L. M. Rabkin

A comparative Raman study of ferroelectric PbTiO3 single crystal and thin film prepared on MgO substrate

A Raman study of a highly oriented PbTiO3 thin film deposited on a MgO single crystal has been performed as a function of the temperature. The results have been compared with those obtained from a single crystal prepared from the same batch and those reported in the literature for polycrystalline thin films and single crystals. A shift in the transition temperature is observed from the soft mode behavior, together with more usual lower values of its frequency, which are both related to effective hydrostatic pressure due to the clamping of grains by their neighbors in the distorted tetragonal structure. The large soft mode linewidth and its integrated intensity, which do not vanish at the ferroelectric to paraelectric transition temperature, have been interpreted in terms of the coupling of this mode (and more generally of all polar phonons) with the static strain fields originating from the clamping effect and the epitaxial conditions.

Polarized Raman spectra of Cs5H3(SO4)4· H2O single crystals

The Raman spectra of the superprotonic conductor Cs5H3(SO4)4·H2O have been investigated in the 300-500 K range. An assignment of bands due to internal and external vibrations in term of approximate type of motions is proposed. The spectroscopic results are used for discussing crystal structures of different phases, the type of structural disorder and phase transitions.

Magnetic and dielectric response of cobalt-chromium spinel CoCr2O4 in the terahertz frequency range

The nature of the phonon and magnon modes in the CoCr2O4 multiferroic with a cubic spinel structure has been studied using submillimeter spectroscopy and infrared Fourier spectroscopy. This paper reports on the first measurement of the evolution with temperature of the exchange optical magnon in the ferrimagnetic (TC = 94 K) and two low-symmetry (TS ≈ 26 K, Tlock-in = 14.5 K) phases of CoCr2O4 down to T = 5 K in zero magnetic field. It has been shown that the detected magnon is not a ferrimagnetic order parameter and originates, most probably, from spin precession in the cobalt sublattices. At the points of the magnetic phase transitions, the oscillator parameters of the two lowest-frequency phonon modes reveal an anomalous temperature behavior, thus evidencing the presence of significant interaction between the magnetic and phonon subsystems. The increase by 25% of the damping parameter of the phonon mode originating from vibrations of the CoO4 tetrahedra during the transition of CoCr2O4 to the multiferroic state (T < TS) suggests structural changes in the lattice involving loss of spatial central symmetry of the medium.

The transition to a proton glass state in Cs5H3(SO4)4 · H2O

Abstract Raman scattering, calorimetric and dielectric spectroscopy data for Cs 5 H 3 (SO 4 ) 4 · H 2 O single crystals taken at 20–300 K are reported. The glass transition temperature, T g , as determined from DSC, shows a remarkable hysteresis: 261 K on heating and 248 K on cooling. The change in the molar heat capacity is 27 J/mol · K. The dielectric data (30 kHz–2.2 MHz) show a pronounced dispersion whose relaxation frequency obeys an Arrhenius law. An increase in the Isotropie component of Rayleigh light scattering was observed below T g . Raman scattering data indicate a conservation of hexagonal crystal symmetry in the glassy phase, and a lowering of the site symmetry. The transition from dynamic to static disorder is discussed.

Raman spectra of crystals in the proton glass state

Polarized Raman spectra of (PCHS) single crystals were measured in the frequency range at 20 - 300 K, i.e. both below and above the glass transition temperature . The evolution of the spectra with decreasing temperature indicates that the orientational disorder of ions and molecules changes at from the dynamical to a static one. A scenario for the glass transition and a model of the resulting glass state is suggested, in which the water molecules (H bonds) play the role of binding constituents of the isolated sulphate complexes .

Influence of the growth mechanism and thermoelastic stresses on the lattice dynamics of heteroepitaxial films of barium strontium titanate

Heteroepitaxial thin films of Ba0.7Sr0.3TiO3 (BST-0.3) solid solutions were grown on single-crystal (001) MgO substrates by high-frequency cathode sputtering of a stoichiometric ceramic target. The parameters of the tetragonal unit cell of a film were determined by x-ray diffraction methods, and the temperature dependence of the parameter c was studied depending on the synthesis conditions in the temperature range 293–520 K. An E(TO) soft mode was observed in the Raman spectra, the frequency of which correlates with two-dimensional stresses arising in films. It is shown that the two-dimensional stresses in a film are controlled not only by the film-substrate lattice constant mismatch and the difference of their thermal expansion coefficients but also are significantly dependent on the heteroepitaxial growth mechanism. It is shown that the phase transition to the tetragonal paraelectric phase during film heating occurs irrespective of the growth mechanism.

Raman scattering in the proton conductor Rb3H(SeO4)2

Abstract Careful polarised Raman scattering studies have been performed with the aim of analysing structural and dynamic aspects of the superionic transition in Rb 3 H(SeO 4 ) 2 . The results, in full agreement with the structural data, show convincingly the off-centre positions of protons in the OH … O bonds and confirm the orientational disorder of the SeO 4 2− groups as well as the formation of a dynamic network of hydrogen bonds in the high-temperature superionic phase.

Raman spectra and phase transitions in some langbeinites

Double sulfates of mono- and divalent metals belong to a family of compounds with a M2+ M22+(S04), overall formula which mainly crystallize in the structural type K2Mg2(SO4)3 (langbeinite). In the high-symmetry phase the langbeinite structure described by the space group T4(P213) with four molecules per unit cell.1 By now, there have been synthesized 26 compounds of this family on 11 of which the phase transitions have been discovered. In some crystals a sequence of transitions has been observed.2

Raman spectra of crystals

Polarized Raman spectra of crystals (x = 0.1 and 0.7) were studied in the temperature range 10-300 K. Thorough site-symmetry analysis of internal and vibrations combined with factor-group analysis based on hexagonal pseudosymmetry allowed us to make an assignment of the observed modes. The spectra of the x = 0.1 sample are compatible with those of pure including the features of the disorder in the paraelectric phase above 220 K (the broad central peak of the relaxator type and high mode damping) and ordering in the ferroelectric phase. The spectra of the x = 0.7 sample show a much smaller disorder at high temperatures (no broad central mode and lower mode damping) due to the lower ammonium content, but clearly confirm the onset of the dynamic dipolar glass transition (short-range correlations inducing the appearance of long-lived dipolar clusters) near 220 K.

Exoelectron emission studies of reduced BaTiO3 ceramics

Abstract Thermostimulated exoelectron emission has been employed for studying the phase transition temperature and defect structure of dielectric and semiconducting BaTiO3 ceramics. It was found that reduction of BaTiO3 ceramic leads to dramatic changes in the TSEE spectra. The increase of current carrier concentration results in appearance of two new TSEE peaks in 420–590 K region. TSEE peaks corresponding to ferroelectric Curie point T c have been observed both for dielectric and semiconducting samples.