Observation of a second order magnetic phase transition in CsFeS2

Mössbauer experiments performed on CsFeS₂ at temperatures between 4.2 K and 300 K show that the orthorhombic high temperature phase undergoes a second order magnetic phase transition near 69 K, when the previously reported first order magnetic and structural transition to a triclinic modification near 75 K is suppressed by lattice defects or internal stresses. The saturation values of the hyperfine fields are 19.1 T for the triclinic and 15.5 and 14.1 T for the orthorhombic phase.     

Orientational order in the vicinity of a second order smectic-A to nematic phase transition

Measurements of the quadrupolar splitting of the central N¹⁴ are reported for the liquid crystal NBOA. The splitting is a measure of the orientational order parameter. At the nematic to smectic-A transition, the orientational order parameter varies continuously as a function of temperature, but with a discontinuous slope, as expected of a second order phase transition. The results are interpreted in terms of the Kobayashi-McMillan theory.     

Electrostriction soliton as a cluster model in the high-temperature phase of a hydrogen-containing ferroelectric

The polarization clusters observed experimentally in the high-temperature phase of ferroelectrics are interpreted as solitons in the microscopic pseudospin formalism. These solitons are the result of modulation of the pseudospin interaction constant by acoustic vibrations, which represents an electrostriction interaction from the phenomenological point of view. The influence of higher-order nonlinearities present in the pseudospin subsystem and the damping of acoustic modes on a soliton is analyzed. Fiz. Tverd. Tela (St. Petersburg) 40, 713–715 (April 1998)     

Magnetic properties of a higher than second order superconducting phase transition

Based on the measurement of the magnetic characteristics of In−Bi, In−Tl, In−Pb, Sn−Bi and Sn−Sb superconducting alloys it was stated that the phase transition type I→ type II superconductivity is of higher than second order. Comparing the experimental data with the Lifshitz’s theory it is shown that the addition falls between the values of 0.63 and 0.77.

     

On the order of the phase transition in KDA type ferroelectric crystals

The observation of a thermal hysteresis indicates that the phase transitions in KDA, KD²A, RDA and CDA crystals are of the first order. The static dielectric properties of these ferroelectrics resemble those of KD²P.     

The approach to a second order nematic-smectic A phase transition using mixtures

Binary mixtures of compounds from the homologous series 4-n-alkoxybenzylidene-4'-n-propylaniline are used to illustrate their use in approaching a second order nematic-smectic A phase transition.     

The effects of uniaxial stress distribution on the ferroelectric properties of thin films with first-order phase transition

Based on the Landau theory, the first-order phase transition properties of ferroelectric thin films have been studied by taking into account uniaxial stress distribution effects. The stress is supposed to decrease from interface to surface exponentially according to the experimental results. It is shown that tensile stress decreases the polarization and the Curie temperature while compressive stress increases the polarization and the Curie temperature. A stress-driven phase transition is found at the critical stress. Our prediction is compared with the available experiment results.     

Structural phase transition in ferroelectric NH4103

The structural phase transformation between the ferroelectric and paraelectric form of NH₄IO, near 83°C is dominated by tilt motions of IO₆-octahedra within a perovskite-like framework. Simultaneously, small displacements of NH₄ along the polar axis give rise to a switchable spontaneous polarization at low temperatures. As the transition mechanism is governed by the tilting of octahedra and not by the appearance of ferroelectricity, NH₄IO₃ is characterised as an improper ferroelectric material. Although the space group of the high-temperature β-phase is polar, the polarization moment is zero. Raman-active thermal soft modes appear in the ferroelectric phase. The phase transition is first order with anomalies of dielectric, piezoelectric and elastic constants.     

Effect of chiral solute on the smectic a to C phase transition of second order

The Landau theory is applied to explain the helicoidal effect of a chiral solute on an ordinary smectic C state below a smectic A-C phase transition of second order. It is shown that T_c increases with the concentration c of the solute, for small c. We suggest that a simultaneous application of electric and magnetic fields perpendicular to the smectic layers might, in some cases, also make the C phase helicoidal.     

The ferromagnetic transition of iron according to a Landau-theory of phase transitions of second order

The properties of iron at its ferromagnetic transition point are described by a type of Landau-theory developed for He II. The theory includes spatial fluctuations of the spontaneous magnetisation which lead to a logarithmic singularity of the specific heat nearT _c in agreement with measurements byKraftmakher andRomashina.     

Magnetocaloric effect in ferromagnetic and ferrimagnetic systems under first and second order phase transition

In this work we present a model to describe the magnetocaloric effect (MCE) in ferrimagnetic arrangements. Our model takes into account the magnetoelastic interactions in the two coupled magnetic sublattices, which can lead to the onset of the first order magnetic phase transition and the giant-MCE. Several profiles of the MCE, such as: the inverse- and giant-MCE were systematically studied. Application of the model to the ferromagnetic compounds GdAl₂, Gd₅(Ge_1.72Si_2.28), Gd₅(Ge₂Si₂), and to the ferrimagnetic compound Y₃Fe₅O₁₂ was performed, showing a good agreement with the experimental data.     

Optical second harmonic generation study of the ferroelectric phase transition in crystals and nanocrystals of lithium heptagermanate

The D _2h -C _2v ferroelectric phase transition in crystals of lithium heptagermanate Li₂Ge₇O₁₅ was revealed by the optical second harmonic generation technique. The phase transition temperature T _c was observed to shift from 10 to 6°C when the samples were doped with chromium (0.05%). No second harmonic signal was detected in the ferroelectric phase of nanocrystalline Li₂Ge₇O₁₅ samples.