Characterization of powellite-based solid solutions by site-selective time resolved laser fluorescence spectroscopy

Abstract

We present a comprehensive study of the solid solution system Ca₂(MoO₄)₂–NaGd(MoO₄)₂ on the molecular scale, by means of site-selective time resolved laser fluorescence spectroscopy (TRLFS). Eu³⁺ is used as a trace fluorescent probe, homogeneously substituting for Gd³⁺ in the solid solution crystal structure. Site-selective TRLFS of a series of polycrystalline samples covering the whole composition range of the solid solution series from 10% substitution of Ca²⁺ to the NaGd end-member reveals it to be homogeneous throughout the whole range. The trivalent ions are incorporated into the powellite structure in only one coordination environment, which exhibits a very strong ligand–metal interaction. Polarization-dependent measurements of a single crystal of NaGd(Eu)(MoO₄)₂ identify the coordination geometry to be of C_2v point symmetry. The S₄ symmetry of the Ca site within the powellite lattice can be transformed into C_2v assuming minor motion in the first coordination sphere.