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J. L. SERVOIN and B. PIRIOU: Infrared Reflectivity and Raman Scattering 677 phys. stat. sol. (b) 56, 677 (1973) Subject classification: 6 and 20.1; 22.8.1 U.E. B. Sciences Fondamentales et Appliqukes, Cniuersite' d'0rlhans ( a ) and Centre de Recherches sur la Physique des Hautes Tempkratures, C.N.R.S., Orldans ( b ) Infrared Reflectivity and Raman Scattering of Mg,SiO, Single Crystal BY J. L. SERVOIN (a) and B. PIRIOU (b)The lattice vibrations of Mg,SiO, forsterite single crystal have been studied using infrared and Raman techniques. The site symmetry method allows to study the degeneracy lifting of the silicate ion vibrational levels and to assign the modes of vibration (45 infrared active modes and 36 Raman active modes), as internal or external. The analysis of the reflectivity data between 150 and 10000 cm-l by classical dispersion theory allows to find the oscillator parameter values. Most of the infrared lattice bands could be identified. All the predicted modes have been observed in the Raman spectra and the vl, v,, and v4 lines have been identified. For some modes, specially in the low frequency range, the difference between internal and external modes is not obvious, taking into account the SiO, tetrahedra deformation.Noiis avons btudiC, par reflexion infrarouge et diffusion Raman. les vibrations de rbseau d'un monocristal de forsterite Mg,Si04. La mkthode du site nous a permis d'etudier la levee de dCg6nbrescence des vibrations de l'ion silicate e t de classer les modes (35 actifs infrarouge et 36 actifs Raman) en internes e t externes. L'analyse des spectres de rkflexion observks entre 150 et 10000 cm-l par les relations classiques de dispersion nous a permis de determiner les parametres associbs aux modes B1 I,, Bzl, et Bsu. Nous avons interprkt6 la majeure partie des modes infrarouge. Par diffusion Raman, nous avons observe la totalitk des raies prkvues e t nous avons identifik les modes issus de Y,, e t v4. Pour certains modes, notamment de basse frkquence, la distinction entre modes externes et internes doit dtre nuanche, compte tenu de la deformation des tetraedres SiO,.

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The temperature dependence of infrared reAection spectra of single crystals of n-type strontium titanate doped with 0.3 to 1.5 at. % niobium are reported from 10 to 5 000 cm in the temperature range 80-900 K. Results are analyzed by means of various factorized forms of the dielectric function. A decoupling scheme allows the temperature dependence of the plasmon to be deduced. The plasma frequency of the carriers is found to increase upon cooling, a very unusual phenomenon which, together with Hall-effect measurements, may imply that the effective mass of the carriers decreases upon cooling. The dependence of the soft ferroelectric mode on temperature and Nb doping is also reported and discussed.

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2014 On compare les données reçues de mesures précises de réflectivité infrarouge et de constantes diélectriques 03B50(T) dans certains ferroélectriques ABO3. Dans BaTiO3, le mode ferroélectrique se sature au-dessus de la transition cubique tétragonale tandis que 03B50(T) tend à diverger. Ceci s'interprète en terme de changement de régime dominant, passant de displacif à ordre-désordre à l'approche de Tc, en accord avec la dimensionalité critique dans les systèmes cubiques dipolaires dc = 4 > d = 3. Dans le composé uniaxe LiTaO3 où d = dc = 3, le phénomène est en effet moins marqué.Abstract 2014 A detailed comparison between accurate infrared reflectivity data and dielectric constant 03B50(T) measurements has been carried out in ABO3 ferroelectrics. In BaTiO3, the ferroelectric mode saturates above the cubic-tetragonal transition, whereas 03B50(T) tends to diverge. This supports an intrinsic crossover from displacive to order-disorder behaviour on approaching Tc because the critical dimensionality in these cubic dipolar systems is dc = 4 > d = 3. In the uniaxial LiTaO3 with d = dc = 3, the crossover is less pronounced.