Showing papers in "Phase Transitions in 1985"
TL;DR: A review of the various instability-based theories of melting with special emphasis on a particular type of instability which is both thermodynamic and elastic in nature can be found in this article.
Abstract: The various instability-based theories of melting are reviewed with special emphasis on a particular type of instability which is both thermodynamic and elastic in nature. This “thermoelastic” instability originates from a decrease in resistance to shear stress, an idea developed by Born, and is characterized by divergent coefficients of thermal expansion (α) and isothermal compressibility at the melting temperature (T m), an ideal first proposed by Herzfeld and Goeppert-Mayer (1934). Evidence is presented from the results of parameter-free equation-of- state calculations for alkali halides and from published thermal expansion results for a variety of materials, which suggests that melting in many materials may be caused by a thermoelastic instability. A study of the potential energy surface for NaF suggests a type of disorder for the liquid phase which is consistent with recent results of molecular dynamics calculations. If α diverges at Tm then the slope of the melting curve can be deduced from...
73 citations
TL;DR: In this article, the a(T) curve for the monodomain and polydomain regions of a flux-grown SrTiO3 crystal was observed below the transition temperature without applying external stress.
Abstract: In a flux-grown SrTiO3 crystal, monodomain and polydomain regions have been observed below the transition temperature without applying external stress. For each region, measurements of the lattice constant as a function of temperature are made by means of high-angle double-crystal X-ray diffractometry (HADOX) with a precision better than one part in 106. The a(T) curve for the monodomain clearly shows a critical contraction in the region Tc ± 2 K, where the curve deviates from the relation 1—a/a 0 ∞ (1-T/Tc )βa . The phenomenon is discussed in terms of the fluctuation of the order parameter and related to the precursor effect reported for KMnF3 and for SrTiO3 Verneuil crystals for T > Tc . This means, on the other hand, that the determination of the critical exponent of spontaneous strain of SrTiO3 is intrinsically difficult. An anomalous temperature dependence of the intensity of the 400 and 004 reflections found for Verneuil crystals is observed in a very similar manner.
54 citations
IBM1
TL;DR: In this paper, it was found that, far above the transition temperature, Tc, certain ferroelectric systems show a temperature dependent index of refraction, n(T), that is qualitatively different from ordinary ferroelectrics.
Abstract: It is found that, far above the ferroelectric transition temperature, Tc, certain ferroelectric systems show a temperature dependent index of refraction, n(T), that is qualitatively different from ordinary ferroelectrics. We review measurements in these systems and show how some aspects of this n(T) data can be quantitatively understood. This is done by considering the polarization in these materials to arise from very small localized clusters of a few unit cells far above Tc. The strongly broken long range symmetry allows these effects to occur. Furthermore, we find a one-to-one correspondence between ferroelectrics that show this unusual high temperature behavior and those that show glass-like excitations at very low temperatures; this clarifies the ambiguities in the low temperature heat capacity and thermal conductivity results.
45 citations
TL;DR: In this article, the volume content of neighbouring phases and the temperature variation of unit cell volume have been investigated in the vicinity of phase transitions between the para-, ferro, and antiferroelectric states for Pb(Zr, Ti)O3 ceramic with Ti content up to 3%.
Abstract: Using X-rays the volume content of neighbouring phases and the temperature variation of unit cell volume have been investigated in the vicinity of phase transitions between the para-, ferro-and antiferroelectric states for Pb(Zr, Ti)O3 ceramic with Ti content up to 3%. The diffuse nature of the phase transitions has been confirmed in the solid solutions.
34 citations
TL;DR: In this article, the authors identify the microscopic tunneling centers in the glassy crystal (KBr)1_x(KCN)x and predict the quantitative experimental behavior of a glassy material starting from a microscopic model.
