Characterization of flux-grown SmxNd1-xVO4 compounds and high-pressure behavior for x = 0.5

Abstract

The crystal structure and the vibrational and optical characteristics of flux-grown mixed lanthanide vanadate compounds SmxNd1-xVO4 (x = 0, 0.1, 0.25, 0.5, 0.75 and 1) are reported. A linear, monotonic decrease of the unit-cell parameters is observed with increasing Sm content, as expected according to the lanthanide contraction phenomenon and Vegard's law. The evolution of Raman phonons across the series is explained by the reduction of the unit-cell size and the length of atomic bonds. UV-vis absorption spectra are found to be a weighted superposition of the absorption spectra of SmVO4 and NdVO4, while the photoluminescence of the mixed compounds is heavily quenched, which is explained by the energy transfer processes occurring between energy levels of Sm3+ and Nd3+. Finally, the influence of pressure on the structure of Sm0.5Nd0.5VO4 is reported through an X-ray diffraction study by synchrotron radiation. There is evidence of two structural phase transitions (zircon-scheelite and scheelite-fergusonite) in the pressure range up to 23.1 GPa. The zircon-scheelite-fergusonite transition is partially reversible, as a mixture of zircon and scheelite is found after pressure release. Enthalpy calculations on isostructural orthophosphates explain the appearance of a metastable monoclinic phase in the range 8-14 GPa. Finally, hints of a possible further phase transition to an orthorhombic structure are put in evidence by the trend of the monoclinic angle of the fergusonite polymorph.