First-principles insight into the UV-driven anisotropy of NaXSiO4 (X = Al, Ga): structural, electronic, elastic and optical analysis

Abstract

The pressing need for advanced ultraviolet (UV) materials, vital for aerospace, optoelectronics, and radiation hardened technologies, demands a deep understanding of structure property relationships in wide bandgap ceramics. This study explores the structural, electronic, elastic, and optical properties of NaXSiO₄ (X = Al, Ga) through a density functional theory (DFT) investigation, with experimental level precision. Using WIEN2k calculations, we show that both compounds, with hexagonal frameworks of corner-sharing [AlO₄]/[SiO₄] and [GaO₄]/[SiO₄] tetrahedra, exhibit indirect bandgaps of 4.599 eV (NaAlSiO₄) and 4.089 eV (NaGaSiO₄), in their electronic band profiles, demonstrating their insulating nature. These materials displayed strong optical anisotropy, with UV peak absorption coefficients of (5.5 × 10³ cm⁻¹) for NaAlSiO₄ and (5.8 × 10³ cm⁻¹) for NaGaSiO₄ at 12.5 eV, surpassing conventional oxides like SiO₂ and Al₂O₃. Mechanistic insights reveal that O-2p → Al/Si-3sp and Ga-4sp interband transitions drive their UV-selective behavior, and a marginally higher reflectivity of NaGaSiO₄ of about 0.58 compared to 0.56 for NaAlSiO₄ in the energy range of 12–13 eV, indicate that NaGaSiO₄ serves effectively in UV-reflective coatings and UV shielding. The loss function analysis confirms that NaGaSiO₄ exhibited an improved dynamic electronic response compared with NaAlSiO₄, and the higher refractive index of NaGaSiO₄ (1.8) as compared to NaAlSiO₄ (1.75), highlights its potential for high-energy environments and UV optical applications, such as lenses and waveguides. Mechanically, NaAlSiO₄ and NaGaSiO₄ exhibit bulk moduluses (143.1458 GPa and 137.1902 GPa), ensuring durability under thermal and mechanical stress. This work establishes NaAlSiO₄ and NaGaSiO₄ as promising candidates for extreme UV applications and provides a DFT framework to accelerate the development of high-performance optical ceramics.