacity, while YH₂ possesses a slightly higher gravimetric hydrogen content. The estimated volumetric hydrogen storage capacities for YH₂ (90.80 kg/m³) and ZrH₂ (121.26 kg/m³) not only fulfill but also substantially surpass the 2025 benchmark set by the U.S. Department of Energy (DOE), which is 40 kg H₂/m³. Optical absorption, reflectivity, and conductivity data suggest that tetragonal XH₂ (X = Y, Zr) compounds are suitable for ultraviolet optoelectronic applications such as UV detectors and radiation shielding. Our findings establish tetragonal XH₂ as a superior alternative to the cubic phase, paving the way for its integration into future nuclear reactors, hydrogen storage systems, and energy technologies. This study provides a fundamental understanding of their intrinsic properties, contributing to the development of sustainable energy materials and enhancing performance under extreme operational conditions.