New Ln[B6O9(OH)3] borates (Ln = Sm-Lu): Structure, properties, and structural relation to cationic Li4[B7O12]Cl conductors (Li-Boracites)

Crystals of new rare earth borates of the composition Ln[B₆O₉ (OH)₃] (Ln = Sm-Lu), sp. gr. R3c are synthesized under hydrothermal conditions. Their crystal structures are determined on single crystals with Ln = Ho, Gd without preliminary determination of their chemical formulas. The polar anionic framework of the crystals consists of BO₃ triangles and BO₄ tetrahedra and has wide channels along the threefold axis of the structure, which are similar to the channels along the a, b, and c axes in cubic Li₄[B₇O₁₂]Cl boracite with Li conductivity. Rare earth atoms are arranged in the structure over the cubic F pseudolattice, whereas the analogous positions in Li boracites are filled with Cl anions. The squared optical nonlinearity of the new crystals is comparable with the nonlinearity of quartz, whereas the electrical conductivity in borates at 300°C exceeds 10⁻⁶ S/cm. __________ Translated from Kristallografiya, Vol. 49, No. 4, 2004, pp. 681–691. Original Russian Text Copyright ? 2004 by Belokoneva, Ivanova, Stefanovich, Dimitrova, Kurazhkovskaya.     

RADIATION-INDUCED CHANGES IN THE STRUCTURE AND FERROELECTRIC PROPERTIES OF Pb5Ge3O11 SINGLE CRYSTALS

Journal of Structural Chemistry - Perfect stoichiometric Pb5Ge3O11 single crystals are irradiated with 60Co γ-rays (1·106 Rad) and high-energy electrons (0.13 e/cm2, 0.89 e/cm2,...     

Switching Channel Development Dynamics in Planar Structures on the Basis of Vanadium Dioxide

The results of the experimental studies and numerical simulation of the switching channel development dynamics in planar structures on the basis of vanadium dioxide are reported. The obtained data on the variation of the temperature in the channel with time and of the current arisen after the pulsed load, and on the times of transition from the high-resistance to the low-resistance state and back are analyzed in order to determine the switching mechanism and to predict the functional characteristics of the switchable vanadium- oxide structures as promising materials for the creation of relaxation generators that can serve as prototypes of neural oscillators. It is shown that the switching behavior is associated with the metal–semiconductor phase transition in vanadium dioxide, which is stimulated by the emission of Joule heat.     

Structure and mutual transformations of anhydrous and hydrous CsMgPO4

A complex study of the synthetic CsMgPO₄ phosphate with a ??-tridymite structure type is performed. It is found that CsMgPO₄ is an unstable phase. At room temperature in air, it absorbs water and turns into the CsMgPO₄ · 6H₂O crystal hydrate. The transformation of cubic CsMgPO₄ · 6H₂O into orthorhombic CsMgPO₄ proceeds upon heating to 150°C by the pattern of the first-order phase transition with the dehydration of the specimen.     

Electrical properties of heterovalently doped apatite-like oxosilicates

Oxygen-ion-conductive apatite-like oxosilicate and silicophosphate ceramics has been prepared by sintering sol-gel synthesis products at temperatures 200°C below those of solid-phase synthesis. The electrical properties of the ceramics obtained have been studied by impedance spectroscopy in the range 400–1000°C. A positive effect of heterovalent substitutions in the silicon sublattice on the bulk and intergrain conductivities is established. The increase in the oxygen-ion conductivity in La_9.6Si_5.7Mg_0.3O_26.1 in comparison with the other studied oxosilicates by more than two orders of magnitude is related to the formation of additional interstitial positions for oxygen ions as a result of partial substitution of the silicon ion, tetrahedrally coordinated by oxygen, with the Mg ion having a characteristic octahedral coordinatio     

Dielectric relaxation in a thermosetting polyimide modified with a thermoplastic polyimide

Relaxation processes in glass-fiber-reinforced composites with a polymer matrix based on blends of thermosetting and thermoplastic polyimides are studied via dielectric-relaxation spectroscopy. For all investigated blends, two relaxation processes related to the β relaxation of different fragments of the polymer chain are found. Linear flexiblechain polyimide incorporated into the polymer matrix serves as a plasticizer.     

Curing of a network polyimide modified with a linear component

The curing and relaxation processes in polymeric composites based on blends of network and linear polyimides applied onto a filler were studied by dielectric spectroscopy. The sensitivity of dielectric spectroscopy to processes occurring in the course of curing was examined.     

Kinetics of formation and growth of antiphase domains during second-order phase transitions

The kinetics of the formation and growth of antiphase domains during second-order phase transitions is investigated theoretically in the Ginzburg-Landau model using a statistical approach. It is shown that depending on the initial conditions both uniform and polydomain-type ordering can be realized in thermodynamic equilibrium. It is found that for small initial inhomogeneities the ordering process can be nonmonotonic in its initial stages. It is established that for special initial conditions long-lived ordered structures of a special type arise in the intermediate stages of the ordering process. The characteristic formation time of the domain structure is estimated. Zh. éksp. Teor. Fiz. 113, 228–239 (January 1998)     

The fluctuation theory of superconductivity for a polyene system

A system of polyacetylene chains is considered in which the chains are in a solution, whose atoms and ions have nonzero spin. It is shown that a superconducting phase may appear in the temperature range T_c2 ? T ? T_c1. A new particle that is responsible for the weak interaction in the nuclear matter is found. The possibility of using the Messbauer effect for abserving the superconducting phase in a magnetic superconductor is also considered.     

Nanosized structural transformation and nonlinear optical properties of lithium niobium germanate glasses

Glass formation in Li₂O-Nb₂O₅-GeO₂ (LNG) system, the structure and crystallization behavior of glasses that have compositions near the ratio Li₂O/Nb₂O₅ ∼ 1 corresponding to stoichiometry of ferroelectric phase LiNbO₃ were examined by differential thermal analysis, X-ray diffraction, small-angle neutron scattering and second harmonic generation (SHG). LNG glasses were subjected to heat treatments at temperatures in the range between T_g and temperature of the first exothermic peak in order to initiate nonlinear optical activity by nanoheterogeneity formation. Transparent nanostructured glasses with second-order optical nonlinearity were obtained for compositions characterized by the Li₂O/Nb₂O₅ molar ratio ranging from 0.83 to 1.2 and GeO₂ 40-45 mol%. As prolonged heat treatments of nanostructured glasses result in crystallization of ferroelectric LiNbO₃ the origin of SHG in transparent LNG glasses is supposed to be connected predominantly with polarity of nanoheterogeneities formed at the initial stage of phase separation.