Ferroelectric phase transition in SbSI type crystals

In this work Lorenz field coefficients F for SbSI type crystals are estimated. Also transverse and longitudinal frequencies (ω _T and ω _l) of optical vibrations are determined. From a study of these quantities it was concluded that a combination of F is an inadequate condition for ferroelectric phase transitions to take place. Taking the electronic structure, chemical bond model, electronic potential and condition ω _T—>0 into account, the phase transition in SbSI type crystals was investigated. The dependence of the electronic potential upon the composition of V-VI-VII crystals and mixed crystals has revealed the factors that change the phase transition temperature T _c and the dynamics of the soft mode.     

Uniaxial pressure influence on dielectric properties of Pb(Zr0.99Ti0.01)O3 single crystals

The effect of uniaxial pressure (0–1000 bars) applied parallel to AC electric field on dielectric properties of Pb(Zr_0.99Ti_0.01)O₃ single crystals has been investigated. It was found that uniaxial pressure significantly influences these properties. With increasing pressure: (i) peak ?(T) decreases, becomes diffuse and shifts to higher temperature, (ii) the thermal hysteresis is reduced, (iii) the hump in ?(T) profile connected with antiferroelectric–intermediate phase transition vanishes, and (iv) the local anomaly in ?(T) profile connected with polar microregions existence above T_m is protruded and slightly shifts towards higher temperature. It was concluded, that applied uniaxial pressure or increasing Ti-ions content in lead zirconate titanate system induces similar effects. The results were discussed in terms of domain switching under pressure.     

Thermopower of beech wood biocarbon

This paper reports on measurements of the thermopower S of high-porosity samples of beech wood biocarbon with micron-sized sap pores aligned with the tree growth direction. The measurements have been performed in the temperature range 5–300 K. The samples have been fabricated by pyrolysis of beech wood in an argon flow at different carbonization temperatures (T _carb). The thermopower S has been measured both along and across the sap pores, thus offering a possibility of assessing its anisotropy. The curves S(T _carb) have revealed a noticeable increase of S for T _carb < 1000°C for all the measurement temperatures. This finding fits to the published data obtained for other physical parameters, including the electrical conductivity of these biocarbons, which suggests that for T _carb ∼ 1000°C they undergo a phase transition of the insulator-(at T _carb < 1000°C)-metal-(at T _carb > 1000°C) type. The existence of this transition is attested also by the character of the temperature dependences S(T) of beech wood biocarbon samples prepared at T _carb above and below 1000°C.     

3D dependence of the dielectric dispersion in a BaTiO3 single crystal

The 3D dependences ?′(log f, T) and tanδ(logf, T) of a perfect BaTiO₃ single crystal grown by the Remeika method have been studied in the ranges f = 1–2 × 10⁷ Hz and T = ?80–130°C. These dependences characterize a transition from the paraelectric phase (121.5°C) as a near-antiferroelectric transition followed by the transition to the tetragonal phase at ～79.5°C. According to a number of signs, the range 121.5–79.5°C corresponds to a metastable phase typical of first-order phase transitions. The unexpected result of this work has been discussed with invoking the hypothesis on the BaTiO₃ structure in the paraelectric phase, according to which it consists of three antiferroelectric states oriented along the crystallographic axes. Using the dielectric properties of BaTiO₃ as an example, the method of direct correct determination of the temperatures of the structural transformations from the anomaly of tanδ(logf, T) has also been demonstrated.     

Effect of regimes of cooling in a magnetic field on the magnetization of a La0.9Sr0.1MnO3 single crystal

Magnetization measurements were performed on a lanthanum manganite La_0.9Sr_0.1MnO₃ single crystal in the temperature interval 4.2–300 K and magnetic field interval 50 Oe-55 kOe in two sample cooling regimes: 1) cooling down to 4.2 K in a high (55 kOe) magnetic field, and 2) cooling in a “zero” field. It is shown that the temperature dependences of the magnetization M(T) are substantially different in these regimes. Pronounced anomalies of M(T) were observed at temperatures T*=103 K and T _c =145 K. The first anomaly is attributed to a structural transition, while the second one corresponds to a ferromagnet-paramagnet phase transition. The magnetization of a La_0.9Sr_0.1MnO₃ single crystal in the cooling regimes studied shows typical “spin-glass” behavior. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 39–43 (10 July 1998)     

