Temperature-dependent Raman spectroscopy studies of phase transformations in the K2WO4 and the MgMoO4 crystals

Temperature-dependent Raman spectroscopy studies of K2WO4 and MgMoO4 polycrystals were performed in order to obtain information about vibrational and structural changes in these materials as a function of temperature. The stability of the monoclinic phase for both K2WO4 and MgMoO4 samples was assessed and our results indicated that this phase is stable in the 295–723 K and 300–770 K ranges for K2WO4 and MgMoO4, respectively. It was observed that both samples underwent two phase transformations above room temperature. The first phase transformations which occur at about 633 K and 640 K for K2WO4 and MgMoO4, respectively, is most likely connected with weak tilting and/or rotations of WO4/MoO4 tetrahedral units that lead to a disorder in the oxygen sublattice. Raman spectroscopy data also indicated that K2WO4 and MgMoO4 exhibited a first-order phase transition at around 723 K and 770 K, respectively, changing from monoclinic to hexagonal symmetry.