Thermal expansion of the Sn2P2(Se0.28S0.72)6 solid solutions

In the present work, we have experimentally studied and analysed the dilatometric properties of Sn₂P₂(Se_0.28S_0.72)₆ crystals during ferroelectric phase transition. A complete matrix of thermal expansion tensors as well as the characteristic surface of the thermal expansion tensor has been obtained. The critical exponent for the volume thermal expansion coefficient in Sn₂P₂(Se_0.28S_0.72)₆ crystals has been determined to be equal to 0.50 ± 0.06, which is peculiar to tricritical behaviour. Non-agreement in the values of the critical exponents evaluated from the temperature dependencies of relative elongation (0.17 ± 0.01) and volume thermal expansion coefficient (0.50 ± 0.06) is shown to be due to the possible temperature rotation of the spontaneous polarisation vector.     

Critical behaviour of Sn2P2S6 and Sn2P2(Se0.28S0.72)6 crystals under high hydrostatic pressures

Basing on the temperature dependences of optical birefringence for Sn₂P₂S₆ and Sn₂P₂(Se_0.28S_0.72)₆ crystals subjected to hydrostatic pressures, we prove unambiguously that Sn₂P₂S₆ reveals a tricritical point on its (p, T)-phase diagram with the coordinates (p, T) = (4.3 kbar, 259 K), so that the second-order phase transition transforms into the first-order one whenever the pressure increases above 4.3 kbar. We also find that increasing hydrostatic pressure applied to Sn₂P₂(Se_0.28S_0.72)₆ leads to the change in the phase transition character from tricritical to first order. Further increase in the pressure up to ~2.5 kbar imposes splitting of the first-order paraelectric-to-ferroelectric phase transition into two phase transitions, a second-order paraelectric-to-incommensurate one and a first-order incommensurate-to-ferroelectric transition.     

Influence of the sample orientation in Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript> crystals on the hydrostatic piezoelectric coefficients

The influence of the sample orientation on the effective value of the hydrostatic piezoelectric coefficients d _h ⁽ⁱ⁾ of Sn₂P₂S₆ crystals has been studied. The hydrostatic piezoelectric coefficients d _h ⁽¹⁾ and d′ _h ⁽³⁾ , were measured, d _h ⁽¹⁾ =(244±3) pC/N and d′ _h ⁽³⁾ =(92±1) pC/N. The hydrostatic piezoelectric coefficient d _h ⁽³⁾ for orthogonal axis system was calculated to be d _h ⁽³⁾ =(87±2) pC/N. The, optimal orientation of the sample has been found as (Xy l)−20°-cut. Maximal value of the effective hydrostatic piezoelectric coefficient d _h ⁽¹⁾ equals 260 pC/N. Double rotated samples were also studied. The orientation of the samples insensitive to the pressure has been found. The theoretical mean value of hydrostatic piezoelectric coefficient (d _h ) _mean corresponding to randomly oriented Sn₂P₂S₆ grains in a poled composite has been calculated to be (d _h ) _mean =136 pC/N.     

Role of the coupling between strains and order parameter gradient in crystals

The manifestation and the role of coupling between order parameter gradient and strains, or equivalently between order parameter and gradient of strains, is reviewed. Dynamical phenomena occurring in perovskites, and in particular in quantum paraelectric phases, evidenced as a coupling between phonon branches at q≠0, and also in several other systems (quartz, Sn₂P₂Se₆, Cs₂HgBr₄, and other systems satisfying the Lifshitz criterion) in relation with the formation of an incommensurate phase, are presented. Besides, the consequences in relation with domain walls, structure of incommensurate phases, and defects are also discussed.     

