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

We have investigated the electronic, vibrational, optical, thermal and piezoelectric properties of LiNbO₃, LiTaO₃ and Li₂NbTaO₆ using the first-principles calculation based on the density functional theory. It also shows structural phase transition below Tc due to ionic displacement that may alter the properties of material. We have checked the structural stability by calculating the tolerance factor and formation energy before proceeding to the further calculations. The ground state electronic band structures and corresponding density of states show its semiconducting nature with a band gap range of 3. 5-3. 7 eV. Optical properties such as dielectric function, absorption coefficient, optical conductivity, refractive index, absorbance and reflectance are calculated using time-dependent density functional theory. Furthermore, the piezoelectric properties and Born effective charges were analyzed to find the correlation between them. In these materials, the distortion induced by the small ionic radius of Li^+ coupled with strong covalent interaction between transition metal and oxygen leads to high spontaneous polarization which can enhance both piezoelectric and optical properties.