Abstract: Lack of information about the local structure of glasses has severely hindered He-tailed understanding of their universal properties. The glassy behavior found in the crystalline systems discussed here is therefore a very exciting theoretical opportunity. In particular, there are no successful microscopic models for the tunneling centers in glasses (which are responsible for the low temperature properties); thus little predictive information about the distribution of tunneling center parameters is available, and sharp tests of the theory are not possible. We have identified the microscopic tunneling centers in the glassy crystal (KBr)1_x(KCN)x. Thus for the first time we predict the quantitative experimental behavior of a glassy material starting from a microscopic model. There are three important experimental features in this system. First, KBr:KCN undergoes a transition into a low temperature state where the cyanide axes are frozen into a disordered configuration. Second, it shows a very broad ...
30 citations
TL;DR: In this article, structural information about first-order transitions in RbCaF3 has been obtained using energy-dispersive X-ray diffraction, and it was found that successive transitions are Oh 1(Pm3m) → D4h 18(14/mcm) and D2h 13(Pmmn) on cooling.
Abstract: Specific heat measurements, dielectric measurements and X-ray diffraction of RbCaF3 show two first-order transitions at 193 K and 41 K. Structural information about these transitions has been obtained using energy-dispersive X-ray diffraction. It was found that successive transitions are Oh 1(Pm3m) → D4h 18(14/mcm) → D2h 13(Pmmn) on cooling. These results were interpreted by successive condensation of an R25 z and an M3 x + y soft phonon mode from the ideal cubic phase.
29 citations
TL;DR: The use of electron paramagnetic resonance (EPR) as a tool to study various kinds of phase transitions in condensed materials is discussed in this article, where representative examples of studies in the different kinds of phases are discussed.
Abstract: The use of electron paramagnetic resonance (EPR) as a tool to study various kinds of phase transitions in condensed materials is discussed. The subject is reviewed by considering representative examples of studies in the different kinds of phase transitions. Structural phase transitions in solids, dynamics of formation of Slater configurations in KDP type ferroelectrics, temperature dependence of ordering in liquid crystals, studies of the dynamics of chain motion in glass to rubber transitions in polymer systems as well as magnetic transitions are discussed.
22 citations
TL;DR: In this paper, the temperature dependences of the molar volume, the ultrasonic velocity and the refractive index in N(n-pentyloxy benzylidene) p-n-decyl aniline were presented.
Abstract: The temperature dependences of the molar volume, the ultrasonic velocity and the refractive index in N(p-n-pentyloxy benzylidene) p-n-decyl aniline, which exhibits nematic, smectic-A and smectic-B, phases, are presented. The isotropic-nematic and smectic-A-smectic-B phase transitions are found to be of first order while the nematic-smectic-A transition is of second order. The thermal expansion coefficient (a) computed from the molar volume data agrees with a second-order N-SA transition. The computed adiabatic compressibility βad, molar sound velocity (or Rao number Rn ) and molar compressibility (or Wada constant A) are presented.
17 citations
TL;DR: In this article, the thermal and rmodynamic properties of configurational states, their temperature, density and time dependence and their possible connection with the low-temperature tunneling states are considered.
Abstract: Thermally activated translational and/or rotational motions of groups of atoms or molecules occur in localized regions of the rigid matrix of glasses, glassy liquid crystals and glassy crystals. These are observed by dielectric and mechanical relaxation spectroscopy at temperatures well below Tg and show features which are remarkably similar amongst the various types of disordered solids. An analysis of the heat capacity and entropy of the three types also shows a substantial non-vibrational contribution from the availability of configurational states in localized regions embedded in a rigid matrix. In this essay, the kinetic and the rmodynamic aspects of such states, their temperature, density and time dependence and their possible connection with the low-temperature tunneling states are considered. It seems clear that the concept of a disordered solid as an elastic continuum is unsatisfactory and that a description of its heat capacity should also include the energy associated with the configur...
16 citations
TL;DR: The problem of analysing data near the critical temperature is investigated using simulated data (with particular reference to specific heat data) and the possibilities of quantitatively comparing competing models is discussed.