Anisotropic thermal expansion of AEFe2As2 (AE = Ba,Sr, Ca) single crystals

We report anisotropic thermal expansion of the parent, AEFe₂As₂ (AE = Ba, Sr, and Ca), compounds. Above the structural/antiferromagnetic phase transition anisotropy of the thermal expansion coefficients is observed, with the coefficient along the a-axis being significantly smaller than the coefficient for the c-axis. The high temperature (200 K ≤ T ≤ 300 K) coefficients themselves have similar values for the compounds studied. The sharp anomalies associated with the structural/antiferromagnetic phase transitions are clearly seen in the thermal expansion measurements. For all three pure compounds, the ‘average’ a-value increases and the c-lattice parameter decreases on warming through the transition, with the smallest change in the lattice parameters observed for SrFe₂As₂. The data are in general agreement with the literature data from X-ray and neutron diffraction experiments.     

Electron spin resonance study of a CuIr2S4 single crystal

A spinel sulphide CuIr₂S₄ single crystal, which exhibits an orbitally induced Peierls phase transition at ～230?K, is investigated by electron spin resonance (ESR) spectroscopy. The phase transition is clearly manifested on the ESR spectra. It is suggested that the ESR signals are produced by a few non-dimerized Ir⁴⁺ ions. Moreover, an extra ESR spectrum appears at low temperature in addition to the paramagnetic ESR signals of Ir⁴⁺ ions, which is suggested to be caused by the Jahn–Teller effect of the non-dimerized Ir⁴⁺ ions. From the ESR results, it is found that the Jahn–Teller splitting energy ΔE _JT is much smaller than the spin-dimerization gap.     

HMX不同晶型热膨胀特性及相变的ReaxFF分子动力学模拟

HMX（octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine）晶体不同晶型对温度的响应对于深入认识含能材料在外加载荷下的感度、稳定性、相变等具有重要意义．采用ReaxFF—lg势函数和等温等压分子动力学（NPT-MD）方法,研究了β-,δ-和α-HMX三种固态晶型在T=303—503K内的晶体结构和分子结构．结果表明,ReaxFF—lg势函数能够合理地描述HMX晶体的热膨胀行为,计算得到的晶体结构和热膨胀系数与实验数据比较符合．三种晶型的线膨胀系数表明,β-HMX晶体具有明显的热膨胀各向异性;δ-HMX晶体C轴的热膨胀特性与a,b轴略有不同;α-HMX晶体三个方向的热膨胀特性基本相同．三种晶型的体膨胀系数为8-HMX〉α-HMX〉β-HMX,表明δ-HMX晶体对温度最敏感,这可能是δ-HMX晶体具有更高感度的原因．在T=443K时,β-HMX晶体的晶胞参数突变,且部分分子构型由“椅式”转变为“船式-椅式”．采用两相热力学模型（2PT）得到的亥姆赫兹自由能表明,β—HMX晶体在T=423—443K时发生相变．β-,δ-和α-HMX晶体分别在T=303—423K,T=443—503K和T=363-423K内稳定存在．     

A genetic approach to the dielectric investigation of the high-temperature phase transition of TEA2MnCl4 crystals

The dielectric function of TEA₂MnCl₄ crystals is studied for a temperature range of 203–263?K in a frequency range of 1 kHz to 10?MHz. The measurement results are processed in the framework of the Cole–Cole model. An influence of the phase transition on the results is widely discussed. A genetic approach is successfully applied to the measured data and it is explained in detail. The method renders it possible to calculate the temperature dependence of dielectric properties in the time domain. The calculation method (the genetic algorithm) proved to be trustworthy and it is recommended to be employed in studying dielectric properties. A structural properties of the high temperature phase (i.e. above the phase transition temperature) will be also discussed and certain presumptions on the low temperature phase will be given.     

Electrical conductivity in undoped and Mn2+-doped NaNO2 single crystals

Electrical conductivity studies in NaNO₂ single crystals with inherent impurities and also in crystals with added Mn²⁺ impurities have been reported. The heating conductivity runs of undoped and doped NaNO₂ crystals have been compared. The decrease in conductivity in cooling following a heating run has been attributed to the oxidation during heating leading to the bulk precipitation of impurities in the host. Above 170°C however the intrinsic defects are responsible for conduction. An anomaly is noticed in both the heating and cooling conductivity runs of the sample at about the Curie temperatures and has been found to show thermal hysteresis.     

