High-Temperature Phase Transition in RbH2PO4

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.