Abstract: The problem of analysing data near the critical temperature is investigated using simulated data (with particular reference to specific heat data). Attention is paid to the fitting problems posed by the many adjustable parameters inherent to theoretically predicted models, and to the statistical significance of resulting parameters. Consideration is given to non-ideal or rounding behaviour near T c and the possibilities of quantitatively comparing competing models is discussed.
13 citations
TL;DR: In this paper, it was shown that the specific heat is dominated by another kind of excitations, which also couple to the phonons and thus determine the universal low temperature thermal conductivity of this class of solids.
Abstract: The low energy vibrational excitations of perfect crystals seen in specific heat and thermal conductivity measurements can be described in terms of Debye phonons. In amorphous solids, however, the specific heat is dominated by another kind of excitations, which also couple to the phonons and thus apparently determine the universal low temperature thermal conductivity of this class of solids. A phenomenological model of tunneling states with a uniform density of states has been very successful in describing a wide range of investigations of these excitations. Of particular importance was the discovery of a logarithmic time dependence of the specific heat, which was predicted by this model. It now appears well established that thermal equilibrium in an amorphous solid at low temperatures cannot be reached on any time scale. A physical model of the tunneling states is still lacking. The major hurdle is that of finding an entity which is common to all amorphous solids, and which leads to a thermal co...
TL;DR: The structure of proustite is investigated by single-crystal x-ray Weissenberg photography, and by neutron powder diffraction at several temperatures in the range 5 K to 300 K as discussed by the authors.
Abstract: The structure of proustite is investigated by single-crystal x-ray Weissenberg photography, and by neutron powder diffraction at several temperatures in the range 5 K to 300 K. The x-ray photographs suggest that proustite retains its room-temperature structure right down to the first-order phase transition at 27 K, and that below this temperature it has a monoclinic structure. Profile refinement of the neutron diffraction data corroborates this hypothesis, and the low-temperature space group is found to be Cc. The spontaneous polarisation and spontaneous strains that occur in this low temperature phase are described. No evidence is found for the existence of a phase transition previously reported to occur at 210 K, and within the accuracy of the measurements, no change in symmetry is observed at 56 K, the temperature at which the second-order phase transition occurs.
TL;DR: In this article, the effect of pressure and temperature on the values of exponents φ and K defined as follows: the magnitude δϵ/E 2 is a measure of the nonlinear dielectric effect (NDE).
Abstract: Presented here are the results of measurements of the effect of pressure and temperature on the values of exponents φ and K defined as follows: The magnitude δϵ/E 2 is a measure of the nonlinear dielectric effect (NDE). The nature of the amplitude variations is also shown. Tests were conducted on a nitrobenzene-hexane solution of critical concentration x c = 0.43 mole fraction of nitrobenzene, for which the curve T c(p) describing the variations of critical temperature has been plotted for a wide range of pressures.
TL;DR: In this paper, it was shown that a variety of disordered crystals exhibit the same anomalous behavior as found in amorphous solids, which arise from localized excitations having broad spectra in both energy and equilibration time.
Abstract: Low temperature measurements of specific heat, thermal conductivity, dielectric dispersion, ultrasonic dispersion and other properties have disclosed that a variety of disordered crystals exhibit the same anomalous behavior as found in amorphous solids. The anomalies arise from localized excitations having broad spectra in both energy and equilibration time. In some disordered crystals, these spectra may be changed systematically by varying the disorder present. The physical origin of the excitations, in crystals or glasses, lacks a satisfactory theoretical explanation. It is expected that the crystalline systems will be more amenable to theoretical study. The crystalline solids found to harbor the excitations include, at this time, certain fast-ion conductors, ferroelectrics, metallic alloys, and rotationally disordered solids.
TL;DR: Barium Indium Niobate and Tantalate have been shown to be ferroelectric and to possess phase transitions above room temperature as discussed by the authors, and their electrical properties and long-range order parameters are investigated.
