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.

The nature of the phonon and magnon modes in the CoCr 2 O 4 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 (T C = 94 K) and two low symmetry (T S ≈ 26 K, T lock in = 14.5 K) phases of CoCr 2 O 4 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 mag netic and phonon subsystems. The increase by 25% of the damping parameter of the phonon mode originat ing from vibrations of the CoO 4 tetrahedra during the transition of CoCr 2 O 4 to the multiferroic state (T < T S ) suggests structural changes in the lattice involving loss of spatial central symmetry of the medium.

Polarized Raman spectra of [N(CH3)4]2ZnCl4 crystals are studied in phase I in the frequency range between 10 and 3500 cm−1. An assignment of the lines observed to motion types is carried out. The sublattice of the TMA ions is found to behave as a quasi‐liquid. The distorsion of the ZnCl 42‐ tetrahedra is insignificant. A considerable orientational disorder is shown to occur in this phase.

The Raman spectra of the superprotonic conductor Cs,H,(S0,),.H20 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.

Polarized Raman spectra of Cs 5 H 3 (SO 4 ) 4 • H 2 O (PCHS) single crystals were measured in the frequency range 0-4000 cm −1 at 20-300 K, i.e. both below and above the glass transition temperature (T g ). The evolution of the spectra with decreasing temperature indicates that the orientational disorder of SO 2− 4 ions and H 2 O molecules changes at T g 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 H 3 (SO 4 ) 5− 4 .

Yu. I. DURXEV et al.: Vibrational Spectra and Phase Transitions (I)

The electroreflectance (ER) of SbSI has been measured in a wide temperature range between 1.6 and 3.3 eV with linearly polarized light. The technique of transverse ER was used and the first and second harmonics of ER were measured simultaneously. The effects of domain reorientation, complementary light illumination, the specimen damping and electric field vector rotation on the sample surface are investigated. It is shown: (a) that the induced reflectivity change is not caused by purely Kern-Harbekeeffect, but depends on the piezoelectric strain; (b) that separated from the deformation component, the ER spectrum is close to the hmodulation spectrum and may be interpreted in a similar way.

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