Unraveling the electronic, vibrational, thermodynamic, optical and piezoelectric properties of LiNbO3, LiTaO3 and Li2NbTaO6 from first-principles calculations

The piezoelectric and optical properties are pivotal in advancing modern microelectronics and smart device technologies. In this context, LiNbO${}_3$ and LiTaO${}_3$ emerge as promising functional perovskites, exhibiting appreciable ferroelectric and nonlinear optical properties with a broad range of applications. In this study, we have investigated electronic, vibrational, optical, thermal and piezoelectric properties of LiNbO${}_3$, LiTaO${}_3$ and Li${}_2$NbTaO${}_6$ using first-principles calculations based on density functional theory. We have checked the structural stability by calculating the tolerance factor and formation energy before proceeding to further calculations. The ground state electronic band structures and corresponding density of states establish their semiconducting nature with a wide band gap range of 3.5–3.7 eV. Optical properties, including the dielectric function, absorption coefficient, optical conductivity, refractive index, absorbance, and reflectance, were simulated using time-dependent perturbation theory. Furthermore, the piezoelectric properties and Born effective charges were systematically investigated to elucidate the underlying correlation between covalency and induced polarization. In these materials, the distortion affected by the small ionic radius of Li${}^{+}$, coupled with the strong covalent interaction between transition metal elements and oxygen, leads to high spontaneous polarization, enhancing their piezoelectric and optical properties.