Phonon properties of β-Ag2MoO4: Raman spectroscopy and ab initio calculations

This paper reports the temperature-dependent study of β-Ag2MoO4 micro-crystals using Raman spectroscopy. The micro-crystals were prepared by the hydrothermal method, and lattice dynamic calculations based on the density-functional theory (DFT) were used to assign the Raman modes observed in the spectrum and to support the understanding of the mechanism driving the vibrational changes undergone by β-Ag2MoO4. The effect of the temperature on the vibrational properties of the β-Ag2MoO4 has been investigated in the temperature range from 300 to 850 K. The Raman data indicated that the crystal exhibits two temperature-induced phase transformations. One of them was assigned to a first-order phase transition (from cubic to unknown symmetry) occurring at about 541 K, connected with some disorder in the MoO4 tetrahedron, therefore, leading to tiltings and/or rotations of the MoO4 units, as revealed by changes in the Raman spectra. The second phase transition was observed at about 700 K, thus leading the crystal to change its symmetry from the unknown phase to the same cubic room temperature symmetry (reentrant phase transition).