Birefringence of SbSI and SbSeI crystals at the region of antiferroelectric phase transition

Anomalous behavior has been observed in the temperature range of T = 140–145 °C. The electric current I(T) of SbSI and SbSeI crystals along the c(z) axis changes significantly at the region of the antiferroelectric phase transition when DC voltage is applied. The peak positions are observed at 140 °C and 145 °C for SbSI and SbSeI, respectively. The birefringence spectra show temperature-dependent changes upon retardation m, confirming the antiferroelectric phase transition for both crystals. The peak positions observed from the optical measurements do not differ from the electrical measurements, showing the transition point at 140?°C and 145?°C for SbSI and SbSeI, respectively.     

Correlation between thermoluminescence emission and phase transitions of tridymite

ABSTRACT

Tridymite exhibits a complex 400?nm-induced thermoluminescence spectra consisting of five groups of components at (i) 90 and 130°C (due mainly to oxygen vacancies, substitution and interstitial atoms and/or impurities that create localized charge losses), (ii) 200°C (linked to a partial phase transition), (iii) 230°C (associated probably with alkali ions moving away from Al that leads to [AlO₄]° hole-like or [GeO₄/Li]° centers) and (iv) 320°C (caused by both phase transition and electron recombination with the unstable holes trapped at oxygen vacancies next to Al sites). The application of the T_m–T_stop protocol confirms a continuum in the trap structure not only because of the successive shifting of the maxima with the increase of the temperature, but also with the enhancement of the activation energy values that have been assessed by means of the initial rise method.     

The synthesis of the chiral non-mesogenic d-seco estrone derivatives and their influence on the phase transitions of cholesteric liquid crystals

The synthesis of new chiral seco-estrone derivatives is presented, as well as their influence on the phase transition of binary mixtures of cholesteryc liquid crystals. The new chiral derivatives do not posses any liquid crystalline phases and were synthesized in several synthetic steps, starting from estrone. We have studied the mixtures of cholesteryl non-anoate (40%) with cholesteryl myristate (40%) and addition of new chiral derivatives 3 4, or 5 (20%). It was concluded that the addition of chiral derivative 3 to the binary mixture stabilizes smectic A and cholesteric phase and shifts the phase transition temperature with respect to pure binary mixture for about 5°C towards lower temperatures. The extension of the temperature range of the cholesteric phase from 5°C to 15°C was established in the case when the derivatives 3 and 4 are added to the binary mixture of cholesteryl nonanoate with cholesteryl myristate. The phase diagrams of investigated compounds are formed on the basis of data obtained by the optical microscopy. Using X-ray diffraction on the crystalline powder of unoriented samples we have determined the molecular parameters: the thickness of smectic and cholesteric layers and average distance between the long axes of neighboring molecules.     

The baddeleyite-type high pressure phase of Ca(OH)2

Abstract

A phase transition from Ca(OH)₂ I (portlandite) to Ca(OH)₂ II at high pressure and temperature has been confirmed, using in situ x-ray diffraction in a multianvil high pressure device (DIA). The structure was determined at 9.5 GPa and room temperature from data collected after heating the sample at 300°C at 7.2 GPa in a diamond anvil cell. Both the Le Bail fit and preliminary Rietveld refinement suggest that the new phase, which reverts to Ca(OH), I during pressure release, has a structure related to that of baddeleyite (ZrO₁); it is monoclinic (P2₁/c) with a= 4.887(2), b= 5.834(2), c = 5.587(2), β = 99.74(2)°. The coordination number of Ca increases from six to seven (5 + 2) across the transition. At 500°C, the phase boundary is bracketed at 5.7 ± 0.4 GPa by reversal experiments performed in the DIA.     

