High-temperature phase transition in RbH2PO4, KD2PO4 and KH2AsO4

The high-temperature phase transition in single crystals of RbH2PO4 (RDP), KD2PO4 (DKDP) with different levels of deuteration, and KH2AsO4 (KDA), has been investigated by measurements of the dielectric constant, electrical conductivity and IR reflectivity, in the temperature range 300-500°K. RDP and KDP with a deuterium concentration higher than 74%, show a proper high-temperature phase transition, associated with crystalline structural change. For RDP, the transition temperature varies between 345-375°K, depending strongly on crystalline perfection, while for DKDP, the transition temperature decreases linearly from 453°K with increasing concentration of deuterium. For DKDP with a deuterium level lower than 74%, the transition temperature is constant at 453°K, and the transition is probably connected with the emission of water vapour. No transition is detected in the case of KDA.