Effects of High Pressure on the Far-Infrared Spectra of Paraelectric KH2PO4 and RbH2PO4

Using an opposed‐anvil‐type diamond cell, the infrared spectra of KH₂PO₄, RbH₂PO₄, and monoclinic RbD₂PO₄ have been measured in the 100–1400‐cm⁻¹ region up to pressures of 60 kbar and at several temperatures. The room‐temperature spectrum of RbH₂PO₄ demonstrates the existence of a new high‐pressure phase where the PO₄ tetrahedra are distorted and the protons are ordered. This phase does not occur in monoclinic RbD₂PO₄. The results with KH₂PO₄ are not as dramatic as RbH₂PO₄. The effects of high pressure in the 300–600‐cm⁻¹ region are similar to those obtained on cooling below the Curie temperature ${\mathrm{(T}}_c)$ , and have been interpreted in terms of resonance interference between a multiphonon quasicontinuum and discrete states of the distorted PO₄ groups.