Ferroelectric phase transition in LaBSiO5 crystals from results of thermal and dielectric measurements

The specific heat of the ceramic and the permittivity of a single-crystal sample of LaBSiO₅, a new ferroelectric in the stilwellite family, were measured in a temperature range which includes the phase transition point (T _C=140 °C). The excess entropy of the phase transition ΔS=1.05 J/mol · K and the Curie-Weiss constant C _C-W=3.2×10³ K were determined. The results indicate that the phase transition in this crystal is of a “mixed” nature and exhibits features of a displacement-type transition and an order-disorder transition. Fiz. Tverd. Tela (St. Petersburg) 40, 1310–1312 (July 1998)     

First observation of a wetting phase transition in low-angle grain boundaries

The wetting phase transition at low-angle intercrystallite grain boundaries has been experimentally observed. In contrast to the high-angle grain boundaries with the misorientation angels θ > 15°, the low-angle grain boundaries (θ < 15°) are not continuous two-dimensional defects, but constitute a discrete wall (network) of lattice dislocations (edge and/or helical). The theory predicts that, depending on θ, either a continuous layer of the liquid phase or a wall (network) of microscopic liquid tubes on wetted dislocation nuclei is formed at completely wetted low-angle grain boundaries. It has been shown that the continuous liquid layers at low-angle grain boundaries in the Cu-Ag alloys appear at the temperature T _wminL = 970°C, which is 180°C higher than the onset temperature T _wmin = 790°C and 50°C lower than the finish temperature of the wetting phase transition at high-angle grain boundaries, T _wmax = 1020°C.     

Raman study of phase transition in ferroelectric Ba0.95Ca0.05TiO3

Raman scattering investigation of phase transition in the ferroelectric Ba_0.95Ca_0.05TiO₃ is reported. The results suggest onset of significant dynamic disorder at 105°C. This corroborates findings of recent structural study regarding large positional disorder associated with Ti and O1 atoms well below the tetragonal to cubic transition temperature (∼150°C).     

On the characteristic changes in the domain structure and conductivity of CsDSO4 crystals near the superionic phase transition

The appearance of a new domain structure against the background of the old domain structure is observed in CsDSO₄ crystals at 3 °C away from the superionic phase transition. It is established that the appearance of the new domain structure is accompanied by a gradual increase in the conductivity by 1.5–2 orders of magnitude, and then the conductivity increases abruptly by another two orders of magnitude at the temperature of the superionic phase transition. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 11, 871–875 (10 June 1996)     

On the revealing of nonequilibrium phase transitions in water

It has been found that the jump and stabilization of the temperature observed in [L.N. Baturov et al., JETP Lett. 93, 91 (2011)] upon the heating/cooling of water near 4°C at a rate of ∼10⁻³ K/s are observed in water of any purification degree. However, we have not found the process of the formation/melting of supermolecular structures assumed by Baturov et al., which is sought in the shift of the center of the OH band of Raman scattering, e.g., at ∼150 cm⁻¹, as at the melting of hexagonal 1h ice [S.M. Pershin and A.F. Bunkin, Opt. Spectrosc. 85, 190 (1998); Patent RF No. 98, 103249 (1998)]. It has been shown that the revealed temperature features are absent in the presence of the mixing of water and artificial limitation of convection, as well as in a thin layer; this indicates that the regularity of the phenomenon is doubtful and that convection plays an important role. The visualization of convection flows by potassium permanganate made it possible to detect the reversion of their circulation over the trajectory of surface-wall-bottom-volume symmetry axis at the transition through a point of 4°C. The observed features have been interpreted as a manifestation of Archimedes’ principle.     

On the low temperature phase transition in (NH4)2SO4

An ultracryostat and multidecameter were used to determine the temperature dependence of the dielectric constant ?′ and dielectric loss ?″ over a wide range of frequencies of single crystals and polycrystalline samples of (NH₄)₂SO₄ in the region of the low temperature phase transition. A sharp increase was observed in the values of ?′ and ?″ at about ?50°C. In addition, a dielectric dispersion was detected and found to be more pronounced in the high temperature phase. This dispersion was attributed to piezoelectric resonance. The observed sudden increase in the values of the dielectric constant and dielectric loss below ? 50°C was attributed to the ferroelectric nature of the low temperature phase of (NH₄)₂SO₄.A DTA thermogram showed a sharp peak at ? 50°C which indicated that the phase transition is one of first order type. A TMA thermogram showed that this transformation was associated with a rapid increase in the expansion coefficient. Such an increase in the lattice parameter might be attributed to the enhanced rotation of electric dipoles associated with the distorted NH₄⁺ and SO₄^2? ions. The distortion of both the ammonium and sulfate ions in addition to their expected orientational motion are suggested to be responsible for the ferroelectric behaviour of ammonium sulfate below ?50°C.A transition to a metastable hexagonal state at about ?40°C is thought to occur, and this transformation is found to be irreversible.     

