Temperature‐induced phase transformations in Na 2 WO 4 and Na 2 MoO 4 crystals

Temperature‐dependent Raman studies on Na 2 WO 4 and Na 2 MoO 4 crystals were performed in order to obtain information on structural changes induced by temperature evolution. The stability of the cubic phase of Na 2 WO 4 and Na 2 MoO 4 crystals was assessed and our results indicate that this phase is stable in the 8–823 K and 15–773 K ranges for Na 2 WO 4 and Na 2 MoO 4 , respectively. The crystal of Na 2 WO 4 shows a phase transition occurring at 833 K. Na 2 MoO 4 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 MoO 4 tetrahedra, which leads to a disorder at the WO 4 and MoO 4 sites. Copyright © 2010 John Wiley & Sons, Ltd.