Vicinity Of Phase Transition Research Articles

Ultrasonic investigations of [(CH3)2NH2]5Cd3Cl11

The temperature dependences of ultrasound velocities along the crystallographic a, b and c axis in [(CH3)2NH2]5Cd3Cl11 were studied in the vicinity of phase transition at 178.5 K. The results obtained for the longitudinal and shear ultrasonic velocities confirmed clearly the existence of the second-order phase transition at this temperature and the quadratic relation between δV4 and δV6 and phase transition parameter was observed.

Temperature Gradients Induced Phase Separation in a Miscible Polymer Solution.

Temperature inhomogeneity is common in nature and technology —yet physical studies of the origins of phase transitions and the resulting morphologies generally concern systems whose temperature is constant everywhere, the contrary situation being viewed as too applied. Indeed, temperature gradients have interesting applications. They can be exploited for controlled crystal growth in zone refining of metals and semiconductors [1], can underly a rich physics of convection [2], and can generate chemical differentiation in an otherwise homogeneous geological mantle [3]. This issue also pertains to a more general intellectual problem, to understand the structure and phase behavior of mixtures when the thermodynamic variables are inhomogeneous [4 ‐ 12]. We report below a connection between temperature inhomogeneity and the actual occurrence of a phase transition above the thermodynamic coexistence temperature. Phase separation induced by temperature gradients has previously been observed when one portion of a sample is below the critical temperature and another portion above it [7,8], but, to our knowledge, not previously when the entire sample is above this temperature, with the possible exception of the cryptic allusion by Lesher and Walker to certain “failed experiments” with geological magmas [13]. Reasons are given to expect such behavior to be especially common for macromolecular mixtures in the vicinity of phase transitions. The experiments show that small temperature gradi

Giant fluctuation magnetoresistance in MnAs thin films

We present the similarity in magnetic and transport properties of MnAs and perovskite La1−xCaxMnO3 thin films which exhibit a colossal magnetoresistance. Submicron thickness magnetic MnAs films exhibiting a continuous magnetic phase transition have been molecular beam epitaxy grown on sapphire substrates. A giant magnetoresistance (of order of 10% at H=10 kOe) arising from critical spin fluctuations in the vicinity of phase transition (T≊40 °C) is observed. The concomitant large pyromagnetic (d ln M/dT=−0.5 K−1) and pyroresistive (d ln R/dT=0.046 K−1) effects along with a high magnetoresistance make MnAs films promising for applications.

On the studies of thermodynamics properties of fast neutron irradiated (LixK1−x)2SO4crystals

Abstract The effect of fast neutron irradiation on the thermodynamic properties of (LixK1−x)2SO4, (x = 0.1, 0.2,˙˙˙˙˙˙˙˙0.5) has been studied. The measurements were carried out in the vicinity of phase transition. The study reveals that as the lithium content decreases the first high temperature phase Tc = 705 K disappears, while the second one is shifted to lower temperature. It is observed also that the specific heat, Cp, decreases sharply with neutron integrated fluence φ and increases once more. Both entropy and enthalpy changes increase with the increase of neutron integrated fluence.

Electronic structure of intrinsic and impurity MN-centres in A2BX4 crystals with β-K2SO4 type structure

A 2 BX 4 single crystals (A = Cs, Rb, N(CH 3 ) 4 ; B = Cd, Zn, Mn, Fe ; X = Cl, Br, I) with β-K 2 SO 4 type structure belong to the compounds with the different phase transitions (PT), including PT into incommensurate structure. The persistence of the transitions metals play an important role in the investigations of PT by the ligands field in the types of crystals, in particular, in the vicinity of phase transition. The nature of the corresponding phase transitions is closely connected with the tetrahedral complexes [B 2+ X 4 ] 2- bounded by A - cations. The Mn 2+ ions were chosen due ithe persistence of the five external d-electrons, which are highly sensitive to the external fields. The present work deal with experimental and theoretical investigation of the electronic band structure and optical parameters of [N(CH 3 ) 4 ] 2 MnCl 4 and Cs 2 CdI 4 single crystals.

Ionic transport and microwave spectra of TGS crystals

Complex investigations have been made in order to show the correlations between the microwave dielectric response dispersion and both thermodepolasation currents and conductivity. It was also shown that activation energy in TGS crystals in the vicinity of phase transition takes on the same value as in superionic conductors in superionic phase. Apparent absence of high conductivity at low frequencies can be explained by the fact of channel conductivity.

Change in the radiative characteristics of substances at phase transitions

An analysis of experimental investigations of the radiative characteristics of solid and liquid bodies and real gases in the vicinity of phase transitions is presented.

