Effect of Water Species on the Phonon Modes in Orthorhombic Y2(MoO4)3 Revealed by Raman Spectroscopy

Abstract

Negative thermal expansion material of yttrium molybdate has been synthesized by rapid solidification with a CO2 laser. It crystallized in the orthorhombic phase with unique microstructures consisting of nanoparticles or nanodendrites with grain sizes around 20−30 nm. Raman spectroscopic and XRD analyses reveal that the presence of water species hinders not only the rocking motion but every type of motion of the corner shared polyhedra. Because of the presence of water species, degeneracy of the Raman bands happens for symmetric stretching, asymmetric stretching, and the bending modes, and the lattice, translational, and librational modes cannot be observed. The release of water species below 403 K (388 K for the newly prepared sample or 383 K in the second temperature cycle) has little whereas that above this temperature has great effect on the stretching and bending vibrations and also the librational and translational modes. This indicates that the water species which are released within the first step do not interact with the MoO4 tetrahedra in the Y2Mo3O12 framework while those released in the second step have a direct interaction. It is suggested that the translational and librational motions are closely coupled with stretching and bending vibrations. They together give rise to the negative thermal expansion.