Ferroelectric and ferroelastic phase transitions in (CH3NH3)3Sb2Br9 crystals

(CH₃NH₃)₃Sb₂Br₉ crystals were studied by pyroelectric method in the temperature range 10 - 200 K. Pyroelectric measurements revealed reversal spontaneous polarization below 141 K along the a-axis. The optical observations showed that MABA possesses ferroelastic domains in all low temperature phases.     

Preferred orientation of the grains in the ceramics of high Tc-superconductors

Highly grain oriented YBa₂Cu₃O₇ and GdBa₂Cu₃O₇ ceramics are prepared by using the controlled calcining and sintering conditions. T_c and ▵T_c on such ceramics were ∼94K and 2K respectively.     

高压下富氢高温超导体的新探索

实现室温超导是超导研究的终极目标.最近科学家在高压下富氢材料中发现了系列高温超导体,特别是首先被理论预测随后被实验证实的新型硫氢化物H3S和镧系氢化物LaH10,它们的超导转变温度均超过200 K.富氢材料已经被认为是室温超导体的最佳候选体系,成为物理、材料、超导等多学科的研究热点.综述了本课题组在此领域的主要成果,包...     

Dielectric and optical investigations of phase transition in TEASbCl4 crystal

Dielectric permittivity, linear birefringence measurement and polarizing microscope studies were performed for TEASbCl₄ single crystal. These investigations showed anomalies of permittivity and linear birefringence at T_c = 274 K and give evidence for the first-order structural phase transition at this temperature. No twinning (ferroelasticity) was observed below T_c.     

Dielectric anomalies related to domain walls in ferroelectric CsD2AsO4

Dielectric measurements of CsD₂AsO₄ confirm the observations of Sidorkin et al. [Sov. Phys. Sol. St. 26, 1910 (1984)] that a peak 15-25K below T_c is present and probably due to wall effects involving pinning by H-ions. However, our values for both T_c and the temperature T_W of the wall anomaly are ≈30k higher than those of the Soviet work. Our value of T_C = 224K agrees with that of Loiacono et al. [Ferroelectrics 14, 761 (1976)]. This higher transition temperature is probably due to a higher D-concentration (∼85%) than is present in the Soviet specimens.     

Crystallographic phase transition of NH4ZnF3 by single crystal XRD

Below T_c=115K, the Pm3m perovskite structure of NH₄ZnF₃ is deformed by two tilts of the coupled octahedra ZnF₆, about two of its tetrad axes. The first tilt of 2.8° is due to the condensation of a M₃ soft mode, while the second one of 1.4° arise from a R₂₅ mode.     

Microstructure Characterization of the (1−x)La2/3TiO3·xLaAlO3 System

Microstructural characterizations on the (1− x )La_2/3TiO₃· x LaAlO₃ (LTLA) system were conducted using transmission electron microscopy. The presence of La₂Ti₂O₇ and La₄Ti₉O₂₄ phases in pure La_2/3TiO₃ is confirmed by the electron diffraction pattern. When x = 0.1, the ordering due to the A-site vacancies could be confirmed by the presence of antiphase boundaries (APBs) and return ½(100) superlattice reflection. As x increases, the ordering decreases and finally disappears when x = 0.6. The tilting of oxygen octahedra could be demonstrated by the presence of the ferroelastic domains in the matrix and return ½(111) and return ½(110) superlattice reflections in selected area electron diffraction patterns. In pure LaAlO₃, only the antiphase tilting of oxygen octahedra is present due to the presence of return ½(111) superlattice reflection. In the LTLA system of x = 0.1, both the antiphase and in-phase tiltings of the oxygen octahedra are involved; however, in the range of x from 0.3 to 0.9, the antiphase tilting of oxygen octahedra has appeared. The growth of the ferroelastic domains is influenced by the APBs in the matrix.     

On the elfctrical and thermal properties of NaKSO4 crystals

The d.c. conductivity of NaKSO₄ crystal is investigated, along various crystallographic axes in the temperature range from 300 to 500 K. Two segments are observe in the temperature dependence of electric conductivity at T_c=453 K. The spontaneous strain η_1s, η_2s and η_3s, deduced from measurements of the linear thermal expansion coefficients along various crystallographic axes a, b and c showed linear temperature dependence below the transition temperature.     