Optical and Acoustic Properties of (NH4)2ZnCl4 Crystals near the Phase Transitions

AbstractOptical activity, electrooptical effect, and ultrasound wave velocities are investigated in the vicinity of phase transitions in the incommensurate ferroelectric crystals (NH4)2ZnCl4. The influence of hydrostatic pressure on the acoustic properties and phase transitions are studied as well. The optical activity and electrooptical effects are found only in the ferroelectric phase. It is shown that the normal‐incommensurate phase transition shifts to high temperatures under the action of hydrostatic pressure. At the same time, the temperature region of modulated phase coexisting becomes narrower and disappears finally at the triple point (Pk = 196 MPa, Tk = 249.5 K). The obtained results are considered within the framework of the phenomenological theory.

On the acoustic properties of CsD2PO4 and RbD2PO4 crystals near phase transitions under hydrostatic pressure

The influence of hydrostatic pressure on the temperature behaviour of acoustic properties in the vicinity of phase transitions of CsD2PO4 and RbD2PO4 crystals is studied. The pressure-temperature phase diagrams are obtained. The results are considered within the framework of phenomenological theory.

Vesicles and Passages Between Lamellas inLαMembrane Phases

ABSTRACTWe consider entropic effects of vesicles and passages confined between lamellas in fluid membrane lamellar (Lα) phases. We show that their presence induces attractive interactions between lamellas which strongly compete with steric repulsion in the vicinity of phase transitions from the lamellar to isotropic membrane phases. An interplay between steric repulsion and entropic interactions mediated by passages and vesicles determines the inter-lamellar distance reached at these transitions. This distance crucially depends on the membrane Gaussian bending rigidity constant.

Stability of localized excitations and domain growth in the vicinity of the first order phase transition

In the frame of the time-dependent Ginzburg-Landau model the evolution of localized excitations in the vicinity of the phase transition is considered. The critical nucleus of a stable phase in a metastable one and the evolution of the isolated domain of a new phase for systems with different space dimensionality are studied both analytically and numerically.

Critical behaviour of uniaxial Sn2P2S(Se)6ferroelectrics

Abstract Critical behaviour of proper uniaxial Sn2P2(SexS1-x)6 ferroelectrics whose phase diagram contains a Lifshitz point and a “virtual” tricritical point is studied by means of measured temperature dependences of birefringence and heat capacity. An anomalous heat capacity change in paraphase is described by an exponential function δC p ∼ τ−α. Far from the phase transition temperature, α = 0.5 and this corresponds to the first fluctuational correction. In the vicinity of the phase transition to ferroelectric phase for Sn2P2S6, α = 3/2, which corresponds to dipoled effects while in the vicinity of phase transition (PT) to incommensurate phase for Sn2P2Se6, α = −0.02, which satisfies the Heisenberg double-component model. The crossover from tricritical behaviour to critical one is displayed in ferroelectric and incommensurate phases. In the vicinity of Lifshiftz point, β = 0.2 for a compound with x = 0.29 within a wide temperature range.

Rigorous analysis of renormalization-group pathologies in the 4-state clock model

We perform an exact renormalization-group analysis of one-dimensional 4-state clock models with complex interactions. Our aim is to provide a simple explicit illustration of the behavior of the renormalization-group flow in a system exhibiting a rich phase diagram. In particular we study the flow in the vicinity of phase transitions with a first-order character, a matter that has been controversial for years. We observe that the flow is continuous and single valued, even on the phase-transition surface, provided that the renormalized hamiltonian exist. The characteristics of such a flow are in agreement with the Nienhuis-Nauenberg standard scenario, and in disagreement with the “discontinuity scenario” proposed by some authors and recently disproved by van Enter, Fernández and Sokal for a large class of models (with real interactions). However, there are some points in the space of interactions for which a renormalized hamiltonian cannot be defined. This pathological behavior is similar, and in some sense complementary, to the one pointed out by Griffiths, Pearce and Israel for Ising models. We explicitly see that if the transformation is truncated so as to preserve a hamiltonian description, the resulting flow becomes discontinuous and multivalued at some of these points. This suggests a possible explanation for the numerical results that motivated the “discontinuity scenario”.

Superionic compound Li 3Sc 2(PO 4) 3: NMR spectra and nuclear magnetic relaxation

The 6,7Li and 31P NMR spectra and the 7LI, 31P spin-lattice relaxation, as well as electrical conductivity and thermal properties have been studied for powder samples of the Li 3Sc 2(PO 4) 3 superionic conductor over a wide temperature range. The quadrupole splitting parameters of the 7Li NMR spectra and the 7Li spin-lattice relaxation rate, T 1 −1, are changed abruptly in the vicinity of phase transition at ⋍530 K. In the high-temperature range the 31P spin-lattice relaxation rate is controlled by a translation motion of lithium ions, while in the low-temperature range it is controlled by a spin diffusion to paramagnetic impurities. A “frozen” EFG model is suggested, which allows to explain a quadrupole structure observed for the 7Li NMR spectra under fast motion of lithium ions over structurally equivalent sites. The 7Li 7Li dipolar relaxation mechanism at a distance close to the sum of the radii of two lithium ions is used to explain T 1 −1 values of 7Li. Three types of lithium ion motion are defined and the corresponding activation energy values have been determined. Anomalous temperature behavior of the 7Li NMR spectra intensity that does not follow the Curie law have been found. Lack of a qualitative agreement between the X-ray data and the parameters of the 7Li NMR spectra observed is discussed.

