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

Kotagiri, G P and Niranjan, Manish K and Asthana, Saket and Karthikeyan, R (2016) Investigation of Raman Modes and Born-Effective Charges in AgNb1/2Ta1/2O3: A Density-Functional and Raman Scattering Study. Journal of the American Ceramic Society, 99 (1). pp. 332-339. ISSN 0002-7820

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Abstract

Using ab initio density-functional theory, the Born-effective charge tensors and zone-center phonon mode frequencies are computed for AgNb1/2Ta1/2O3 in monoclinic P2/m and orthorombic Pbcm symmetries. The experimental mode frequencies are obtained from deconvolution of Raman spectrum of prepared AgNb1/2Ta1/2O3 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 AgNb1/2Ta1/2O3 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.

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