Local distortion of PO4 tetrahedra in the paraelectric phase in low temperature under high pressure in KH2PO4

The ν1–ν4 internal modes of PO4 tetrahedra of crystalline KH2PO4 (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 PO4 tetrahedra is found to be C2 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 S4 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 C2 or S4 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.