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A b s t r a c t . The paper presents the results of the microwave dielectric investigations of the ferroelectric PbHP04. The full dielectric spectrum of the ferroelectric Debye-type relaxational dispersion over the range 1 O6 to 10' Hz confirms that the relaxational soft mode triggers the order-disorder type phase transition and accounts for the static permittivity.

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The dielectric properties are investigated of TlH2PO4 crystals over the frequency region 1 MHz to 78 GHz. Dielectric permittivity is almost independent on frequency up to the millimeter range. In the millimeter range a low‐frequency lattice mode causes dielectric dispersion. The mode (νs = 367 GHz at T = 360 K) contribution defines the high static dielectric permittivity ϵ′b of TDP, its softening causes the anomalies of ϵ;′b and ϵ″b at the upper ferroelastic phase transition temperature Tc2 = 357 K and the temperature dependence of the permittivity in the ferroelastic phase.