Superionic phase transition in disordered sublattice on the ferroelectric solid solution

The spontaneous formation of the dipole defects as the evidence of its coperative behaviour in paraelectric crystal is discussed.     

Carbon concentration dependence of the superconducting transition temperature and structure of MgCxNi3

The crystal structure of the superconductor MgC_xNi₃ is reported as a function of carbon concentration determined by powder neutron diffraction. The single-phase perovskite structure was found in only a narrow range of carbon content, 0.88<x<1.0. The superconducting transition temperature was found to decrease systematically with decreasing carbon concentration. The introduction of carbon vacancies has a significant effect on the positions of the Ni atoms. No evidence for long-range magnetic ordering was seen by neutron diffraction for carbon stoichiometries within the perovskite phase stability range.     

Preparation of solid solutions of deuterated triglycine sulphate and concentration dependence of some of their ferroelectric properties

A number of ferroelectric solid solutions (N[H, D]₂CH₂COO[H, D])₃[H, D]₂SO₄ have been prepared in the concentration range of the initial components of 0 to 94·2%. Moreover, the parameters of the Curie-Weiss law have been measured as a function of the concentration of the initial components. The course of the phase transition position and of the Curie constant as a function of the concentration is discussed by means of Janovec's thermodynamic theory of ferroelectric solid solutions. Mixed crystals cannot be classified as perfect solid solutions, but their behaviour may be expressed by regular approximation.     

Selectivity and temperature dependence of phase and phase transition in diblock copolymer solution

In order to study the effects of solvent selectivity and temperature on phase behavior and transition of diblock copolymer solution, self-consistent field theory is modified to incorporate the short-range interaction and non-local effects. Inhomogeneous free-energy density is shown to be dependent on solvent selectivity, temperature and copolymer concentration. Enthalpic quantity and entropic contributions are crucial to phase diagrams of diblock copolymer solution. Three selective strengths of solvent --weak, moderate and strong-- are chosen for comparison. For a weakly selective solvent, theoretical and experimental results illustrate the same variation tendency in the phase boundary of the order-disorder transition for a symmetric diblock of polystyrene and polyisoprene. Self-consistent field equations can be used to calculate the exact FCC-BCC structural phase transition temperatures in moderately and strongly selective solvents. Detailed comparison with the experimental phase diagrams including lamellar, cylindrical and spherical structures is presented.     

On the transition temperature of ferroelectric and the Blume-Hubbard scheme

A way to derive upper and lower bounds for the transition temperature of a general system is introduced and applied to the transverse Ising model using the Blume-Hubbard decoupling scheme but without invoking the fluctuation-dissipation theorem.     

BST solid solutions, temperature evolution of the ferroelectric transitions

Solid solutions Ba_1 − xSr_xTiO₃ (BST) are of high technological importance, particularly in microwave domain. Barium titanate has “naturally” three transitions, between four stable ferroelectric phases: (C) cubic, (T) tetragonal, (O) orthorhombic, (R) rhombohedral. Jaffe et al. [B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, 1971] has given the dependence of the transition temperatures up to 30% of Sr content. We have extrapolated these temperatures and we have found that some phases might disappear at higher Sr concentrations. A family of solid solutions with x = 25, 50, 75, 90% was prepared by standard solid-state reaction and sintered at 1230 and 1260 °C, respectively. The permitivities and the dielectric losses were measured with a self-acting bridge (1 kHz), on a large temperature range (±200 °C). The composition x = 25% shows three peak values of permittivity as expected, while the composition x = 50%, only two peak values, corresponding to phase transitions cubic-tetragonal-rhombohedral, phase orthorhombic being excluded. Compositions with x ≥ 63%, Sr shows only one peak value corresponding to a genuine transition cubic-rhombohedral. The cubic transition to several lower phases shows almost a linear decrease of the Curie point with the increase of Sr fraction. For Sr concentration x ≥ 80%, the Curie point appears to fall more rapidly than linear. To our best knowledge, there is for the first time, this effect is reported.     

The temperature dependence of the deformation strength of solid Cu-Al solutions

The temperature dependence of the deformation strength, of the parameters ₀ and K of Pötsch's equation, and of the discontinuous deformation in solid Cu-Al solutions of various concentrations have been studied. It was shown that the temperature dependence of and ₀ changes substantially when the alloy concentration is increased (under otherwise equal conditions). The nature of the processes which lead to the disappearance of the deformation discontinuity and to a sharp high-temperature recrystallization by process annealing is discussed.     

Size dependence of phase transition temperatures of ferromagnetic, ferroelectric and superconductive nanocrystals

With the miniaturization of devices, size and interface effects become increasingly important for the properties and performances of nanomaterials. Here, we present a thermodynamic approach to the mechanism behind size-induced unusual behavior in the phase stabilities of ferromagnetic (FM), antiferromagnetic (AFM), ferroelectric (FE), and superconductive (SC) nanocrystals, which are different dramatically from their bulk counterparts. This method is based on the Lindemann criterion for melting, Mott’s expression for the vibrational melting entropy, and the Shi model for the size-dependent melting temperature. Simple and unified functions, without any adjustable parameter, are established for the size and interface dependences of thermal and phase stabilities of FM, AFM, FE and SC nanocrystals. According to these analytic functions, as the size of nanocrystals is reduced, the thermal and phase stabilities may strengthen or weaken, depending on the confluence of the surface/volume ratio of nanocrystals and the FM(AFM, FE or SC)/substrate interface situations. The validity of this model is confirmed by a large number of experimental results. This theory will be significant for the choice of materials and the design of devices for practical application.     