Metal insulator transition characteristics of macro-size single domain VO2 crystals

The metal insulator transition (MIT) characteristics of macro-size single-domain VO₂ crystal were investigated. At the MIT, the VO₂ crystal exhibited a rectangular shape hysteresis curve, a large change in resistance between the insulating and the metallic phases, in the order of ～10⁵, and a small transition width (i.e. temperature difference before and after MIT) as small as 10^?3°C. These MIT characteristics of the VO₂ crystals are discussed in terms of phase boundary motion and the possibility of controlling the speed of the phase boundary, with change in size of crystal, is suggested.     

On the special points on the (P,T, x)-phase diagram for Sn2P2(Se x S1− x )6 crystals: acoustic studies of Sn2P2(Se0.28S0.72)6

On the basis of temperature dependences of the acoustic wave velocity and the acoustic attenuation in Sn₂P₂(Se_0.28S_0.72)₆ crystals under normal conditions and hydrostatic pressures, we show that increasing pressure causes these crystals to move away from the tricritical point and enter into the region of first-order phase transitions (PTs) in (P,?T,?x)-space. Still higher hydrostatic pressures lead to an increase in the attenuation of the acoustic wave with the velocity v ₂₂ and broadening of its anomaly in the PT region, thus hinting at splitting of the PT, with the appearance of an intermediate incommensurate phase approximately at P?=?3.3?kbar. We offer a (P,?T,?x)-phase diagram for the solid solutions Sn₂P₂(Se _x S_{1? x} )₆, which includes two special lines of PTs, tricritical and triple ones, which can intersect at the Lifshitz point at negative hydrostatic pressures. The variations of the surface of acoustic wave velocities occurring with changing temperature are obtained for Sn₂P₂(Se_0.28S_0.72)₆ crystals under the atmospheric pressure.     

The thermal metal-insulator phase transition in (EDO-TTF)2PF6

ABSTRACT

The thermal metal-insulator phase transition in the π-stacked (EDO-TTF)₂PF₆ charge transfer salt is of the Peierls type. It is related to geometrical reorganisations and charge ordering phenomena. We report that dimerising displacements are involved in the mechanism of this transition. By using periodic quantum chemical calculations, we find a double well potential in which dimerisation and charge localisation become manifest. By analysing the nuclear wavefunctions we discuss the mechanism of the phase transition in terms of thermal fluctuations.     

Influence of crystal synthesis on the incommensurate phase and lock-in transition of Rb2ZnCl4 crystals

A comparative study of several crystals of Rb₂ZnCl₄, obtained by different crystal growing methods, has allowed us to determine the influence of growth defects on the incommensurate phase and on the lock-in transition of these samples. X-ray diffraction has allowed us to complete previous dielectric measurements realized on the same samples and to relate the crystalline quality to the evolution of the modulation as a function of the temperature. The principal influence of an increasing defect density seems to be a stronger pinning of the modulated phase and this induces a lower lock-in temperature and a wider hysteresis.     

γ-LiAlO2晶体生长、改性和热学性质研究

采用快速提拉法生长出了透明、完整的γ-LiAlO₂晶体，但是晶体的高熔点和易 挥发性限制了γ-LiAlO₂晶体质量.采用气相传输平衡法（vapor transport equilibra tion technique,VTE）工艺对晶体改性，半高宽（FWHM）值从1169arcsec降至442arcsec，继续升高VTE处理温度至1300℃，FWHM值反而升高至552arcsec.快速提拉法生长出来晶体，100］方向和［001］方向的热膨胀系数分别为172398×10^-6/K，107 664×10^-6/K.经过三步VTE处理后［100］和［001］方向热膨胀系数降至16 6539×10^-6/K和101784×10^-6/K. 关键词： 2')" href="#">γ-LiAlO₂ 气相传输平衡法 热膨胀系数     

