An EPR study of local fluctuations in the vicinity of the ferroelectric phase transition in Li2Ge7O15

EPR spectra of Mn²⁺ ions have been studied in the temperature interval containing the ferroelectric transition in crystalline lithium heptagermanate Li₂Ge₇O₁₅. The EPR linewidth and shape were measured as functions of temperature and orientation. It is shown that the critical resonance-line broadening observed to occur in the vicinity of the phase transition is due to fluctuations in the local order parameter with frequencies below 10⁷ Hz.     

Neutron scattering study of the ferroelectric phase transition of SbSI

The paraelectric-ferroelectric phase transition of SbSI has been studied using neutron scattering. Several low frequency phonon branches have been observed. In particular, it has been shown that the [100] and [010] transverse acoustic modes polarized along the [001] direction with the same symmetry as the soft mode, exhibit very anisotropic dispersion relations in reciprocal space. For large q values, an anomalous dispersion relation and a very large temperature dependence have been observed. All these effects are shown to be due to a coupling with a soft mode-central component entity. This entity was observed away from the zone center, but at the zone center, in contrast with the light scattering measurements, no well-defined soft mode response could be observed.     

Raman scattering study of ferroelectric phase transition in ammonium sulphate

The interesting features of the temperature and pressure dependence of the Raman spectrum of ammonium sulphate are discussed with respect to its ferroelectric transition at 223 K. These include, the observation of quasi-elastic wing scattering and rapid line-width broadening of both internal and external phonon modes. A model of the transition based on these and previous results is indicated.     

Low-frequency Raman scattering study of the ferroelectric phase transition in the DKDP crystal

The low-frequency Raman scattering spectra of the DKDP ferroelectric crystal are studied in the temperature range 30–393 K. At temperatures above 150 K, the Raman spectra exhibit a central peak which reflects the lattice relaxation susceptibility. The width and integrated intensity of the central peak are derived from the experimental spectra. The critical slowing down of the relaxation response predicted by the Ginzburg-Landau-Devonshire theory is observed throughout the temperature range in which the central peak persists. Its integrated intensity does not, however, follow the predictions of the theory and reveals a strong temperature dependence in the ferroelectric phase and a weaker dependence in the paraphase. It is shown that the thermal activation law describes well the temperature dependence of the intensity of the central peak. An interpretation is proposed according to which the intensity of order parameter fluctuations is related to the activation barrier whose height is proportional to the deviation from the phase transition temperature.     

EPR of the ferroelectric phase transition in Li2Ge7O15:Cr3+ crystals

A Cr³⁺ EPR study of lithium heptagermanate crystals, Li₂Ge₇O₁₅ (LHG), close to the phase transition is reported. Orientation dependences of the spectra in the paraelectric phase of LHG have been measured. An anomalous broadening of the resonant lines accompanied by a crossover in their shape was found in the vicinity of the transition point. Doublet splitting of the EPR lines was observed to occur below T _C as a result of emergence of two structurally nonequivalent positions for Cr³⁺ ions. Fiz. Tverd. Tela (St. Petersburg) 40, 1102–1105 (June 1998)     

Dielectric study of the ferroelectric phase transition of KH2PO4

We report high precision measurements of the dielectric constant of Potassium Dihydrogen Phosphate as a function of temperature and electric field. Landau model parameters are evaluated and the biasing field E_c which drives the transition second order is determined.     

Optical study of the spontaneous ferroelectric phase transition in lead scandoniobate single crystals

A study is reported of the optical transmission and small-angle light scattering (SAS) in ordered and disordered stoichiometric single crystals of lead scandoniobate (PSN) with a zero and a dc electric field applied. It is shown that the spontaneous phase transition occurring in both crystals is accompanied by a sharp peak in SAS intensity, which implies the percolation nature of this transition. A field-temperature phase diagram has been constructed for the PSN single crystals studied in the work.     

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.     

