P. Becker

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.

Low temperature phase transitions in zinc tris (Thiourea) sulfate (ZTS) determined by Raman scattering

Phase transitions in zinc tris (thiourea) sulfate (ZTS), occurring below room temperature at (60 ± 2) K and (122 ± 2) K, respectively, are investigated by measuring polarized Raman scattering between 10 and 300 K in the 400 to 800 cm -1 range. A careful examination of the evolution of the wavenumber, intensity, and width of thermosensitive bands, appearing at 520 cm -1 in the spectra at 10 K, near the transition temperature, is helpful in deciding about the type of phase transition. The transitions are found to be of an order-disorder nature. The values of the estimated critical exponents are 2β = 0.57, δ = 0.92 for the transition at 60 K and 2β = 0.92, δ = 0.77 for the transition at 122 K. It is determined that the mechanism of the transition appears to be closely related to the behavior of the thiourea molecule.

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.

Thermal and Spectroscopic Study of Dehydration of Lithium Formate Monohydrate Single-crystals

Differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) of lithium formate monohydrate (LiHCOO·H2O) were performed in the temperature range 300–700 K. The DSC/TG measurements show that the dehydration process to anhydrous lithium formate (LiHCOO) is complex and occurs in two stages. The data are correlated to the structure and to the arrangement of the molecules in the crystal, including the hydrogen-bonding. Infrared transmittance and Raman spectra of this crystal are reported and commented on.

Crystal growth and vibrationnal spectra of lithium hydrogen malate LiH3(C4H4O5)2

Abstract Single crystals of Lithium Hydrogen Malate (LHM) have been grown by slow evaporation of aqueous solutions. The crystals structures have been controlled by X-rays diffraction experiments. Differential calorimetry (DCA) and Thermogravimetry (TGA) analysis performed on LHM showed no thermal anomaly from room temperature to the start of decomposition at 440K indicating the absence of any high temperature phase transition. The Raman and Infrared reflectivity spectra of LHM were measured at room temperature and at 10K, they did not reveal either any low temperature structural phase transition.

Vibrational spectra of a Ca-Nb-Ga garnet (CNGG) single crystal studied by Raman and infrared reflectivity spectroscopy

Calcium niobium gallium garnet (CNGG), a new non-linear optical material, is studied by Raman and infrared reflectivity spectroscopy at low and at room temperature for various scattering configurations. No structural phase transition is observed in the temperature range of the experiments. The phonons are classified within their symmetry species in the space group assumption; their dynamical parameters such as wavenumber, damping and oscillator strength are obtained on the basis of the infrared reflectivity spectra analysis. The comparison of the infrared and Raman activity of the phonon modes is in favour of the aforementioned space group; however, Pockels electrooptic coefficient room temperature measurements did not allow us to confirm this space group.