Temperature-induced phase transformations in Na2WO4 and Na2MoO4 crystals

Temperature-dependent Raman studies on Na2WO4 and Na2MoO4 crystals were performed in order to obtain information on structural changes induced by temperature evolution. The stability of the cubic phase of Na2WO4 and Na2MoO4 crystals was assessed and our results indicate that this phase is stable in the 8-823 K and 15-773 K ranges for Na2WO4 and Na2MoO4, respectively. The crystal of Na2WO4 shows a phase transition occurring at 833 K. Na2MoO4 undergoes a sequence of three phase transitions, which were observed at 783-803, 823-913 and 943-950 K. In both crystals, a strong first-order phase transition occurs as indicated by the behavior of the Raman modes: the Raman bands split and new bands appear below 100 cm(-1). These transitions are connected with tilting and/or rotations of the WO(4)and MoO4 tetrahedra, which leads to a disorder at the WO(4)and MoO4 sites. Copyright (C) 2010 John Wiley & Sons, Ltd.