Thermal-wave investigation of the ferroelectric phase transition in colemanite

Thermal waves with penetration depths ranging from 150 m to 3 m and amplitudes smaller than 0.1 mK were used to investigate the ferroelectric phase transition in colemanite. The critical behavior of the pyroelectric current is found to be independent of the penetration depth. It agrees well with the prediction from mean field theory for the temperature derivative of the bulk order parameter. The results are discussed in terms of a possible application of the thermal wave method to the investigation of surface critical phenomena.     

EPR study of the low temperature ferroelectric phase transition in Cu doped Rb2ZnCl4 single crystals

Temperature dependent EPR measurements on copper doped Rb₂ZnCl₄ single crystals allowed us to evidence and study the P2₁cn↔C1c1 structural phase transition that takes place in this compound at 74.6 K. From the two types of Cu²⁺ centers localized at different anionic sites, called Cu²⁺(I) and Cu²⁺(II), which are formed in this compound, only the Cu²⁺(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition.     

EPR study of the ferroelastic phase transition in CsLiSO4

EPR spectra of SO^-₃ ion-radical in X-ray irradiated CsLiSO₄ single crystals were used for the study of the ferroelastic phase transition at T_c = 203.0 K. The splitting (ΔH) of the SO₃ line in the low-temperature ferroelastic phase has been interpreted as proportional to the square of the order parameter. The splitting shows the temperature dependence ΔH ∞ (T_c ? T) 1.01^± 0.01     

X-ray diffraction study of phase transition of ferroelectric liquid crystal DOBAMBC

Cu₆PS₅Br superionic crystals were implanted by sulphur ions. The effect of ion implantation on the phase transition (PT) temperatures in Cu₆PS₅Br is observed from isoabsorption studies of the optical absorption edge. The dependences of the ferroelastic PT temperature on the ion fluence are analysed.     

Raman study of phase transition in ferroelectric Ba0.95Ca0.05TiO3

Raman scattering investigation of phase transition in the ferroelectric Ba_0.95Ca_0.05TiO₃ is reported. The results suggest onset of significant dynamic disorder at 105°C. This corroborates findings of recent structural study regarding large positional disorder associated with Ti and O1 atoms well below the tetragonal to cubic transition temperature (∼150°C).     

Investigations on the soft-mode mechanism of the ferroelectric phase transition in

Dielectric and Raman spectroscopic measurements have been performed to investigate the ferroelectric phase transition in . Single crystals were grown by the zone melting method. The frequency dependence of the dielectric permittivity from 1 MHz to 1 GHz has been studied in a temperature range between 265 and 285 K. A Debye like dielectric dispersion was found, showing a critical slowing down around K. Polarized Raman spectra have been taken between 220 and 310 K. Two softening modes have been found, one of A- and another one of B / B g-symmetry. The phase transition mechanism in can be classified as partially order-disorder and partially displacive, confirming former structural results. It resembles strongly that of monoclinic . Received: 7 April 1998 / Revised: 5 June 1998 / Accepted: 16 June 1998     

High pressure investigation of ferroelectric phase transition in PbSnTe

The anomalies in electrical transport properties at the displacive phase transition in Pb_1?xSn_xTe (x=0.18?0.86) has been investigated. The results of applying pressure point out the multiphonon scattering mechanism caused by the soft TO phonons as responsible for the resistivity anomaly.     

Proton conductivity and ferroelectric phase transition in hydrogen-bonded ferroelectric NH4IO3

We report on the ac dielectric permittivity (ε) and the electric conductivity (σ_ω), as function of the temperature 300?K?T4IO₃. The main feature of our measured parameters is that, the compound undergoes a ferroelectric phase transition of an improper character, at (368?±?1)K from a high temperature paraelectric phase I (Pm2₁ b) to a low temperature ferroelectric phase II (Pc21n). The electric conduction seems to be protonic. The frequency dependent conductivity has a linear response following the universal power law (σ_{( ω )}?=?A(T)ω ^{s (T)}). The temperature dependence of the frequency exponent s suggests the existence of two types of conduction mechanisms.