Comparison of thermomagnetic history effects in weakly pinned single crystals of R 3Rh4Sn13 (R=Yb, Ca)

A comparative study of the thermomagnetic memory effects of J _c in two weakly pinned low T _c superconductors, Ca₃Rh₄Sn₁₃ (CaRhSn) and Yb₃Rh₄Sn₁₃ (YbRhSn), is presented. In both the systems, the peak effect (PE) phenomenon appears as an order-disorder transformation through stepwise amorphization of the flux line lattice (FLL). However, in CaRhSn. we can witness another disorder-driven transition (Bragg glass (BG) to a vortex glass (VG) in a distinct manner as in a single crystal of high T _c YBa₂Cu₃O_7-δ for H‖c.     

Optical properties of Mn-activated Na2SnF6 and Cs2SnF6 red phosphors

Alkaline hexafluorostantanate red phosphors Na₂SnF₆:Mn⁴⁺ and Cs₂SnF₆:Mn⁴⁺ are synthesized by chemical reaction in HF/NaMnO₄ (CsMnO₄)/H₂O₂/H₂O mixed solutions immersed with tin metal. X-ray diffraction patterns suggest that the synthesized phosphors have a tetragonal symmetry with the space group D_4h¹⁴ (Na₂SnF₆:Mn⁴⁺) and a trigonal symmetry with the space group D_3d³ (Cs₂SnF₆:Mn⁴⁺). Photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and the Raman scattering techniques are used to investigate the optical properties of the phosphors. The Franck-Condon analysis of the PLE data yields the Mn⁴⁺-related optical transitions to occur at ∼2.39 and ∼2.38 eV (⁴A_2g→⁴T_2g) and at ∼2.83 and ∼2.76 eV (⁴A_2g→⁴T_1g) for Na₂SnF₆:Mn⁴⁺ and Cs₂SnF₆:Mn⁴⁺, respectively. The crystal field parameters (Dq) of the Mn⁴⁺ ions in the Na₂SnF₆ and Cs₂SnF₆ hosts are determined to be ∼1930 and ∼1920 cm⁻¹, respectively. Temperature-dependent PL measurements are performed from 20 to 440 K in steps of 10 K, and the obtained results are interpreted by taking into account the Bose-Einstein occupation factor. Comprehensive discussion is given on the phosphorescent properties of a family of Mn⁴⁺-activated alkaline hexafluoride salts.     

Thermal infrared cross-sections of C2F6 at atmospheric temperatures

Spectral absorption cross-sections, k_ν (), have been measured in the 8.0 and bands of C₂F₆. Temperature and total (N₂-broadening) pressure have been varied to represent the conditions specified in various models of the terrestrial atmosphere so that the absorption cross-sections can be applied directly to the optical remote-sensing of C₂F₆ in the atmosphere. The measured absolute intensities of the 8.0 and the bands are (1.636±0.003)×10⁻¹⁶ and (, respectively.     

Raman study of KMnF3 perovskite crystals doped by Na+

A study of the influence of cationic Na⁺ substitution in the archetype KMnF₃ perovskite crystal was performed using the Raman method. The Raman spectra of mixed K_1-xNa_xMnF₃ crystals with x = 0.029, 0.048 and 0.065 were recorded versus temperature and fully interpreted in terms of a “one mode” behaviour. In addition to the soft mode not completely vanishing close to Tc, attention was especially paid to evidence of static and dynamical disorder. From this point of view the behaviour of the hard Raman modes versus temperature has been studied together with two unexpected Raman bands in the cubic phase. The interpretation has been made within the more general framework of structural disorder existing in such perovskites with anisotropic interactions.