Raman measurements have been carried out on the soft mode of thiourea and the wave vector and temperature dependence of the soft mode frequency ( 0 s ) are observed. The longitudinal ( w~L ) and transverse (OJST) frequencies are used to derive a comparison between the dielectric constant obtained from the LST relation and that obtained from standard measurements of the capacitance.Nous avons btudie en diffusion Raman la frbquence 05 du mode mou du monocristal de thiouree en fonction de la temperature et du vecteur d'onde. Nous avons observe les frequences longitudinale OSL et transverse WST en fonction de la temperature. Nous avons compare la valeur de la constante diblectrique statique 8; obtenue 21 partir de la relation de LST et celle obtenue par mesure de capacite.
The infrared reflection spectra of a single crystal of thiourea polarized along the ferroelectric axis [IOO], are extended up to very low frequencies with the help of E data obtained by soine authors. The whole reflectivity spectrum is fitted with a generalized dielectric constant model a t several temperatures. So for the first time all the dispersion parameters of the soft mode ( a comparison with Raman data is made) are obtained and the existence of two Debye relaxations is shown. One is attributed to domain all motion and the other to reorientation of molecules. With the help of a generalized LST relation, it is proved that the discontinuity of ~( 0 ) a t 169 I< is due to a relaxation process.Le spectre de reflexion infra-rouge dn cristaI de thiouree suivant l'axe ferroklectrique [ 1001 a pu 6tre prolong6 jusqu'aux tres basses frkquences en utilisant les valeurs de e dhterminkes par divers auteurs. Le spectre coniplet de reflcxion de 1000 Hz jusqu' B 100 cm-l a 6te aJust6 i pInsieurs tempkraturos & I'aide d'un modkle gknkralis6 de la constante di6lectrique. Ceci nous a perms d'obtenir pour la premiere fois, d'une part tous les pardmetres dc dispersion du soft mode (et de 1es comparcr avec les mesures obtenues en diffusion Kaman) et d'autre part, de montrer qu'il existe deux relaxations de type Debye dont I'une est attribube ail mouvement des parois de domaine et I'autre aux rkorientations des molhcule. A I'aide d'une relation de LST g6nbralisbe il est inontrb quc la discontinuit6 de la constante di6lectrique statique B 169 K est due essentiellomcnt & une relaxation de Debye.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.