G. V. Kozlov

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.

Proton modes in the crystals of KH2PO4 family

Abstract According to the present conceptions, excitation of the proton subsystem is of principle importance for the dynamics of phase tran- sitions of the KH2P04 type crystals and, therefore, it is an object of thorough experimental investigation. The present paper is concerned with the investigation of proton modes by means of dielectric spectroscopy in submillimetre wave range, Just in this spectrum range lies the region of fundamental ferroelectric dispersion of the KH2p04 family crystals.

Submillimeter dielectric spectra of SC(NH2)2 and BaMnF4 single crystals

The dielectric spectra of thiourea and BaMnF4 were determined in all phases (85-300 K) in the 3-30 cm-1 region using the monochromatic spectrometer with tunable oscillator. Moreover, far infrared (30-450 cm-1) reflectivity of BaMnF4 at 90 and 300 K was measured and evaluated to obtain the dielectric spectra. In the case of thiourea a new extra mode near 7 cm-1 was observed in the paraelectric region. A rather complex dielectric dispersion was found in incommensurate phases in qualitative accord with theoretical expectations. In the case of BaMnF4, two low-frequency modes which partially soften at Tc were found in agreement with recent Raman results.

Far infrared and millimetre dielectric response of incommensurate and ferroelectric K2SeO4

Dielectric response of K2SeO4 in the spectral region 5–460cm−1 was determined using transmissivity and reflectivity measurements as a function of temperature between 80 and 300K. The spectral features above 20cm−1 are interpreted using results of lattice vibrational analysis in three known commensurate phases. The low-frequency dielectric anomaly in the incommensurate phase can be roughly described by critical slowing-down of a Debye relaxation given rise to by the overdamped infrared active phason mode which softens at the incommensurate-commensurate transition.

Soft-mode spectroscopy of tris-sarcosine calcium chloride

We have measured soft modes associated with the second-order ferroelectric phase transition in tris-sarcosine calcium chloride, (CH3NHCH2COOH)3 CaCL2, in both paraelectric D2h16 (Pnma) phase, and ferroelectric C2v9 phase. In the paraelectric phase the mode decreases from 11.2 cm-1 at 167K to 3.2 cm-1 at 130 K (TC = 127 K) and is underdamped over most of this range. A Curie constant of 25 ± 2 K is deduced from these infrared data, in fair agreement with that of 58 K measured in the megahertz regime via conventional dielectric studies. In the ferroelectric phase the mode is measured from 30.5 cm-1 at 4 K to 2.3 cm-1 at 126 K. The system is unique inasmuch as the longitudinal optical phonon decreases in energy by 90% between T = 0 and T = TC, in contrast to normal ferroelectrics in which LO energy is nearly independent of temperature; this shows that the inherent instability in the lattice is in the short-range forces and not to any subtle cancellation of short-range and long-range forces, as proposed in Coc...

Millimeter wavelength spectroscopy of the ferroelectric phase transition in tris-sarcosine calcium chloride

Abstract We have measured the soft mode responsible for the ferroelectric phase transition in tris-sarcosine calcium chloride: (CH3 NHCH2 COOH)3 CaCl2, by both far infrared and Raman techniques. In the ferroelectric C9 2v phase the soft TO varies from 30. 5 cm−1 at 4K to 2. 3 cm−1 at TC - 1K = 126 K; in the paraelectric D16 2h phase the soft TO has B2u symmetry varies from 11. 2 cm−1 at 167 K to 3. 2 cm−1 at TC + 3 K = 130 K. A Curie constant of 25 ± 2 K is deduced, which differs from that of 57 K measured in the MHz regime, indicating significant dispersion. This ferroelectric is unique in that the A1(LO) and B2u(LO) modes also soften substantially; in the C2v phase A1(LO) varies from 31 cm−1 at 4 K to 4 cm−1 at TC - 1K. This shows that ferroelectricity in TSCC does not arise from a subtle cancellation of short-range and long-range forces, as in Cochrans theory, but from an intrinsic instability in the short-range force associated with the N-H-Cl hydrogen bonds.

Ferroelectric dynamics of ortho-phosphate crystals

Abstract The present paper is concerned with the theoretical and experimental investigations of dynamic properties of ferroactive subsystems of ortho-phosphate crystals.