Raman scattering study of KDP under high pressure with a diamond anvil cell

Abstract

Raman scattering spectra of the ferroelectric KDP (KH₂PO₄) have been observed under hydrostatic pressure up to 52.0 kbar using a diamond anvil cell at room temperature. A curious dispersion relation of the soft mode at the Γ point of the Brillouin zone is derived from the low-frequency components of the B₂(z) spectra in z(yx)z + δ and x(yx)y geometries after numerical fitting based on the coupled phonon model. The dispersion relation suggests breakdown of the soft-mode model of KDP. Each of the frequencies of the internal v₁, v₂ and v₃ modes of the PO₄ ion show a linear dependence on pressure with the mode Grüneisen parameter, 0.32, 0.31 and a small value of 2.5 × 10⁻², respectively. Increase of the frequencies of the v₁ and v₂ modes corresponds to pressure dependence of the P–O distance. A new pressure-induced phase transition appears at 40.0 kbar with a growing relaxational central peak in the spectra.

Key Words:

• Ferroelectric
• KDP
• Raman scattering
• central peak
• phase transition
• high pressure.