Thermal expansion anisotropy of β-TlInS2 crystals in the course of phase transitions

Basing on the thermal expansion studies performed for β-TlInS₂ crystals in the course of their phase transitions, we have found that the crystals aged for one year are composed from at least two polytypes, which undergo phase transitions at different temperatures. The detailed analysis of anisotropy of the thermal expansion demonstrates that all of the diagonal components of the thermal expansion tensor associated with the eigensystem have almost the same modules but differ by their signs. This leads to appearance of an elliptical conical surface of zero thermal expansion. The orientation of this surface does not depend on the temperature in the temperature interval under study. We have also found that the thermal expansion coefficients along the crystallographic a and b axes are close to zero.     

Study of electrical conductivity and phase transition in Bi2O3–V2O5 system

The solid solutions of bismuth–vanadate were prepared by the conventional solid-state reaction. The sample characterization and the study of phase transition were done by using FT-IR, X-ray diffraction (XRD) and DSC measurements. AC impedance measurements proved that the oxide ion conductivity predominantly arises from the grain and grain boundary contributions as two well-defined semicircles are clearly seen along with an inclined spike. The electrical conductivity of Bi₂O₃–V₂O₅ has been studied at different temperatures for various molar ratios. The isothermal conductivity increases with an increase in the concentration of V₂O₅ due to the vacancy migration phenomenon. It has been found that the conductivity of different compositions of Bi₂O₃–V₂O₅ increases and shows a jump in the temperature range 230–260°C due to the phase transition of BiVO₄ from monoclinic scheelite type to that of tetragonal scheelite type. The endothermic peak in DSC at around 260°C reveals the phase transition, which is also confirmed by the XRD and FT-IR analysis. The XRD patterns confirmed the monoclinic structure of BiVO₄.     

Activation energy and conductivity of glasses in the P2O5–V2O5–Li2O system

The conductivity of glasses in the 50\textP_\text2 \textO_\text5 - x\textV_\text2 \textO_\text5 - ( 50 - x )\textLi_\text2 \textO50{\text{P}}_{\text{2}} {\text{O}}_{\text{5}} - x{\text{V}}_{\text{2}} {\text{O}}_{\text{5}} - \left( {50 - x} \right){\text{Li}}_{\text{2}} {\text{O}} system was studied as a function of temperature and composition. For all compositions, the conductivity variation as a function of temperature followed an Arrhenius type relationship. Isothermal variation of conductivity as a function of composition showed a minimum for a molar ratio x near 20. Probable mechanisms for decrease of conductivity with decrease of vanadium oxide concentration were explained. The minimum in room temperature was attributed to increase of V⁴⁺/V⁵⁺ with decrease of vanadium oxide in specific concentrations of vanadium oxide. Activation energy increased with decrease of V₂O₅ content. This behavior was attributed to increase of average spacing between vanadium ions.     

Influence of atmospheric humidity on the symmetry and phase transitions of layered potassium oxyfluorides K2NbOF5 · H2O

Crystals of K₂NbOF₅ · H₂O have been grown, polarization optical investigations have been performed, and the birefringence and rotation angle of the optical indicatrix have been measured in the temperature range 100–400 K. It has been found that, depending on the degree of atmospheric humidity, the layered K₂NbOF₅ · H₂O crystal at room temperature can be in three states, namely, A, B, and C, which differ in symmetry and properties of the crystal. The K₂NbOF₅ · H₂O crystal in the A state exists at a relative humidity RH = 90–100% and undergoes a first-order improper ferroelastic phase transition P $\bar 1$ ? C2/m, which is accompanied by strong anomalies of the optical characteristics, twinning, and shear strain x ₆ at temperatures T ₂ ^↓ = 308 K and T ₂ ^↑ = 313 K. The most stable state of the K₂NbOF₅ sdH₂O crystal is the B state (RH = 20–90%), which retains the monoclinic symmetry C2/m in the temperature range 100–370 K. In a dry atmosphere (RH = 0–20%) or at T ₁ ≈ 370 K, the crystal becomes anhydrous (K₂NbOF₅) with the symmetry P4/nmm (the C state). The difference between the crystals in the states A and B is explained by the presence or absence of water molecules in interlayer spaces.     

