Far-infrared spectra of alkali germanate glasses and correlation with electrical conductivity

Abstract

The infrared spectra of 0.20${\mathit{R}}_{2}$O-0.${80\mathrm{G}\mathrm{e}\mathrm{O}}_{2}$ (R=Li,Na,K,Rb,Cs) and x${\mathrm{Rb}}_{2}$O-(1-x)${\mathrm{GeO}}_{2}$ (0x\ensuremath{\leqslant}0.27) glasses were measured in the reflectance mode and analyzed by the Kramers-Kronig technique to investigate the nature and composition dependence of metal ion sites in germanate glasses. The deconvolution of the far-infrared profiles showed that in glasses of low alkali content (x\ensuremath{\leqslant}0.075 for R=Rb) alkali ions occupy one type of sites (M), while for higher alkali contents two types of site (L and H) were found. The ion motion frequencies in these sites are in the order ${\ensuremath{\nu}}_{\mathit{L}}$${\ensuremath{\nu}}_{\mathit{M}}$${\ensuremath{\nu}}_{\mathit{H}}$, and increase with increasing alkali oxide content. Factor group analysis of the alkali motion modes in analogous crystalline germanate compounds showed that the H band in glass can be assigned to ion motion in sites similar to those in the crystal. The low-frequency band (L) was attributed to ion motion in ``secondary'' energetic sites, whose coordination numbers and charge density are correspondingly larger and smaller than their optimum values. The presence of L sites is the cause of the extra absorption exhibited by glasses at low far-infrared frequencies, as compared to the crystals of similar composition. M-type sites were shown to be the precursors of H sites, but for the organization of the latter a minimum alkali oxide content is required. The comparison of activation energies for conductivity calculated on the basis of the free-ion model with observed values suggests that long-range ion movement is probably facilitated along M and H-type sites. \textcopyright{} 1996 The American Physical Society.