Effect of negative chemical pressure on some displacive-type ferroelectrics

The effect of embedded helium on the phase-transition temperature and properties of displacive-type ferroelectrics has been studied. It is shown that helium embedded in crystals produces an effect opposite to that of hydrostatic pressure, namely, the temperatures of the ferroelectric phase transitions increase, and those of the antiferroelectric ones, decrease. Fiz. Tverd. Tela (St. Petersburg) 41, 1293–1296 (July 1999)     

EPR of a non-Kramers iron ion in KTaO3

The EPR spectrum of the non-Kramers iron ion Fe⁴⁺ (S=2) in a KTaO₃:Fe crystal appearing after illumination of the sample in the visible has been detected and studied. Because of the large initial splitting (|D|=4.15 cm⁻¹), only transitions within the |±1〈 and |±2〈 doublets are seen experimentally. Superhyperfine structure in the spectrum of a non-Kramers ion in perovskites has been detected for the first time. A structure is proposed for the center responsible for the new EPR spectrum, which represents a complex of a Fe₄₊ ion substituting for Ta with an oxygen vacancy at the nearest anion site. Fiz. Tverd. Tela (St. Petersburg) 41, 1424–1427 (August 1999)     

Dipole moment and mobility of molecules in nematic liquid crystals of the 4-n-butyl ester of [4′-n-hexyloxyphenyl] benzoic acid in the absence of external orienting fields

Dielectric polarization was studied in a nematic liquid crystal of the 4-n-butyl ester of [4′-n-hexyloxyphenyl] benzoic acid (BE[HOP]BA) in the absence of external orienting fields in the isotropic and mesophase states in a frequency range of 10³–10⁷ Hz. In the isotropic melt, three regions of dielectric absorption of a relaxation origin were revealed. It is shown that two of them are related to a reorientation motion of individual molecules about the longitudinal axes (process I) and about the short axes (process II). Processes I and II have relaxation times τ ～ 10^?9 and 10^?8 s and activation energies ΔU ～ 16 and 23 kcal/mol, respectively; the energy of activation of process II in the liquid-crystal phase increases to ΔU ～ 38 kcal/mol. In the isotropic melt, in addition to processes I and II, process III occurs in the low-frequency range, which is characterized by greater relaxation times τ ～ 10^?7 s and an activation energy ΔU ～ 28 kcal/mol. In order to establish the nature of process III, temperature dependences of the dipole moments and Kirkwood correlation factor g were studied in both BE[HOP]BA phases. The magnitude of the Kirkwood factor in the isotropic phase of the BE[HOP]BA near the transition temperature from the mesomorphic state (g ～ 0.88) indicates the retention of the orientational ordering of molecules of the low-molecular liquid crystal and the compensation of their dipole moments intrinsic to the liquid-crystal state. This circumstance suggests that the third process of the relaxation of dipole polarization is due to a cooperative mode of motion of molecules in mesophase nuclei in the isotropic melt.     

Observation of superhyperfine structure in the ESR spectra of two tetragonal Fe centers in KTaO3 and a new model for the center

Superhyperfine structure is observed in the lines of the ESR spectra of two tetragonal iron centers in the KTaO₃ crystal, namely, “Fe 4/2” and Fe _Ta ³⁺ -V_O. Analysis of the superhyperfine structure of the former of these centers shows that the iron ion replaces Ta⁵⁺ and is found in the charge state 5+. Justification is given for assuming that the tetragonal symmetry of the center is due to the displacement of Fe⁵⁺ from the Ta⁵⁺ site along a 〈100〉 direction to an off-center position. Fiz. Tverd. Tela (St. Petersburg) 39, 626–629 (April 1997)     

Self-synchronization dynamics in the process of the generation of a second harmonic in BaTiO3

A model of self-synchronization in the process of second-harmonic generation in a quadratic medium with a small coherence length is verified by investigations of the dependences of the output intensity on the intensity and the repetition frequency of excitative pulses.     

Coloring and breakdown of barium titanate

Spontaneous increase in current in strong fields is found to be related to motion of color centers. Electron injection increases the number off, centers, which reduces the electrical strength.Read at the All-Union Conference on Recent Insulator and Semiconductor Techniques, Leningrad.