Raman scattering of high-temperature phase transition in KH2PO4

Raman scattering was applied to study the high-temperature phase transition (near 175°C) in KH₂PO₄. Drastic temperature-dependent changes were observed to take place in the normal modes of B₁ symmetry between 1000–3400 cm^?1. The disintegration of the dominant broad feature near 2500 cm^?1 when temperature rises beyond 150°C suggests that the alteration of the hydrogen-bond network is closely connected with this high-temperature phase transition.     

Analysis of phase transition behavior of BaCeO3 with thermal analyses and high temperature X-ray diffraction

Structural phase transitions in BaCeO₃ have been investigated with combination of differential scanning calorimetry (DSC), dilatometry and high temperature X-ray diffraction with high sensitivity and resolution. In DSC curve at heating procedures, baseline shift, endothermic peak and another baseline shift were observed at 260 °C, 385 °C and 895 °C, respectively. From DSC curve at cooling procedure, it was revealed that all the baseline shifts and peak were reversible. No hysteresis was observed in the both baseline shifts indicating second order phase transition at 260 °C and 895 °C with variation of specific heat capacity, ΔC_p, of 10 J/mol K and 7 J/mol K, respectively; whereas the order of the phase transition at 385 °C was revealed to be the first since hysteresis was detected around 370–385 °C. Variation of enthalpy, ΔH, at the phase transition was 45 J/mol. High temperature X-ray diffraction measurements have revealed that the crystal structure of BaCeO₃ changes from primitive orthorhombic perovskite through body-centered one, rhombohedral distorted one to cubic one around 280 °C, 400 °C and 900 °C, showing correspondence with DSC curves. Dependence of molar volume on temperature estimated from high temperature X-ray diffraction showed agreement with thermal expansion behavior observed with dilatometry.     

Two-Dimensional Correlation Analysis of Temperature-Dependent FT-IR Spectra of Oleic Acid

ABSTRACT

In the present study, variable temperature FT-IR spectroscopic investigations were used to characterize the spectral changes in oleic acid during heating oleic acid in the temperature range from ?30°C to 22°C. In order to extract more information about the spectral variations taking place during the phase transition process, 2D correlation spectroscopy (2DCOS) was employed for the stretching (C?O) and rocking (CH₂) band of oleic acid. However, the interpretation of these spectral variations in the FT-IR spectra is not straightforward, because the absorption bands are heavily overlapped and change due to two processes: recrystallization of the γ-phase and melting of the oleic acid. Furthermore, the solid phase transition from the γ- to the α-phase was also observed between ?4°C and ?2°C. Thus, for a more detailed 2DCOS analysis, we have split up the spectral data set in the subsets recorded between ?30°C to ?16°C, ?16°C to 10°C, and 10°C to 22°C. In the corresponding synchronous and asynchronous 2D correlation plots, absorption bands that are characteristic of the crystalline and amorphous regions of oleic acid were separated.     

A structural phase transition in squaric acid

Squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) is found to undergo a second order antiferrodistortive tetragonal to monoclinic phase transition at 97.7°C. The transition temperature for the fully deuterated compound is 243°C. The crystals are optically biaxial negative at room temperature, and the partial birefringence decreases with temperature as (n_z ? n_y)α(T_C ? T)^β, where β_H = 0.34 ± 0.02 and β_D = 0.37 ± 0.04.     