Abstract: Barium Indium Niobate and Barium Indium Tantalate are both shown to be ferroelectric and to possess phase transitions above room temperature. The electrical properties and long-range order parameters are investigated. The ability to change the degree of long-range order is demonstrated for BIT.
TL;DR: In this paper, thermal microscopy analysis of N-(p-n-alkoxybenzylidene) p-n butylanilines has confirmed the existence of a mixed smectic phase, SAG.
Abstract: Re-examination of the N-(p-n-alkoxybenzylidene) p-n-butylanilines, especially the pentyloxy homologue, by thermal microscopy has confirmed the existence of a mixed smectic phase, SAG. The compound N-(p-n-pentyloxybenzylidene) p-n-butylanilineexhibits nematic, smectic A, smectic AG and smectic G phases.
TL;DR: In this article, the electrical resistivity of polycrystalline samples of MnCr2S4 at room temperature and at constant temperature was investigated. But the electrical resistance was not shown for the case of the noncollinear ferromagnetic interaction.
Abstract: The Seebeck coefficient for polycrystalline samples of: 1. Mn1-x Cu x Cr2S4 (0.0 ≤ x < 1.00) at 313 K and 2. MnCr2S4 between 313–393 K is presented. The electrical resistivity of Mn1-x Cu x Cr2S4 as a function of x at the room temperature is also presented. The n-p phase transition is observed in two cases: 1. on changing x at constant temperature (313 K) for 0.0 ≤ x ≤ 0.1 and 2. on changing temperature for MnCr2S4 at about 326 K. The first case is probably connected with the noncollinear ferromagnetic interaction of the chromium 3+ and 4+ ions. The second case follows from the strong ferrimagnetic interaction of the Mn2+ and Cr3+ ions in pure MnCr2S4.
TL;DR: In this article, the dielectric constant variation with temperature of N-(p-n-butoxybenzylidene)p- n-butylaniline is presented in the isotropic, nematic, smectic A and smective B phases.
Abstract: The dielectricconstant variation with temperature of N-(p-n-butoxybenzylidene)p-n-butylaniline is presented in the isotropic, nematic, smectic A and smectic B phases. A new transition in the nematic phase is reported. The derived parameter, molar susceptibility anisotropy, Δσ, is also determined.
TL;DR: In this paper, four phases in the temperature interval 312 and 77 K were observed in the infrared and Raman spectra of the title compound and the phase changes concerned the proton potential function of the short hydrogen bond connecting both carboxylate moieties.
Abstract: Four phases in the temperature interval 312 and 77 K are observed in the infrared and Raman spectra of the title compound. The phase changes concern the proton potential function of the short hydrogen bond connecting both carboxylate moieties. The optical spectroscopic observations are correlated with literature NQR and our own DSC data.
TL;DR: In this article, the phase transitions of the liquid crystal p-n-nonyloxybenzoic acid were investigated with density and ultrasonic velocity measurements, and the variation of density with temperature confirmed that the isotropic-nematic and nematic-smectic C transitions are of first order.
Abstract: Phase transitions of the liquid crystal p-n-nonyloxybenzoic acid have been investigated with density and ultrasonic velocity measurements. The variation of density with temperature confirms that the isotropic-nematic and nematic-smectic C transitions are of first order. All the phase transitions are confirmed by the variation of ultrasonic velocity with temperature. The adiabatic compressibility (βad), and Rao number (Rn) are also estimated.
TL;DR: The ultrasonic velocity in N-(p-n-pentyloxybenzylidene) p-n butylaniline is measured as a function of temperature from the isotropic liquid phase to the smectic G phase.
Abstract: The ultrasonic velocity in N-(p-n-pentyloxybenzylidene) p-n-butylaniline is measured as a function of temperature from the isotropic liquid phase to the smectic G phase. A mixed smectic phase (SAG) for a temperature interval of 2°C is reported. The derived parameters such as adiabatic compressibility (βad). Rao number (Rn) and molar compressibility (B) are also reported.
