Ramanstreuung an gemischtvalentem Thuliumselenid und einigen anderen Chalkogeniden seltener Erden

Abstract

The wellknown macroscopic elastic anomalies of interme¬ diate valence compounds have stimulated wide speculations about the existence of phonon anomalies, but direct evidence from experiments has been missing. Therefore a Raman investi¬ gation of the phononic properties of mixed valent Tm com¬ pounds was performed. The Raman spectra of metallic chalcogenides with fee crystal structure show a defectinduced one phonon density of states. The trivalent Gd and La chalcogenides were used as reference materials without specific phonon anomalies to study the basic scattering mechanism and the in¬ fluences of sample stoichiometry and preparation on the Raman spectra. The Raman bands could be identified with distinct phonon branches by comparison with the wellknown semiconduc¬ ting europium chalcogenides. In the ideal intermediate valent system Tm Se the valence mixing can be controlled in a wide range by the sample stoi¬ chiometry. In the Raman spectra two phonon anomalies are ob¬ served: a softening of the zone boundary LO frequency linear with the degree of valence mixing and an anomalous scattering peak in the acoustic phonon range around 60 cm . The fully symmetric r. Raman tensor contributes most of the scattering intensity. The anomalies are discussed in terms of a breathing 13 12 deformation model. The degeneracy of the 4f and the 4f 5d states with different ionic radii gives rise to an enormous monopolar charge deformability of the Tm ions. The LO phonons at the L point of the Brillouin zone and the LA branch half¬ way between the r and L point couple most strongly to this r. deformability. The strong coupling of phonons and charge fluc¬ tuations is responsible for a frequency softening and the ob¬ served shift of the LO band. Due to a symmetry dependent en¬ hancement of the electron-phonon matrix elements an increased scattering contribution from these modes is projected into the Raman density of states. The renormalized LA(CSC) phonons ap¬ pear as a distinct band around 60 cm in the Raman spectra. This view is further established experimentally by the inve¬ stigation of the mixed systems TmSe, Te and Tm._ Eu Se, A~X X J. X X where the degree of valence mixing is changed by chemical substitution. In the semiconducting Tm, Eu Se the defectinduced den¬ sity of states is superimposed by a multiphonon structure with to. = 152 cm . From the temperature dependence and the polarization analysis which displays a predominant antisym¬ metric r,s scattering, a spin disorder scattering from zone boundary LO phonons is established. Concerning the old con¬ troversy between the spin disorder process and hot lumines¬ cence as the scattering mechanism in the europium chalcogenides an argument in favour of the spin disorder model is deduced from the investigation of impurity spectra of Tel¬ lurium compounds.