Active Lattice Modes Research Articles

Temperature dependence of the Raman lines of α-Al 2O 3

The temperature dependence of the frequencies and line-widths of the Raman active lattice modes of α-Al 2O 3 has been measured between 77° and 1200°K. The temperature dependence of the frequencies is compared with the results of Schauer based on a thermal expansion model and the temperature dependence of the linewidths is compared with the results from a simple two and three phonon decay model.

Infra-red lattice bands of trigonal tellurium and selenium

The room temperature reflectivity spectra of trigonal Te and Se have been fit using a harmonic oscillator formalism. The effective charge associated with the infra-red active lattice modes is high (∼ 0.15 e to 2.2 e) and is attributed entirely to charge redistribution effects. A calculation of the polaron coupling constants for Te and Se indicates that polar mode scattering is important in room temperature transport phenomena.

Lattice vibrations of orthorhombic polyoxymethylene

The lowest line group fundamental vibration of orthorhombic polyoxymethylene has been located. Optically active translational lattice modes have been found at 130 and 89 cm −1. The in-phase rotational mode occurs at 83 cm −1. Remarkable shifts toward higher frequencies have been observed when the temperature is lowered to ∼120°K.

Normal Modes in Hexagonal Boron Nitride

Pyrolytic boron nitride, a rather anisotropic material among the III-V compounds, has been investigated in reflection and transmission, using polarized infrared radiation in the spectral range from 100 ${\mathrm{cm}}^{\ensuremath{-}1}$ to 3000 ${\mathrm{cm}}^{\ensuremath{-}1}$. The reflection spectra have been analyzed by means of a Kramers-Kronig analysis and a classical oscillator fit. The results indicate one infrared active lattice mode for in-plane motion of the particles and one for out-of-plane motion, in accordance with the boron-nitride structure reported by Pease and the resulting group-theoretical selection rules. In addition, one Raman line has been found corresponding to a lattice mode for in-plane motion. Furthermore, the absorption spectrum exhibits several minor peaks which are probably due to 2-phonon combination processes.

Dielectric Dispersion of Some Perovskite Zirconates

Abstract : The dielectric properties of perovskites have been the subject of many investigations, but a direct correlation between the dielectric constant and a normal optically active lattice mode has been convincingly established only in the case of the paraelectric strontium titanate. The identification of this so-called 'soft' mode is still debated. Such an assignment of the normal-mode spectrum in perovskites is now achieved by a comparison of the normal modes of titanates and zirconates. In addition, evidence is presented that also in the case of an antiferroelectric material, lead zirconate, the low-frequency dielectric constant is the result of a low-frequency optically active mode, at K = O, whose contribution tothe dielectric constant accounts for about 90% of its value at room temperature. (Author)