High-Temperature Phase Transition in RbH2PO4

Abstract

The high-temperature phase change in tetragonal RbH2PO4 at 86°C is found to result in a quasi-irreversible transition to a monoclinic modification which seems to be isomorphous with monoclinic RbD2PO4 and KD2PO4. The transition is accompanied by a distortion of the PO4 groups and an ordering of the protons. From the analogy with monoclinic KD2PO4 it seems probable that the protons attach to one upper and one lower corner of a PO4 group giving rise to pyroelectric behavior along the b axis. The similarity of the infrared spectra of the high-temperature forms of RbH2PO4 and KH2PO4 demonstrates that the above statements are valid for KH2PO4 as well.