Piezoelectric activity and phase transitions in (PMN)0.69(PT)0.31 single crystal

Piezoelectric and dielectric investigations have been performed on a (PMN)_0.69(PT)_0.31 single crystal. Low frequency (100?Hz) dielectric permittivity measurements revealed distinct anomaly at 129°C (T _εmax) corresponding to the structural transformation from the tetragonal to cubic phase. Two other anomalies have been detected at 90 and 96°C. After preliminary polarization in the d.c. electric field, switched on above T _εmax and switched off inside the tetragonal phase, the piezoelectric activity has been observed in function of temperature. Values of the piezoelectric resonance frequencies changed markedly at 96°C (on cooling) and 124°C (on heating) showing clear softening of the elastic properties near these temperatures. Values of the piezoelectric and electromechanical coupling coefficients obtained were respectively of the order of 800?pCN^?1 (d ₃₁) and 0.35?(k ₃₁). Piezoelectric activity was detected tens of degrees above the temperature T _εmax and disappeared at temperature at which the dispersion of the dielectric permittivity due to the presence of polar nanoregions is negligible. It was found that strong softening of the elastic properties accompanies phase transitions to the tetragonal and monoclinic phase.     

Effect of external pressure on Tc of as‐grown and thermally treated superconducting Rbx Fe2–ySe2 single crystals

We report on the effect of external pressure on the superconducting transition temperature (T_c) of as‐grown and thermally treated single crystals of superconducting iron chalcogenide Rb_0.85Fe_1.9Se₂. The superconducting transition temperature of 27.1 K at ambient pressure for the as‐grown sample was found to increase up to 33.2 K for the sample annealed for 3 h at 215 °C in vacuum. An increase of T_c up to 28.2 K was observed for the as‐grown sample at a pressure of 0.83 GPa. For all the studied crystals, annealed in the temperature range between 215 °C and 290 °C, the external pressure seems to decrease the superconducting transition temperature and a negative pressure coefficient of T_c was observed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase transitions in SbSI

In ferroelectric SbSI the phase transitions are studied by means of precise birefringence measurement. It is observed that the transition from D^162h phase to C⁹_2v phase occurs at 15.70°C. At ? 36.61°C, the second order transition from C⁹_2v phase to C¹_s or C²_s phase is observed. From the behavior of birefringence near the second order transition point, the mechanism of the phase transition in SbSI is considered.     

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.     

Variety of LaSrMnO structures induced by growth conditions and laser irradiation

Crystallographic phase transitions in perovskite-like LaSrMnO metallic oxides are studied. The transitions are induced when internal stresses generated during film synthesis (at temperatures between 450 and 730°C) vary (decrease or increase) upon subsequent irradiation by a KrF laser emitting in the UV range. As the synthesis temperature T _s grows, the rhombohedral-to-orthorhombic phase transition occurs at 650–670°C. The resistivity is shown to be either temperature-independent, ρ(T)=const, at T<T _crit, or varies and reaches a maximum, ρ(T)=ρ_max, at the Curie temperature T _c. Optical transmission spectra taken at photon energies ℏω=0.5–2.5 eV exhibit both a high (0.8–0.9) and low (0.1–0.3) transmission coefficient t, depending on the synthesis temperature. As follows from X-ray diffraction data, the laser irradiation causes a phase transition only in LaSrMnO films grown at T _s<650°C. Phases of different size scales appear: the long-range-order orthorhombic matrix and mesoscopic-range-order rhombohedral clusters are observed in the films grown at T _s=450–550°C and the rhombohedral matrix with orthorhombic clusters, in the films grown at T _s=550–650°C.     

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.     

Dynamical studies of highly deuterated betaine phosphate near the antiferroelectric phase transition

The dielectric response and the Raman spectra of single crystals of deuterated betaine phosphate are studied around the antiferroelectric phase transition. The dielectric data between 10 MHz and 11 GHz can be explained on the basis of a simple Debye-relaxation with a critical slowing-down of the relaxation rate on approachingT _C . Using the Cole-Davidson form of the dielectric function we succeeded in fitting the data in the whole frequency range from 10 MHz to 11 GHz and from 64–400 GHz over a temperature range from 145–280 K. Raman spectra clearly indicate that the doubling of the unit cell does not take place at the antiferroelectric transition temperature, but some degrees below.     

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.     

Phase transitions in CuBiP2Se6 crystals

Investigation results of dielectric (20?Hz–1?MHz) properties of layered CuBiP₂Se₆ crystals are presented. The temperature dependence of the static dielectric permittivity reveals the first-order “displacive” antiferroelectric phase transition at T _c?=?136?K. In the paraelectric phase, at low frequencies, dielectric spectra are highly influenced by the high ionic conductivity with the activation energy of 2473?K (0.21?eV). In the antiferroelectric phase the electrical conductivity and its activation energy (531.1?K (0.045?eV)) are considerably smaller. At low temperatures, the temperature behaviour of the distribution of relaxation times reveals complex freezing phenomena. A part of long relaxation time distribution is strongly affected by external direct current (DC) electric field and it is obviously caused by antiferroelectric domain dynamics.     