Radiation damage in triglycine sulphate due to fast neutron irradiation

Abstract The change in electrical properties of TGS crystals due to induced defects created by fast neutron irradiation of two different energies (2 and 14 MeV) and different integrated neutron fluxes have been studied in the vicinity of phase transition. It is observed that the electrical conductivity increases with increase of neutron fluence up to 1.7 × 1010 n · cm−2 and the values of the relative change of electrical conductivity in case of 2 MeV are higher than that of 14 MeV neutrons at the same neutron fluence (φ)

Electroresistance of some semiconductors in the phase transition region at high pressure

Abstract In apparata of liquid type piston-cylinder up to 2GPa and toroid with solid anvil up to 25 GPa pressure (P) electrial resistance R(P) in sphere of phase transition (PT) CdSnAs2,SnTeInP,GaAs is investigated, and GaP at T = 300K, and also R(T,P) VTSP YB2Cu306+x in sphere of superconductive transition. In order to describe the substance behaviour in the vicinity of PT, that is, the region of gapped change of R the approximation geterophased structure is used - the effective environment (model GSEE) which checks different configuration of phases inserts and their dependence from P. In order to picture the solid substance behaviour in the vicinity of point PT at high pressure (HP) different methods are used [1–8].

Physical nature of the anomalies in the temperature dependence of the probability of the M�ssbauer effect near phase transitions

A mechanism for microstructural changes in the vicinity of phase transitions is proposed, which explains the anomaly in the temperature dependence of the Mossbauer effect in high temperature superconducting ceramics with 1-2-3 composition before the transition to the superconducting state, and in Ti(Ni, Fe) alloys before the martensite transformation into the R-phase.

Ellipsometric studies of behenic acid at the air‐water interface

AbstractThe rich phase behaviour of monolayers of behenic acid (=docosanoic acid) has been analyzed by null ellipsometry in isothermic and isobaric measurements. By using structural data known from x‐ray measurements the anisotropy and refractive indices in the vicinity of the S‐CS phase transition have been calculated with a minimum of model assumptions. The calculated values are: nsx=1.47. ncsx=1.48. nsz=1.54. ncsz=1.55. and anisotropy γ=nx/nz ≈︁0.95. It is also shown that ellipsometry is a very sensitive and convenient method of detecting phase transitions. Its value for quantifying the roughness of the monolayer in the close vicinity of phase transitions due to thickness and density fluctuations is shown by the measurement of the parabolic intensity variations of the ellipsometric angle δΔ. The intensity minimum at the phase transition is much wider than in the adjacent homogeneous phases.

Investigation of the second ultrasonic harmonic generation in Sn2P2S6and Sn2P2Se6crystals

Abstract The results of the second longitudinal ultrasonic harmonic measurements in ferroelectric semiconductive Sn2P2S6 and Sn2P2Se6 crytals in the vicinity of phase transitions are presented for X crystallographic direction. It is shown, that there are unusual anomalies of the second harmonic amplitude at the phase transition region. Such harmonic behaviour is determined by the increase of acoustic nonlinearity and also by the ultrasonic attenuation maximum at the phase transition temperature. The temperature dependences of the nonlinear elastic coefficients for Sn2P2S6 and Sn2P2Se6 crystals have been calculated using ultrasonic data. The elastic nonlinearity is markedly higher in the temperature interval where an incommensurate phase exists

Determination of a complete dielectric tensor of polymerizable pTS diacetylene

The complete dielectric tensor for single crystals of polymerizable pTS diacetylene was determined as a function of temperature and monomer-to-polymer conversion. A pronounced elongation of the ellipsoid of permittivity is observed in the direction of growing polymer chains, whereas the cross section perpendicular to this direction is almost isotropic, being affected by the polymerization to a minor degree. To within experimental error two principal axes of the tensor coincide with the crystallographic axes, the orientation of the tensor being independent of temperature and polymer content. Phase transitions in monomer, polymer and mixed monomer-polymer crystals are visible on the ϵ( T) dependences. The changes of permittivity outside the region of phase transitions may be understood within models based on concepts of the Lorentz or Onsager local fields. In the vicinity of phase transitions the dependence of permittivity on temperature and polymer contents is explained employing the Landau-Devonshire-Kittel theory for antiferroelectric-type phase transitions.