Relaxation processes in the paraelectric phase of ceramic oxides with a perovskite structure

Solid solutions of barium, strontium, and lead titanate ceramics with a perovskite structure are studied using impedance spectroscopy in the frequency range 10²–10⁶ Hz at temperatures from 20 to 450°C. The experimental data represented by the dispersion of the electric modulus are compared with the results of calculations performed for different mechanisms of the non-Debye relaxation in terms of the Havrilyak-Negami empirical formulas and the Jonscher universal dielectric response with the aim of determining a more adequate approach. The activation energy of relaxation processes in the paraelectric phase of the samples under investigation are calculated.     

Thermal hysteresis and memory effects in TlGaSe2 crystal with incommensurate phase

Temperature dependencies of dielectric permittivity of TlGaSe₂ have been measured under various thermal cycles. Peculiarities of anomalies in temperature dependencies of dielectric permittivity corresponding to structural phase transitions at 108 and 115?K are discussed. The coexistence of two different incommensurate structures in TlGaSe₂ was proposed. The phase transitions at 108 and 115?K are considered as commensurate lock-in transitions. As a result a new model of the structural phase transitions in TlGaSe₂ has been suggested.     

Nuclear quadrupole resonance spectrum and transition from the incommensurate phase to the commensurate phase of a crystal

The relationships describing the nuclear quadrupole resonance spectrum in the vicinity of the transition from the incommensurate phase to the commensurate phase of a crystal are derived in the adiabatic approximation. It is demonstrated that the intensity of the nuclear quadrupole resonance peaks depends on the mutual orientation of the order parameter and the electric field gradient.     

Existence of an incommensurate phase in a Li2B4O7 crystal

The x-ray diffraction spectra of Li₂B₄O₇ single crystals are investigated in the temperature range 80–300 K, and the lattice parameter c is determined in the same temperature range in the presence of a periodically varying temperature field. An incommensurate phase is not observed anywhere in the temperature range investigated, regardless of whether the crystals are subjected to a periodic temperature field. Fiz. Tverd. Tela (St. Petersburg) 39, 1461–1463 (August 1997)     

Linear defects in incommensurate phases

It is shown that the boundary between incommensurate (I) and commensurate (C) phases, or between two I regions with different periodicities, consists of an array of linear defects. A topological classification of these defects is given for one-component complex order parameters and their significant in interpreting C-I transitions and properties of I phases is outlined.     

Topological defects in incommensurate magnetic and crystal structures and quasi-periodic solutions of the elliptic sine-Gordon equation

Vortex-type singular solutions with a topological charge of the elliptic sine-Gordon equation have been studied. One- and two-dimensional vortex lattices on a homogeneous and periodic background are constructed in the explicit form using the Bäcklund transformation. The interaction of vortices is investigated and finite energy configurations are found. On the basis of the obtained results new topological defects in incommensurate magnetic and crystal structures are predicted and described. The interaction of vortex magnetic structures with nonlinear spin waves is considered.     

Identification of a TGBA liquid crystal phase via its defects

We have shown that cholesteryl nonanoate, a thermotropic compound which is well known to exhibit pretransitional effects at the smectic A (SmA) cholesteric (N*) transition (W.L. McMillan, Phys. Rev. A 4, 1238 (1971); 6, 936 (1972)), has in fact a TGBA phase in between. Our arguments rely on the observation of new TGBA defects, either in Robinson spherulites cooled from the N* phase or in free-standing films. The same new defects can be obtained in a well-documented TGBA phase of a tolane compound. We analyze qualitatively the TGBA defects in both geometries, in particular their relation to the disclination radius of the N* Robinson spherulites. Received 12 February 2001     

Self-organization processes and topological defects in nanolayers in a nematic liquid crystal

Atomic force microscopy is used to study the self-organization processes that occur during the formation of topological defects in nanomolecular layers in a nematic liquid crystal with the homeotropic orientation of its molecules with respect to the substrate. In this case, a smectic monolayer with a thickness of one molecule length (about 2.2 nm) forms on the substrate, and a nanomolecular layer of a nematic liquid crystal forms above this monolayer. In such virtually two-dimensional layers, numerous different nanoclusters, namely, hut structures, pyramids, raft structures with symmetry C _nm (where n = 2, 4, 5, 6, 7, …, ∞), cones, and nanopools, form [1]. They have a regular shape close to the geometry of solid crystals. Modulated linear structures and topological point defects appear spontaneously in the nanopools and raft structures.     

Influence of irradiation defects on the incommensurate phase of Ba2NaNb5O15

Preliminary observations of the incommensurate phase of barium sodium niobate under irradiation are reported here. Defects induce the stabilization of the incommensurate phase at room temperature although the effect of temperature on the incommensurability δ is still visible.     

On the satellite extinction in incommensurate phase

For ZnP₂ and CdP₂ crystals in incommensurate phase selective superstructure X-ray reflections are observed. As shown a phason and amplitudon excitation modes do not result in this selective extinction of satellites. Apparently, it is associated with a modulation wave form in the incommensurate phase. The X-ray spectrum peculiarity of incommensurate crystals is discussed taking into consideration systematic and selective extinction of satellites.     

Dynamics of relaxation processes in a triglycine selenate single crystal in a ferroelectric phase

The deviation of the relaxation spectrum from the Debye type is studied experimentally and the presence of the non-Debye relaxation is shown, which is defined in different temperature ranges either by proton multiposition transitions below the Curie point T _c or relaxation domain and interphase boundaries in the vicinity of T _c. For triglycine selenate, the methods of analysis of dielectric non-Debye spectra are used.     

Ba-IV-type incommensurate crystal structure in group-V metals

The long-unknown crystal structure of Bi-III has been solved. It comprises a body-centered-tetragonal (bct) "host" and a bct "guest" component made up of chains that lie in channels in the host; the guest is incommensurate with the host along the tetragonal c axis. Diffraction data for Sb-II reveal that it too can be fitted with the same composite structure. The structures of these two high-pressure phases of Bi and Sb are similar to those reported recently in the alkaline-earth metals Ba and Sr.     

Dynamics of the domain structure in ferroelectrics with an incommensurate phase

The dynamics of the domain structure of ferroelectrics with an incommensurate phase (like sodium nitrite) is considered. The polarization properties of these ferroelectrics are described using the pseudospin formalism. The equation for the polarization is numerically solved together with the equation for acoustic waves. The obtained results are analyzed for various values of the parameters of the crystal and different initial conditions. It is shown that, after a sufficiently large time, the domain structure of the crystal becomes localized.     

Zero-field incommensurate spin-Peierls phase with interchain frustration in TiOCl

We report on the magnetic, thermodynamic, and optical properties of the quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been discussed as spin-Peierls compounds. The observed deviations from canonical spin-Peierls behavior, e.g., the existence of two distinct phase transitions, have been attributed previously to strong orbital fluctuations. This can be ruled out by our optical data of the orbital excitations. We show that the frustration of the interchain interactions in the bilayer structure gives rise to incommensurate order with a subsequent lock-in transition to a commensurate dimerized state. In this way, a single driving force, the spin-Peierls mechanism, induces two separate transitions.     

Melting of a dust crystal with defects

The effect of defects on the melting of a two-layer dust Wigner crystal in the electrode layer of a radio-frequency gas discharge is investigated by Langevin molecular dynamics. Two types of defects have been included in the consideration: (a) point defects and dislocations and (b) particles levitating above and below a two-layer crystal (so-called strong defects). It is shown that local melting of a two-layer crystal found experimentally is explained by the occurrence of strong defects located above the crystal.