Combined experimental and theoretical Raman scattering studies of alpha -quartz-type FePO sub(4) and GaPO sub(4) end members and Ga sub(1-x)Fe sub(x)PO sub(4) solid solutions

alpha -Quartz-type Ga sub(1-x)Fe sub(x)PO sub(4) solid solutions (0.04 less than or equal to x less than or equal to 0.28) and pure GaPO sub(4) and FePO sub(4) end members were studied by X-ray diffraction (XRD) and Raman spectroscopy to investigate the influence of iron content on the GaPO sub(4) structure, and to have a better understanding of the lattice dynamics of the Ga sub(1-x)Fe sub(x)PO sub(4) solid solutions. The unit cell parameters and structural distortion (i.e.inter-tetrahedral bridging [thetas], and tetrahedral tilt delta angles) of Ga sub(1-x)Fe sub(x)PO sub(4) 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 GaPO sub(4) decoupled modes are centered at 98, 237, 365 and 183 cm super(-1) and respectively assigned to E(TO1), E(TO5), E(TO7) and A sub(1), while the vibrational modes located between 400 and 1200 cm super(-1) could be coupled. The coupled A sub(1) bending mode centered at 457 cm super(-1) in pure alpha -GaPO sub(4) 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 sub(1-x)Fe sub(x)PO sub(4) solid solutions.