High-temperature phenomena in RbD2PO4 and CsH2PO4 polymeric transformations or polymorphic phase transitions?

X-ray analysis has been performed for RbD2PO4 and CsH2PO4 over the temperature range from 288 K to 537 K and 507 K, respectively. The refinement of the crystal structure of RbD2PO4 at 430 K has revealed that the high-temperature paraelectric phase of this crystal is isomorphic with the monoclinic P2(1)/m paraelectric phase of CsH2PO4. The X-ray diffraction CCD images obtained for RbD2PO4 have proved that the high-temperature paraelectric phase is stable up to approximately 525 K. At this temperature, polycrystallisation of the single-crystal samples and their subsequent decomposition has been observed. CsH2PO4 undergoes a structural phase transition at 504 K, from the monoclinic paraelectric phase to a cubic superionic phase (P2(1)/m transforms to Pm-3m symmetry). The reversibility of the superionic phase transition in the crystals is a strong evidence for a polymorphic character of this solid-solid transition.