New transitions in the β-decay of 36Ca

The β-decay of the T _z = - 2 nucleus ³⁶Ca was studied at the LISE3 magnetic spectrometer at GANIL. Two new proton-emitting states have been detected and the other nine known βp and βγ transitions have been remeasured with improved resolution. A simulation with the GEANT code has been applied to this experimental setup. A comparison with shell model calculations is given. Received: 19 September 2000 / Accepted: 28 February 2001     

The Mössbauer effect in the points of phase transitions

The temperature dependence of the intensity of the Mössbauer line and the shift of its centre on passing through the points of phase transitions was studied. Both quantities exhibit discontinuity at transitions of the first kind while only the shift of the line centre does so at transitions of the second kind. The possibility of using the Mössbauer effect for the exact localization or even classification of phase transitions is shown.     

What is the origin of chirality in the cholesteric phase of virus suspensions?

We report a study of the cholesteric phase in monodisperse suspensions of the rodlike virus fd sterically stabilized with the polymer polyethylene glycol (PEG). After coating the virus with neutral polymers, the phase diagram and nematic order parameter of the fd-PEG system then become independent of ionic strength. Surprisingly, the fd-PEG suspensions not only continue to exhibit a cholesteric phase, which means that the grafted polymer does not screen all chiral interactions between rods, but paradoxically the cholesteric pitch of this sterically stabilized fd-PEG system varies with ionic strength. Furthermore, we observe that the cholesteric pitch decreases with increasing viral contour length, in contrast to theories which predict the opposite trend. Different models of the origin of chirality in colloidal liquid crystals are discussed.     

On the ferroelectric phase transition in polytypes of β-TlInS2 crystals

Permittivity measurements and x-ray diffraction study were performed for polytypes c and 2c of β-TlInS₂ crystals in the temperature range T = 160–250 K. Substantial differences are revealed in the temperature, sequence, and character of the structural phase transitions associated with the formation of incommensurate modulated structures and the occurrence of a ferroelectric state in these polytypes.     

Protonic conduction and diffusion in the hydrous oxides V2O5·nH2O,Nb2O5·nH2O,Ta2O5·nH2O and CeO2·nH2O

Conductivity (ac and dc) measurements are reported for hydrous V₂O₅, Nb₂O₅, Ta₂O₅ and CeO₅ in the range 250<T/K<320. Ambient temperature conductivities increase with water content and (SEM) agglomerate size, the most highly conductive material being Ta₂O₅·3.92 H₂O (α₂₉₈=3×10⁻⁴S cm⁻¹). There is no simple composition dependence of conductivity activation energy. ¹H NMR relaxation time measurements in the range 170<t/K<330 are also reported. The data consistent with chemical exchange between a range of interparticle protonic environments, but there is no simple link with conductivities.     

Ordering of hexagonal layers in phase β- and β′-Mg2Al3

Two phases, β and β′, in Mg₂Al₃ have been compared. Structural rod-like domains composed of sets of hexagonal layers have been described. The main set containing 11 layers does not change during β′–β transformation. The short sequences of layers on the β′ phase transform into clusters in the β-phase. The centres of the domains form a superstructure with a modulation vector equal to 3/22.     

FT-IR Study of the Adsorption of Propane and Propene on V2O5/γ-Al2O3 Catalyst

It has been observed on the oxidized V₂ O₅/γ-Al₂ O₃ that C₃H₆ form alkoxides which were converted to acetone during the desorption of adsorbate phase at 373 K. C₃ H₈ may form π-complexes on the reduced form of the same catalyst. However, the intensities of the bands in the adadsorption of C₃ H₈ were very weak as compared with the case of C₃ H₆ adsorption. Therefore, it was not reasonable to assign these bands clearly as any of the surface type species.     

