VIBRATIONAL-SPECTRA OF HIGH-SILICA GLASSES OF THE SYSTEM K2O-SIO2-LA2O3

Parallel and perpendicular-polarized Raman spectra and KBr transmission IR spectra of glasses of the compositions 10K2O.50SiO2 - nLa2O3 (n = 0, 1, 5, 10 mol) and (10 - x)K2O-50SiO2-(x/3)La2O3 (x = 0, 6, 7.5) are presented. Increasing lanthanum concentration in glasses with fixed K:Si produces partially polarized high-frequency bands at 1030, 940 and 860 cm-1, in order of appearance. These are assigned to the symmetric stretching modes of Si-O bonds in SiO4 tetrahedra containing 1, 2 and 4 non-bridging oxygens, respectively, whose non-bridging oxygens coordinate with lanthanum. Since no disilicate or metasilicate crystals occur in the La2O3-SiO2 binary, lanthanum coordinates with silicate anions in glasses inaccessible to it in binary silicate crystals. Lanthanum forms coordination polyhedra of non-bridging oxygen without the aid of potassium; therefore, the solution chemistry of lanthanum in these glasses is governed by homogeneous equilibria between different silicate anions bonded to lanthanum. Glass-glass phase separation of 10K2O-50SiO2-10La2O3 when heated to 900-degrees-C indicates that these homogeneous equilibria produce regions within the glass relatively rich in La-O-Si bonds but depleted in Si-O-Si linkages.