Raman and infrared structural investigation of xRb(2)O center dot(1-x)GeO2 glasses

Infrared reflectance and Raman spectra of rubidium germanate glasses, xRb(2)O .(1 - x)GeO2, have been measured and studied as a function of Rb2O mole fraction in the range 0 less than or equal to x less than or equal to 0.60. The spectra of corresponding crystalline germanate phases, GeO2-quartz, GeO2-rutile, Rb2Ge4O9, K4Ge9O20, K2Ge2O5, and K2GeO3, have been also measured for a better understanding of the glass spectra. At very low content (x less than or equal to 0.01) Rb2O was found to induce a redistribution of GeO4 tetrahedra in rings smaller than those encountered in quartz- and glassy GeO2. The spectral manifestations of this trend are the frequency increase of the Raman active symmetric stretching vibration of Ge-O-Ge bridges and the parallel decrease of the infrared frequency of the asymmetric stretching of Ge-O-Ge bridges, Beyond the composition x = 0.01 and up to x approximate to 0.15-0.20, the predominant modification mechanism involves the transformation of GeO4 tetrahedra into GeO6 octahedra, with characteristic Raman scattering at 315, 590, and 630 cm(-1). At higher alkali content and up to the limits of glass formation, tetrahedral Q(n) species with nonbridging oxygens are formed ((4 - n) is the number of nonbridging oxygens per tetrahedron). Thus, Q(3) appears beyond x = 0.10, giving place to Q(2) (x > 0.30), Q(1) (x > 0.50), and Q(0) (x = 0.60). The characteristic Raman bands of these germanate tetrahedra were measured at 865, 765, 740, and 720 cm(-1), respectively, Based on semiempirical models, the spectroscopic results were quantified to yield the average coordination number of Ge atoms, as well as the fraction of Ge atoms in 6-fold coordination. Both were found to exhibit a maximum value at approximately 15 mol % Rb2O.