Internal friction in directed-crystallization (Cu-Sn)-Nb alloys

The distinctive features of the low-frequency internal friction Q ⁻¹(T) of (Cu-Sn)-Nb composites at high temperatures (up to 400°C) are investigated for strains in the range 10⁻⁵–10⁻⁴. Considerable hysteresis of Q ⁻¹(T) in the heating-cooling cycle is recorded, including the presence of a minimum at ∼175°C when the sample is heated to 400°C and two peaks P ₂ (at 280°C) and P ₁ (at ∼100°C) when the sample is cooled from 400°C. The activation energy of the anomalous internal friction background (up to 175°C), the oxygen diffusion parameters, and the oxygen concentration in the niobium fibers (all of which govern the peak P ₂) are calculated, and the value and temperature dependence of the yield point of the bronze matrix (which govern the peak P ₁) are estimated. Zh. Tekh. Fiz. 68, 114–117 (November 1998)     

Effect of the annealing temperature of thin Hf0.3Zr0.7O2 films on their energy storage behavior

With increasing annealing temperature (T_anneal), the magnitude of the electric fields for the antiferroelectric‐to‐ferro‐electric (E_AF) and ferroelectric‐to‐antiferroelectric (E_FA) transition of a 9.2 nm thick Hf_0.3Zr_0.7O₂ film decreased. The energy storage densities of the Hf_0.3Zr_0.7O₂ films crystallized at 400 °C, 500 °C, and 600 °C were as large as 42.2 J/cm³, 40.4 J/cm³, and 28.3 J/cm³, respectively, at the electric field of 4.35 MV/cm. The maximum dielectric constant of the Hf_0.3Zr_0.7O₂ film crystallized at 600 °C was the largest (～46) as it had the smallest E_AF and E_FA, whereas the leakage current density of the film crystallized at 400 °C was the smallest. The 400 °C of T_anneal was the optimum condition for energy storage application. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Structure-mediated transition in the behavior of elastic and inelastic properties of beach tree bio-carbon

Microstructural characteristics and amplitude dependences of the Young modulus E and of internal friction (logarithmic decrement δ) of bio-carbon matrices prepared from beech tree wood at different carbonization temperatures T _carb ranging from 600 to 1600°C have been studied. The dependences E(T _carb) and δ(T _carb) thus obtained revealed two linear regions of increase of the Young modulus and of decrease of the decrement with increasing carbonization temperature, namely, ΔE ～ AΔT _carb and Δδ ～ BΔT _carb, with A ≈ 13.4 MPa/K and B ≈ ?2.2 × 10^?6 K^?1 for T _carb < 1000°C and A ≈ 2.5 MPa/K and B ≈ ?3.0 × 10^?7 K^?1 for T _carb > 1000°C. The transition observed in the behavior of E(T _carb) and δ(T _carb) at T _carb = 900–1000°C can be assigned to a change of sample microstructure, more specifically, a change in the ratio of the fractions of the amorphous matrix and of the nanocrystalline phase. For T _carb < 1000°C, the elastic properties are governed primarily by the amorphous matrix, whereas for T _carb > 1000°C the nanocrystalline phase plays the dominant part. The structurally induced transition in the behavior of the elastic and microplastic characteristics at a temperature close to 1000°C correlates with the variation of the physical properties, such as electrical conductivity, thermal conductivity, and thermopower, reported in the literature.     

Search for the NN → 6q phase transition in the np-polarized measurements at T kin = 1–6 GeV

The measurements of np-spin observables at 0° have been performed for the first time on the Delta-Sigma experimental facility of LHE JINR up to P _n = 4.5 GeV/c using the monochromatic neutron beam. They include detailed measurements of the Δσ _L(np) spin differences and the study of the np → pn elastic charge-exchange process. In the Δσ _L(np) and −Δσ _L(I = 0) energy dependencies over the energy region T_kin = 1.2–3.7 GeV the peculiarity at 1.8 GeV was observed. Such energy behavior was predicted by the QCD approach as a signal of the NN → 6q phase transition. For the exhaustive investigation of this effect it is necessary to measure the energy dependence of the complete set of np observables with both longitudinal (L) and transverse (T) polarizations of the neutron beam and proton target. This will allow Direct Reconstruction of all three NN forward Scattering Amplitudes (DRSA) to be performed, and the observed peculiarity to be checked around T_kin = 1.8 GeV and at the higher energies using the Argand diagrams method.