Far infrared and submillimetre dielectric dispersion in ferroelectric PbHPO4 and PbHAsO4

Abstract Polarized far-infrared reflectivity and submillimetre transmittance measurements revealed that the soft mode in PbHPO4 is of a relaxational type with the relaxation time τ ≃ 5.3 · 10−11 (T-Tc )-1s for T>Tc =310 K and τ ≃ 1.3 · 10−11 (Tc-T)−1 s for T<Tc . Unusual strong changes in E ⊥ Ps spectra show a large degree of anharmonicity and make the detailed interpretation difficult.