Combined experimental and theoretical Raman scattering studies of α-quartz-type FePO4 and GaPO4 end members and Ga1−xFexPO4 solid solutions

Abstract

α-Quartz-type Ga_1−xFe_xPO₄ solid solutions (0.04 ≤ x ≤ 0.28) and pure GaPO₄ and FePO₄ end members were studied by X-ray diffraction (XRD) and Raman spectroscopy to investigate the influence of iron content on the GaPO₄ structure, and to have a better understanding of the lattice dynamics of the Ga_1−xFe_xPO₄ solid solutions. The unit cell parameters and structural distortion (i.e. inter-tetrahedral bridging θ, and tetrahedral tilt δ angles) of Ga_1−xFe_xPO₄ solid solutions were found to vary linearly with the iron content. Coupled and decoupled vibrational modes were identified using Raman spectroscopy and the calculation of the localization entropy using density functional theory. The GaPO₄ decoupled modes are centered at 98, 237, 365 and 183 cm⁻¹ and respectively assigned to E(TO1), E(TO5), E(TO7) and A₁, while the vibrational modes located between 400 and 1200 cm⁻¹ could be coupled. The coupled A₁ bending mode centered at 457 cm⁻¹ in pure α-GaPO₄ was found to vary linearly as a function of iron content at room temperature. This mode could be therefore used to determine the iron content in Ga_1−xFe_xPO₄ solid solutions.