Phase transformation in the Anortite-Gehlenite glass system

The effects of replacing SiO2 by CaO (mol basis) in glasses from the pseudobinary anorthite-gehlenite system on the glass structure and devitrification behavior were studied by DTA and x-ray diffraction anal. Glass structure stability (resistance to devitrification) of intermediate compns. was high compared with that of end-member glasses

LOW-FREQUENCY DYNAMICS OF GLASSES AND CRYSTALS OF SIMILAR COMPOSITIONS

Wide range (10–1500 cm-1) dielectric and Raman spectra of crystals and glasses of similar compositions were analysed with the use of both literature (a-quartz, cristobalite, LaBGeO5, Pb5Ge3O11, As2Se3, etc.) and original (Li2Ge7O15, LiBaPO4) data. For all structural types (island, chain, layer or frame like), the boson peak (BP)in the glass spectrum (GS) corresponds to one or several bands in the low frequency (LF) spectrum of the corresponding crystal. In this sense, the LF range (<200 cm-1) is not a specific feature of GS as compared to higher frequency range of stretching and bending modes, where the crystal spectral bands are widened due to disordering and/or vitrification up to their full conversion into a diffuse GS. Therefore the origin of LF bands in GS can be identified taking into account the nature of corresponding bonds in the crystal spectrum by analogy with well known procedure of interpretation of stretching and bending vibrations in GS. The approach proposed is exemplified by different systems. Lead germanate glasses reveal several highly overlapping LF peaks, which originate from optical vibrations of lead atoms by analogy with Pb5Ge3O11 crystal spectrum. The Li2Ge7O15 glass and crystal LF peaks correspond to vibrations of oxygen in layer fragments of the structure. