Insight into the effect of Mg-substitution on the electronic, optoelectronic, and hydrogen storage density of NbH2 fluorite structured: a DFT study†

In the quest for multifunctional hydrogen storage materials, this study investigates the structural, electronic, and optical properties of NbH₂, MgH₂, and a series of Mg-substituted NbH₂ compounds (Mg-NbH₂, Mg₂-NbH₂, and Mg₃-NbH₂) using first-principles density functional theory (DFT) based on GGA/PBE and HSE03 methods. The motivation stems from the need to overcome the well-known limitations of MgH₂, particularly its high desorption temperature and poor reversibility, by introducing Mg into the NbH₂ fluorite framework. Structural optimization revealed a fluorite-type geometry, with Mg substitution inducing moderate lattice distortion and increasing unit cell volume from 97.22 to 103.45 ų. The Mg-NbH₂ system achieved a high density of 10.78 g cm⁻³ and exhibited a favorable hydrogen gravimetric capacity of 3.33 wt%, offering a promising trade-off between storage potential and structural stability. Electronic structure analysis confirmed metallicity across all substituted systems, while MgH₂ retained a non-metallic nature. A progressive decrease in total density of states was observed from 7.0 (NbH₂) to 2.0 (Mg₃-NbH₂), suggesting tunable electronic characteristics. Optical studies revealed that Mg-NbH₂ displayed the strongest dielectric response (ε₂ ≈ 85), the highest refractive index (n₁ ≈ 3.2), and reduced optical losses compared to its parent compounds. Notably, it retained a high optical conductivity (∼13 S m⁻¹) and strong absorption in the visible range, making it a potential candidate for photocatalytic and optoelectronic applications. These results demonstrate that Mg substitution into NbH₂ significantly enhances its multifunctional behavior, offering a viable pathway to improve hydride-based materials for advanced hydrogen storage and light-harvesting technologies.