High-temperature phase transitions in incommensurate Rb₂WO₄

Raman experiments were performed on Rb₂WO₄ crystals, between room temperature and 850 K, including the high-temperature incommensurate phase. The experimental results observed in the incommensurate (T_I = 746 K) and the lock-in (T_C = 660 K) phase transitions can be explained by an order-disorder apex model, where the gradual up/down ordering of the WO₄^2- apexes is responsible for the structural changes. The as-grown sample exhibits a monoclinic-orthorhombic phase transition that is suppressed by heat treatment of the sample, and this result is confirmed by differential scanning calorimetry measurements.