Variation of LiTaO3 critical temperature with partial oxygen pressure

The variation of the critical temperature, T_c, of LiTaO₃ as a function of changing partial pressure of oxygen has been measured, from 1 to 10 kp/cm², following the changes in the dielectric constant peak in the vicinity of the transition, both with rising and lowering temperature. Between 1 and 3 kp/cm², T_c raises nearly 3°K and for higher partial oxygen pressures the increase slows down towards saturation. The possible interpretation of this behavior in terms of a decrease in the number of oxygen defective crystal unit cells as the oxygen pressure rises is discussed.     

Cation Ordering and Domain Boundaries in Ca[(Mg1/3Ta2/3)1−xTix]O3 Microwave Dielectric Ceramics

Cation ordering and domain boundaries in perovskite Ca[(Mg_1/3Ta_2/3)_{1− x} Ti _x ]O₃ ( x =0.1, 0.2, 0.3) microwave dielectric ceramics were investigated by high-resolution transmission electron microscopy (HRTEM) and Rietveld analysis. The variation of ordering structure with Ti substitution was revealed together with the formation mechanism of ordering domains. When x =0.1, the ceramics were composed of 1:2 and 1:1 ordered domains and a disordered matrix. The 1:2 cation ordering could still exist until x =0.2 but the 1:1 ordering disappeared. Neither 1:2 nor 1:1 cation ordering could exist at x =0.3. The space charge model was used to explain the cation ordering change from 1:2 to 1:1 and then to disorder. A comparison between the space charge model and random layer model was also conducted. HRTEM observations showed an antiphase boundary inclined to the (111) _c plane with a projected displacement vector in the 〈001〉 _c direction and ferroelastic domain boundaries parallel to the 〈100〉 _c direction.     

Microstructural Characteristics of Strontium Magnesium Niobate

Microstructural studies were conducted on the domain boundaries in Sr(Mg_1/3Nb_2/3)O₃ (SMN) complex perovskite compound using X-ray diffractometry and transmission electron microscopy. Both the 1:2 chemical ordering of B-site cations and the tilting of oxygen octahedra were involved in SMN. SMN had a 1:2 ordered monoclinic unit cell, which was distorted by the antiphase tilting of oxygen octahedra. Two types of domain boundaries were found: the antiphase boundaries (APBs), which are not confined crystallographically, and the ferroelastic domain boundaries, which were parallel to the crystallographic planes. SMN had the superlattice reflections of type ±⅙[111] and ±½[111] in the electron diffraction patterns, which cannot be indexed in terms of the 1:2 ordered trigonal phase with only a hexagonal unit cell. The presence of the ferroelastic domains that contained both the 1:2 ordered and the antiphase tilting had been verified by a high-resolution transmission electron microscopy lattice image. The structure of SMN was well explained by a model proposed by other researchers. The formation of the 1:2 ordered domains preceded the ferroelastic domains. Normally, the growth of the ferroelastic domain is not affected by APBs, but it is interrupted by them when the driving force for growth is insufficient, resulting in the stoppage of the domains at APBs.     

Thermal conductivity of Gd2(MoO4)3 crystal from 0.5 to 80 K

Thermal conductivity of the improper ferroelastic Gd₂(MoO₄)₃ crystal has been studied in the temperature range from 0.5 to 80 K by the steady-state heat-flow method. The ferroelastic domain walls were found to be strong centers of phonon scattering. At temperatures lower than 40 K, the thermal conductivity of the singledomain samples was by almost one order of magnitude greater than that of the multidomain ones.     

Stress–Strain Behavior During Compression of Polycrystalline La1−xCaxCoO3 Ceramics

Mechanical stress–strain behavior of LaCoO₃, La_0.8Ca_0.2CoO₃, and La_0.7Ca_0.3CoO₃ was studied under compression at 25° and 300°C. A hysteresis in the stress–strain relationship due to reorientation of ferroelastic domains (deformation twins) was observed, and a remanent strain is measured after unloading. The cohersive stress, defined as the maximum in effective elastic compliance during first loading, increases with substitution of Ca for La and decreases with increasing temperature. Domain reorientation was confirmed by X-ray diffraction of surfaces parallel and perpendicular to the loading direction. LaCoO₃ can be regarded as a soft ferroelastic material while the 30% Ca-substituted material is a hard ferroelastic. The hysteresis of the stress–strain relationship was clearly dependent on both composition and temperature.     

