The valence fluctuations which are related to the charge disproportionation of phosphorous ions P 4+ + P 4+ → P 3+ + P 5+ are the origin of ferroelectric and quantum paraelectric states in Sn(Pb)2P2S6 semiconductors. They involve recharging of SnPS3 or PbPS3 structural groups which could be represented as half-filled sites in the crystal lattice. Temperature-pressure phase diagram for Sn2P2S6 compound and temperature-composition phase diagram for (PbySn1−y)2P2S6 mixed crystals, which include tricritical points and where a temperature of phase transitions decrease to 0 K, together with the data about some softening of low energy optic phonons and rise of dielectric susceptibility at cooling in quantum paraelectric state of Pb2P2S6 are analyzed by GGA electron and phonon calculations and compared with electronic correlations models. The anharmonic quantum oscillators model is developed for description of phase diagrams and temperature dependence of dielectric susceptibility.

The structure of TlGaSe2 has been studied by triple-crystal X-ray scattering techniques. At room temperature in the paraelectric phase the structure is believed to be C2/c, but the authors found that some of the (h, h, l) Bragg reflections, those with h odd, are broadened along the c* direction. This is shown to arise because of faults in the stacking of the atomic sheets perpendicular to c*. A simple model is developed for the scattering which suggests that on average a stacking fault occurs in the single crystal about once every four planes. Between 117 and 110 K the crystal has an incommensurate structure with a wavevector ( delta , delta , 1/4) where delta approximately=0.02 reciprocal lattice units. At 110 K, delta jumps discontinuously to zero to produce a commensurate phase. These results confirm the earlier suggestions of an incommensurate phase in TlGaSe2.

High-temperature coherent neutron powder diffraction experiments were carried out on Li2B4O7 with boron isotope ratio 11B : 10B as high as 99.6 : 0.4%. Neither traces of phase transformations nor discontinuous changes of physical properties were observed. Anomalies in the thermal expansion of lithium tetraborate were considered in terms of first-order Grüneisen approximation. Extended bond length analysis revealed significant modifications of the boron–oxygen framework which is supplemented by dynamic lithium disorder. Impedance spectroscopy studies revealed a complicated conduction mechanism in single crystalline lithium tetraborate. The lithium diffusion pathway for bulk conductivity along the polar axis was established using both maximum entropy and anharmonic refinement techniques.

Relaxor ferroelectric PbMg 1/3 Ta 2/3 O3 ceramics and thin films were investigated by means of broad-band dielectric, time-domain terahertz (THz) and Fourier-transform infrared (IR) spectroscopy in the frequency range 100 Hz -90 THz at temperatures 100 -490 K, the THz and IR spectra were studied from 20 to 900 K. Diffused and strongly temperature dependent peak in the complex permittivity is caused by a dielectric relaxation due to the dynamics of polar clusters. The relaxation appears below Burns temperature T d in the THz range, slows down on cooling through the microwave and MHz range and anomalously broadens. The shortest and longest relaxation times of the distribution of relaxation times follow Arrhenius and Vogel-Fulcher law, respectively. The degree of B-site order has only a small influence on the parameters of the dielectric relaxation and almost no influence on the phonon parameters. Below Tm ∼ =180 K the distribution of relaxation frequencies becomes broader than our experimental spectral range and frequency independent dielectric losses develop below 100 GHz in the spectra. Although the macroscopic crystal structure is cubic, IR spectra give evidence about the lower local symmetry which can be assigned to the presence of polar clusters below T d . Infrared spectra above T d still reveal more modes than predicted by selection rules in the paraelectric phase of the F m3m space group so that we suggest selection rules which take into account chemical inhomogeneity in the β"-perovskite sublattice.

The dielectric susceptibility measurements are usually interpreted in terms of the relaxation times of various dynamical processes. Using the simple examples of the simulated spectra it is shown how the distribution of these relaxation times can be obtained by means of the integral equations solved with the Tikhonov regularization technique, and the criteria for the choice of the regularization parameter is discussed.

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