Local distortion of PO 4 tetrahedra in the paraelectric phase in low temperature under high pressure in KH 2PO 4

Abstract

The ν 1– ν 4 internal modes of PO 4 tetrahedra of crystalline KH 2PO 4 (KDP) have been observed by Raman scattering at high pressures and low temperatures. On the basis of the selection rule analysis of the polarized Raman spectra, the local and momentary site symmetry of PO 4 tetrahedra is found to be C 2 in both the paraelectric and ferroelectric phases around 1.2–1.8 GPa. This situation is similar to that at ambient pressure. The symmetry becomes S 4 in the high-pressure and low-temperature region between 296 and 150 K above 3 GPa. At temperatures lower than 150 K above 3 GPa, these spectra are entirely different from those for C 2 or S 4 even in the paraelectric phase. This fact suggests two possibilities: (1) that a structural phase transition takes place, and/or (2) that non-equilibrium proton configurations remain disordered with the residual entropy at such high pressures and low temperatures.