C. Carabatos-Nédelec

Order–disorder and structural phase transitions in solid-state materials by Raman scattering analysis

The temperature dependence of the vibrational wavenumbers, of the anharmonic and/or order–disorder origin of the damping and of the Raman intensities are briefly presented. On that basis, it is then shown that one can obtain information such as the height of the potential barrier, the orientational correlation time, the spatial correlation length and the anharmonic contribution to the Raman peak evolution with temperature. The concepts were applied and exploited with success in three characteristic cases, diethylenetriammonium chlorocadmate [(denH3)CdCl5] single crystals, zinc thiourea sulphate [ZTS] single crystals and lead–lanthanum zirconate–titanate [PLZT] ceramics in which order–disorder and structural phase transitions are observed.

Raman and Infrared Spectra of Bis(Thiourea)Zinc Chloride Zn[CS(NH2)2]2Cl2 Single Crystal

Single crystals of bis(thiourea)zinc chloride, Zn[CS(NH 2 ) 2 ] 2 Cl 2 (BTZC), were grown by slow evaporation of an aqueous solution at room temperature. The crystal has been studied at room temperature by Raman spectroscopy between 20 and 1700 cm -1 and 3000 and 3500 cm -1 and by infrared transmittance in the 400 to 4000 cm -1 wavenumber range. The vibrational modes are classified by the factor group analysis method and the spectral bands are compared with those of parent in order to propose a tentative assignment of the observed modes. Preliminary second harmonic generation measurements have allowed to identify the C 2v 9 space group symmetry of the compound.

Raman scattering in PbZr1-xTixO3 single crystals with low Ti content and a study of the Ti influence

Investigations of the Raman scattering spectra of PbZr0.99Ti0.01O3 single crystals were performed over a large temperature range from 10 to 650 K and compared with those of pure PbZrO3. The main modifications of the Raman spectra have been found to appear near the transitions between the antiferroelectric-intermediate and the intermediate-paraelectric phases. In particular, a polarisation effect of the light scattering in the para-electric phase has been detected and attributed to the special role of Ti ions introduced in the host lattice of lead zirconate. These ions can be regarded as defects breaking the local symmetry and leading to the creation of polar clusters in the para-electric phase. Below a defined temperature in this phase, a long-range interaction appears and causes cluster ordering. The change in the Raman spectra and in the light scattering intensity and polarisation is analysed and connected with the dielectric and pyroelectric properties investigated on the same sample.

Structural and spectroscopic studies of niobium doped PZT 95/5 ceramics

Abstract The ceramic solid solution of PbZr0.95Ti0.05O3:1%wtNb2O5 has been studied by X-rays texture and structure measurements, by electron scanning microscope observations, by Raman scattering and by infrared reflectivity experiments. We conclude for the isotropy of the material, the absence of phase transitions at low temperature and the coexistence of two phases at room temperature.

Low Temperature Raman Spectra of Zinc Tris(Thiourea) Sulphate (ZTS) Single Crystals

The Raman spectra of Zinc tris(Thiourea) Sulphate (ZTS) have been recorded at low temperature (10 K) in the wavenumber range 0-1700 cm -1 and 3000-3500 cm -1 . The spectra were compared to those at room temperature and to those of similar compounds containing thiourea and sulphate molecules in order to propose an assignment of the observed vibrational modes at 10 K. The lattice vibrations of a ZTS crystal are analysed with the help of group theory.

Single Crystal Raman Scattering, Powder Infrared Spectroscopy and Thermal Properties of Rubidium Hydrogen Tartrate

Single crystals of rubidium hydrogen (+)-tartrate [RbH(C 4 H 4 O 6 )] (RbHT) were grown by slow evaporation of an aqueous solution at room temperature. The crystals have been studied by Raman scattering; infrared transmittance at room temperature was measured on powder samples. Tentative assignments have been made for both the internal and external vibrations by comparison with similar compounds. The vibrational modes are classified by the factor analysis method. We equally report the thermal behaviour of the substance in the temperature range of 300 to 700 K, using thermogravimetric analysis and differential scanning calorimetry techniques. RbHT is thermally stable up to 475 K; it starts decomposing without fusion at 570 K.

Raman scattering investigation with temperature of the phase transitions in (Pb0.825Ba0.175)ZrO3 and (Pb0.65Ba0.35)ZrO3 ceramics

Abstract Raman scattering investigations of lead-barium zirconate are presented for two barium concentrations, as a function of temperature from 10 to 450 K. An order-disorder phase transition is observed at 130 K for the low barium concentration, for which an antiferroelectric-ferroelectric phase transition is equally observed at 308 K and a ferroelectric-paraelectric one at 433 K. The ceramic containing a high barium concentration is in a ferroelectric state up to 403 K, above this temperature begins the paraelectric phase of the material. The order parameters of the two types of transitions, order-disorder and antiferroelectric-ferroelectric, are evaluated.

RAMAN AND INFRARED SPECTRA OF LITHIUM HYDROGEN MALATE LI+H(C4H4O5-)(C4H6O5)

Abstract Single crystals of lithium hydrogen malate (LHM) were grown by slow evaporation of aqueous solutions. The crystal structure was controlled by X-ray diffraction experiments. The Raman spectra of LHM were measured at room temperature and at 10 K. A tentative assignment of the various internal modes is proposed by comparison with similar compounds and characteristic frequencies of the organic groups. The Raman spectra did not reveal any low-temperature structural phase transition. The infrared reflectivity spectra were measured at 10 K and compared to the low-temperature polarized Raman spectra. The infrared spectral analysis revealed relatively weak LOTO splittings.

Correspondence between electro-optic properties and the raman scattering in oxydic perovskites

Abstract The physical origin of the large electro-optical properties exhibited by the oxygen-octahedra ferroelectrics is discussed in relation with the dynamical properties, as determined by Raman and infrared spectroscopy. The largest electro-optic coefficient is shown to correspond to the symmetry of the most intense Raman peak. The soft polar optical phonon is pointed out to be dominant in the ionic displacements contribution of the electro-optic coefficient. The role of the oxygen polarizability in the soft mode mechanism, the Raman intensity as well as in the electro-optic effect is emphasized for the ABO3 materials.

Accurate Measurement Of The Electro-Optic Coefficients : Application To LiNbO 3

A technique issued from the Senarmont method is developped in order to determine the electro-optic (E/0) coefficients of solid compounds. The transfer function of the transmitted beam is given by : I/Io=(1/2)[1±sin(Γ-2B], where Γ is the phase shift induced by the natural birefringence and the applied (AC +DC) electric field in the sample I and Io are, respectively, the output and input beam intensities, while B represents the analyser azimuthal angle. We show the various methods derived from the Senarmont arrangement which can be used to determine the E/0 coefficients. The method which provides both good accuracy and correctness is based on a direct determination of the phase shift when the DC and AC fundamental components of I are equal to zero and the frequency of the demodulated output transmitted beam is double of the AC frequency. The measurements take into account of the optical absorption of the set-up and the temperature dependence of the natural birefringence. This method is applied to the determination of the rc coefficient in LiNbO3 around room temperature.