Investigation of Raman Modes and Born‐Effective Charges in AgNb1/2Ta1/2O3: A Density‐Functional and Raman Scattering Study

Using ab initio density‐functional theory, the Born‐effective charge tensors and zone‐center phonon mode frequencies are computed for AgNb_1/2Ta_1/2O₃ in monoclinic P2/m and orthorombic Pbcm symmetries. The experimental mode frequencies are obtained from deconvolution of Raman spectrum of prepared AgNb_1/2Ta_1/2O₃ samples and are compared with computed mode frequencies. The Raman modes with high (>350 cm^?1) and low frequencies (<90 cm^?1) correspond to Ag and O vibrations, respectively. The modes in intermediate frequency band (120–350 cm^?1) are dominated by Nb(Ta)–O vibrations. The computed effective charge tensors of cations at A (Ag) and B (Nb, Ta) sites are found to be diagonal. The off‐diagonal components of charge tensor are found sizeable only for O ions in orthorhombic AgNb_1/2Ta_1/2O₃ with Pbcm symmetry. Further, charge tensor structure of O ions is found to depend on site symmetry in the unit cell. Charge tensor components for Nb, Ta, and O ions differ significantly from their nominal ionic values suggesting (1) large local dipole moments induced by off‐centering of Nb(Ta) ions and tilting(rotations) of Nb(Ta)O₆ octahedra, (2) hybridization between d‐orbitals of Nb(Ta) and p‐orbitals of O atoms. Furthermore, the electronic structure, directional dependence of effective charges and performance of LDA (GGA) exchange‐correlation functionals with regard to computed values are also discussed.