Determination of inelastic critical scattering at a first order phase transition in the CDW structure of 1TTaS2 using Mössbauer diffraction

We report Mössbauer diffraction measurements of the temperature dependence of the elastic and inelastic intensities at the (100) Bragg reflection in 1TTaS₂. These measurements use a newly developed microfoil conversion electron (MICE) spectrometry. They cover the temperature range from 19°C to 100°C, bracketing the first order 1T₁ to 1T₂ phase transition in the charge density wave structure at 79°C. The elastic Bragg peak shows a discontinuity at the phase transition as reported by Moret and Colella. The inelastic scattering shows a significant peak near the phase transition. This peak is interpreted as inelastic critical scattering at this first order phase transition.     

On the decomposition of the transition rhombohedral phase in the Al-30 wt.% Zn alloy

The decomposition of the transition rhombohedral α′_R-phase at the temperatures between 85 and 250 °C was investigated by transmission electron microscopy and by X-ray diffraction on single crystals. Below 161 °C the direct decomposition of α′_R-phase into the equilibrium hexagonal β-phase without the formation of the transition cubic α′-phase was found. Both transformation sequences α′_R → β and α/′_R→α′ → β were observed in the temperature range from 161 to 180 °C whereas only the previously known sequence α′_R → α′ → β was detected on ageing the alloy between 180 and 250 °C. The precipitation process at the temperatures from 161 to 180 °C is characterized by the decomposition of α′_R-phase into the equilibrium β-phase prior to the formation of the transition α′-phase and by the increased rate of decomposition of α′-precipitate. The observed transformation processes are related to the variation of strains at the partially coherent interface between α′_R-phase and α-matrix with the temperature in correlation with the metastable α′_R-phase boundary. These considerations allowed to estimate the relative stabilities of precipitated phases and their activation energies of formation and thus to discuss the decomposition mechanism of α′_R-phase at various temperatures.     

Martensitic transformation and soft modes in KAlF4

Abstract

The structural phase transition observed at ?13°C in KAlF₄ is shown to be martensitic. The results of an investigation of the phonon spectrum by inelastic neutron scattering are reported. It is shown that the transition is preceded by the softening of a flat phonon branch. A model is proposed to explain how such a softening is related to the martensitic transition.     

Differential scanning calorimetric investigations of naturally dehydrated aqueous sucrose solutions

Abstract

Differential scanning calorimetric investigations have been conducted on sucrose solutions for temperatures T<?100°C and in the sucrose concentration range 74 < c < 97. A novel method is adopted for sample preparation. For c > 92, the room temperature phase is amorphous, as indicated by its x-ray diffraction pattern. A melting transition is reported for the first time for these concentrations. A second-order phase transition is recorded for T < 0°C, which is clearly observed only during heating cycles, indicating a continuous phase transition during cooling. These transitions have not been reported so far, suggesting that the phase formed due to the procedure adopted for sample preparation may be different from that reported in the literature.     

Structural phase transitions in CdTiO3

The reconstructive phase transition from the ilmenite-like CdTiO₃ modification to the perovskite modification is investigated thoroughly. It is revealed that the reconstructive transition is determined by size effects, results in the formation of the closest packing of the ilmenite CdTiO₃ structure, and is irreversible with a decrease in temperature. The perovskite CdTiO₃ modification undergoes displacive structural phase transitions at temperatures of 110, 220, and 380°C. The first displacive phase transition is isostructural, whereas the second and third transitions are associated with rotations of oxygen octahedra.     

Study of electrical conductivity changes and phase transitions in Co3O4 doped ZrO2

The electrical conductivity of ZrO₂ doped with Co₃O₄ has been measured at various temperatures for different molar ratios. The conductivity increases due to the migration of vacancies created by doping. The conductivity is also found to increase with rise in temperature up to 120°C, and after attaining a maximum the conductivity decreases due to a collapse of the lattice framework. A second rise in conductivity around 460°C in all the compositions confirms the phase transition in ZrO₂ from monoclinic to tetragonal symmetry. X-ray powder diffraction and DTA studies were carried out for confirming the doping effects and the transition in ZrO₂.