TL;DR: In this article, the motion of the propionate ion in relation to the phase transition was studied in Dicalcium Barium Propionate (DBP), and the second moment of the proton spin-lattice relaxation time was measured in the range 77-350 K.
Abstract: Dicalcium Barium Propionate (DBP) undergoes a first order phase transition at 267 K and a second order one at 204 K. Dicalcium lead propionate (DLP) also shows two phase transitions—a second-order one at 333 K and a first-order one at 191.5 K. The proton second moment $(M_2)$ in the temperature range 160-300 K and the proton spin-lattice relaxation time $(T_1)$ in the temperature range 77-350 K have been measured to study the motion of the propionate ion in relation to the phase transition. In DBF, the $M_2$ variation with temperature shows two transitions corresponding to the two phase transitions reported. On the other hand the $T_1$ measurements yield only a single minimum and a slope change at the second-order phase transition temperature. In DLP, the $M_2$ study yields a transition in the region 180-190 K, near the first-order phase transition, but no $M_2$ transition at the second-order phase transition, $(T_1)$ measurements show a single minimum, as well as a slope change at the second-order phase transition. At the first-order phase transition, $T_1$ changes smoothly. These observations are discussed in relation to propionate ion dynamics.
TL;DR: In this paper, specific heat data near the critical temperature of gadolinium were analyzed for consistency with theoretical models, and the authors concluded that their data are not consistent with the parameter values predicted by theory for a simple (n = 3, d = 3) Heisenberg ferromagnet, although the general form of the model appears compatible.
Abstract: The problems encountered, when analysing specific heat data near the critical temperature, are tackled with particular reference to the critical specific heat of gadolinium. Experimental results are tested for consistency with theoretical models. We conclude that our data are not consistent with the parameter values predicted by theory for a simple (n = 3, d = 3) Heisenberg ferromagnet, although the general form of the model appears compatible. A model allowing the additional concept of a discontinuity at T c appears to produce a significant improvement in the resultant fit.
TL;DR: In this article, the existence of the thermodielectric effect at phase transitions of mesogenes with smectic phases was confirmed, and significant sudden variations of the electric current were detected at all phase transitions.
Abstract: In this paper we confirm the existence of the thermodielectric effect at phase transitions of mesogenes with smectic phases: terephthal-bis-butyl-aniline (TBBA) and cholesteryl myristate (CM). Significant sudden variations of the electric current were detected at all phase transitions. Thus, transition temperatures are identified in good agreement with literature. We have also determined the polymorphism of the mixture TBBA 25% + CM. 75% by weight.
TL;DR: In this article, the relative effectiveness of electric and magnetic fields in the nematic phase of PBBA was investigated by measuring the dielectric constant at 1 MHz, and the relative strength of these fields was discussed.
Abstract: The molecular alignment in the nematic phase of N-(p-n-pentyloxybenzylidene) p-n-butylaniline (PBBA) in the presence of electric and magnetic fields is investigated by measuring the dielectric constant at 1 MHz. The relative effectiveness of electric and magnetic fields in the nematic phase of PBBA is discussed.
TL;DR: In this paper, a model of mobile, magnetically ordered microregions was proposed for the dispersion of the magnetic permeability spectrum in CdCr2Se4 near Tc and two dispersion areas were detected at about 14 MHz and 400 MHz.
Abstract: In CdCr2Se4 near Tc two dispersion areas of the magnetic permeability spectrum were detected at about 14 MHz and 400 MHz. In the pre-transition region (in the paramagnetic phase) mobile, magnetically ordered regions are found to exist. Based on a model of mobile, magnetically ordered microregions, a qualitative interpretation of both dispersion mechanisms as a function of temperature and external magnetic field is postulated.