窄点温差及工质物性对跨临界有机朗肯循环性能的影响

利用约束热源入口及出口温度的热力学模型,将循环热效率及净输出功统一为一个参数,计算41种工质在473.15K废热烟气驱动的跨临界有机朗肯循环中的热力学表现,分析蒸发器内窄点温差及工质物性对循环性能的影响.结果表明,临界温度低于烟气出口温度的工质,及高于0.88倍烟气入口温度的工质,临界温度是循环效率的主要影响因素;临界温度在上述范围之间的工质,干湿性对循环效率影响显著,湿工质效率明显高于干工质.所有循环中,该临界温度范围内的湿工质热效率最高.临界温度高于0.88倍烟气入口温度的工质,窄点温差可能出现在蒸发过程中或蒸发器出口,从热力性能角度看,窄点出现在蒸发过程中的循环明显优于窄点出现在蒸发器出口的循环.改变热源入口及出口温度不会影响上述结论.     

Critical slowing down in betaine phosphate as studied by nuclear magnetic relaxation

Proton and phosphorus spin-lattice relaxation time (T₁) measurements were applied to study the antiferroelectric phase transition in betaine phosphate (CH₃)₃NCH₂COO·H₃PO₄ at 86 K. In the whole temperature range studied, the T₁ or ¹H was governed by a dipolar fluctuation caused by the methyl- and the whole butyl-group reorientations. In the vicinity of the phase transition the T₁ of ³¹P shows a sharp dip which is a characteristic of critical slowing down. The analysis of the T₁ based on the order-disorder mechanism with respect to the position of the acidic protons leads to the critical exponent for the correlation length v=0.5.     

Ferroelasticity in CSIO4

Optical examination of orthorhombic CsIO₄ crystals has revealed the existence of ferroelastic domains. That they are ferroelastic domains was confirmed by subjecting the crystal to external stresses. Our results strongly suggest that the transition at 150°C is of the species 4/mmmFmmm in Aim's terminology.     

Pyroelectric properties of valine doped TGS single crystals

TGS was grown by a slow cooling technique adding several kinds of dopants (10 and 20 mole% alanine, 10% alanine + LiVO₃, 10% and 20% valine) to an aqueous solution. The temperature dependent pyroelectric coefficient, p and the spontaneous polarization, P_s, of doped-TGS crystals were measured to study the transition temperature, P_max and room temperature pyroelectric coefficient depending on the dopants and to calculate the figure of merit, (p/K), (K is a dielectric constant). Furthermore, several pieces were selected to investigate the positional inhomogeneities of the same sample. Moderate figure of merit (p/K) was obtained in valine doped TGS because of low temperature dielectric relaxation around -10°C, although valine doped TGS has a three times higher pyroelectric coefficient (p = 1.671μC/cm²K) at transition temperature (T_c) than that of alanine doped TGS.     

Observation of the ferroelastic domain structure near the phase transition point

Temperature and time dependences of the ferroelastic domain structure were observed near the ferroelastic phase transition temperature in [N(CH₃)₄]₂ZnBr₄ single crystal. It was, found that the domain structure reaches a metastable state within about 20sec after quenching. The domain size increases with decreasing temperature.     

Relaxational dielectric anomaly in the ferroelectric phase of K2ZnCl4

The temperature dependence of dielectric constant of K₂ZnCl₄ was measured at 10, 20, 40 and 100 kHz in the temperature range between 120 and 250 K. A relaxational dielectric anomaly was observed near 210 K for the first time. It is ascribed to domain wall pinning induced by ferroelastic micro-domains which begin to form around 200 K and behave as defects. The dielectric anomaly disappeared after several thermal cycles of heating and cooling. This may be due to the remnant ferroelastic matrix in the ferroelectric phase.     

Dielectric discontinuities and multipolar interactions in ferroelectric perovskites

Ferroelectric perovskites, in particular KNbO₃, PbTiO₃ and BaTiO₃, are characterized by discontinuities in the dielectric constant at the tetragonal-cubic transition (T*) as well as by the abrupt disappearance of the spontaneous polarization which goes from a minimum finite value P_s* =P_s (T) to zero at T=T*. It is shown that, taking into account quadrupolar and higher order terms into the effective field acting on the elementary dipoles lattice, good fits to the observed P_s(T) in a wide temperature range below T* and to the inverse dielectric constant discontinuity (C/T_c)[ε₊^-1(T*)-ε_-^-1(T*)] at T⋍T* are obtained for these three perovskites.