Size dependence of phase transition temperatures of ferromagnetic ,ferroelectric and superconductive nanocrystals

With the miniaturization of devices, size and interface effects become increasingly important for the properties and performances of nanomaterials. Here, we present a thermodynamic approach to the mechanism behind size-induced unusual behavior in the phase stabilities of ferromagnetic (FM), antiferromagnetic (AFM), ferroelectric (FE), and superconductive (SC) nanocrystals, which are different dramatically from their bulk counterparts. This method is based on the Lindemann criterion for melting, Mott’s expression for the vibrational melting entropy, and the Shi model for the size-dependent melting temperature. Simple and unified functions, without any adjustable parameter, are established for the size and interface dependences of thermal and phase stabilities of FM, AFM, FE and SC nanocrystals. According to these analytic functions, as the size of nanocrystals is reduced, the thermal and phase stabilities may strengthen or weaken, depending on the confluence of the surface/volume ratio of nanocrystals and the FM(AFM, FE or SC)/substrate interface situations. The validity of this model is confirmed by a large number of experimental results. This theory will be significant for the choice of materials and the design of devices for practical application.      

Strain-rate dependence of the brittle-to-ductile transition temperature in tungsten

We have investigated the strain-rate dependence of the brittle-to-ductile transition (BDT) temperature in pre-cracked tungsten single-crystals and polycrystals. There is an unambiguous Arrhenius relationship over four decades of strain rate, giving an activation energy for the process controlling the BDT of 1.05?eV. This is equal to the activation energy for double-kink formation on screw dislocations, suggesting that their motion controls the brittle–ductile transition.     

Concentration dependence of the transition temperature in spin glasses

The transition temperature T_f of randomly distributed spins interacting via the RKKY interaction (spin glasses) has been calculated numerically within the spherical approximation. The dependence of T_f on the concentration c of magnetic impurities and on the damping of the interaction due to a finite mean free path of the conduction electrons is investigated for 10^?5 < c < 10^?1.     

外延铁电薄膜相变温度的尺寸效应

考虑外延钙钛矿型铁电薄膜内的等效应力、表面晶格变化和表面电荷引起的退极化效应等机电耦合边界条件,利用铁电薄膜系统的动态金茨堡-朗道方程(DGL),系统分析和讨论了外延铁电薄膜相变温度与临界相变厚度的尺寸效应.结果表明,铁电薄膜相变温度与临界相变厚度完全依赖于各种与薄膜厚度相关的力电耦合边界条件.也给出了BaTiO₃外延铁电薄膜相变温度在各种边界条件下随厚度的变化,从结果看出,本文的分析与结论更符合实验数据. 关键词： 尺寸效应 外延铁电薄膜 相变温度 力电耦合边界     

Vibrational modes and phase transition of Si-Ge solid solution

We present the simplified treatment where the lattice vibrations of Si or Ge atoms in the Si-Ge solid solution are replaced with that of pure Si or Ge crystal at lattice constants of the alloy. Considering the volume effect on the force constants of the pure constituent, we obtain the phonon dispersion curves of the local and band modes for Si_0.91Ge_0.09 and Si_0.11Ge_0.89 systems and the concentration x-dependence of the local and band modes frequencies in the Si_1?xGe_x solid solutions. Then, from the calculation of the effective mode Grüneisen parameter γ_i for the average phonon modes in the Si_1?xGe_x systems, we obtain the predominant correlation between TA mode Grüneisen parameter γ^X_TA at the point X and the phase transition pressure P_t, and the softening of TA modes is related to the pressure-induced phase transition of the Si-Ge solid solution.     

The superconductive transition temperature of binary and ternary solid solutions in thallium

The transition temperatures of various binary alloys and of ternary alloys in the system Tl Hg _x Sn _y were investigated. By analysing the results different anisotropy parameters were obtained. This was explained by a nonlinear valence dependence of the transition temperature, which can be caused by changes in the topology of the Fermi surface.     

Dispersion of the permittivity of a uniaxial ferroelectric solid solution in the morphotropic region

A phenomenologic al theory of the permittivity dispersion fk(ω) in tetragonal ferroelectrics with polarization directed normally to a selected axis was developed for the first time. The permittivity dispersion was calculated for single-domain single crystals in both symmetry-allowed low-symmetry phases. A simple model is proposed for calculating the permittivity dispersion of solid solutions belonging to the morphotropic region of T-c phase diagrams. This model includes the results obtained for single-domain crystals, which in combination with the special averaging method yield the equality (fk(ω)) = ((fk^-1 (ω)))^-1.     

Angular dependence of the superconducting transition temperature in ferromagnet-superconductor-ferromagnet trilayers

The superconducting transition temperature T(c) of a ferromagnet (F)-superconductor (S)-ferromagnet trilayer depends on the mutual orientation of the magnetic moments of the F layers. This effect has been previously observed in F/S/F systems as a T(c) difference between parallel and antiparallel configurations of the F layers. Here we report measurements of T(c) in CuNi/Nb/CuNi trilayers as a function of the angle between the magnetic moments of the CuNi ferromagnets. The observed angular dependence of T(c) is in qualitative agreement with a F/S proximity theory that accounts for the odd triplet component of the condensate predicted to arise for noncollinear orientation of the magnetic moments of the F layers.