FT-IR Study of the Adsorption of Propane and Propene on V2O5/γ-Al2O3 Catalyst

It has been observed on the oxidized V₂O₅/γ-Al₂O₃ that C₃H₆ form alkoxides which were converted to acetone during the desorption of adsorbate phase at 373 K. C₃H₈ may form π-complexes on the reduced form of the same catalyst. However, the intensities of the bands in the ad-adsorption of C₃H₈ were very weak as compared with the case of C₃H₈ adsorption. Therefore, it was not reasonable to assign these bands clearly as any of the surface type species.     

Electrical conductivity of the Li2OB2O3 system with V2O5

New Li ion vitreous conductors have been synthesized using simultaneously B₂O₃ and V₂O₅ as glass forming network. The ionic conductivity of the Li₂OB₂O₃V₂O₅ system for N=[B+V]/[M] and Y=[B]/[b+V, where 1⩽N⩽1.5 and Y⩽1.0 is experimentally determined. The results show a “mixed former effect” for the ratio Y=0.9. The Li₂OB₂O₃V₂O₅ system for the ratio Y=0.9 shows an enhancement in conductivity by an order of magnitude compared to the binary system Li₂OB₂O₃.     

Structures and phase transitions in some Pb2B′B″O6 complex perovskite-type compounds

In order to study the influence of substitutions and of the ionic radii of B’ ions on structure and phase transitions, we present some physical measurements and data on Pb₂B′B″O₆ (B”=W, Nb, Mo) complex perovskite compounds. By Mn and Fe substitution for Mg in Pb₂MgWO₆ compound a change in lattice parameters and shift of antiferroelectric transition temperature is observed. It appears that in all three series of compounds a linear dependence between ferroelectric or antiferroelectric transition temperature and B’ ionic radii is observed.     

Geometric phases and quantum phase transitions in inhomogeneous XY spin-chains:Effect of the Dzyaloshinski-Moriya interaction

We study geometric phases of the ground states of inhomogeneous XY spin chains in transverse fields with Dzyaloshinski--Moriya (DM) interaction, and investigate the effect of the DM interaction on the quantum phase transition (QPT) of such spin chains. The results show that the DM interaction could influence the distribution of the regions of QPTs but could not produce new critical points for the spin-chain. This study extends the relation between geometric phases and QPTs.     

Features of sound propagation in the vicinity of “symmetry-dictated” isostructural phase transitions in ferroelastics

Using a proper ferroelastic phase transition of the tension-compression type as an example, it is shown that, if the order parameter characterizing a structural phase transition allows the existence of a third-power invariant in the Landau potential, then there must be “symmetry-dictated” isostructural phase transition lines in the vicinity of the line of that structural phase transition. These isostructural transitions may manifest themselves both directly and as supercritical anomalies in the behavior of elastic moduli and lattice parameters. These effects are discovered and investigated without invoking the perturbation theory in terms of which the second-order phase transitions are commonly described. A hypothesis is made on the basis of the results obtained that the sound velocity anomalies observed in orthoclase and sanidine crystals are due to the super-critical behavior of the lattice parameters in the vicinity of a symmetry-dictated isostructural phase transition in the prototype phase of these crystals.     

Dielectric properties and phase transitions in crystals of TlInS x Se2 − x solid solutions

This paper reports on a study of the dielectric permittivity and electrical conductivity of single-crystal TlInS _x Se_{2 − x} solid solutions as functions of temperature and composition. It has been found that the dielectric permittivity ɛ and electrical conductivity σ decrease with increasing x and increase with growing temperature. The temperature dependences ɛ = f(T) and σ = f(T) for TlInS _x Se_{2 − x} crystals have been demonstrated to reveal anomalies in the form of maxima, which suggests that these crystals undergo phase transitions. The temperatures of the phase transitions increase with increasing x.     

Order–disorder transformation of the O phase in Ti2AlNb alloys

The free energy of the O₂ (Ti₂AlNb) phase, based on the Gorsky–Bragg–Williams (G–B–W) approximation considering nearest-neighbour interactions, has been utilized to derive expressions for the transition and instability temperatures. The relations among the long-range order parameter, transformation temperatures and free energy as a function of the concentrations of Al and Nb have been established. The results are compared with those of earlier experimental and theoretical investigations.