FAR-INFRARED SPECTRA OF ALKALI GERMANATE GLASSES AND CORRELATION WITHELECTRICAL-CONDUCTIVITY

The infrared spectra of 0.20R(2)O-0.80GeO(2) (R=Li,Na,K,Rb,Cs) and xRb (2)O-(1-x)GeO2 (0<x less than or equal to 0.27) glasses were measured in the reflectance mode and analyzed by the Kramers-Kronig technique t o 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 less than or equal to 0.075 for R=Rb) alkali ions occupy one type of sites (M), while for hi gher alkali contents two types of site (L and H) were found. The ion m otion frequencies in these sites are in the order nu(L)<nu(M)<nu(H), a nd increase with increasing alkali oxide content. Factor group analysi s of the alkali motion modes in analogous crystalline germanate compou nds showed that the H band in glass can be assigned to ion motion in s ites similar to those in the crystal. The low-frequency band (L) was a ttributed to ion motion in ''secondary'' energetic sites, whose coordi nation numbers and charge density are correspondingly larger and small er than their optimum values. The presence of L sites is the cause of the extra absorption exhibited by glasses at low far-infrared frequenc ies, as compared to the crystals of similar composition. M-type sites were shown to be the precursors of H sites, but for the organization o f the latter a minimum alkali oxide content is required. The compariso n of activation energies for conductivity calculated on the basis of t he free-ion model with observed values suggests that long-range ion mo vement is probably facilitated dong M and H-type sites.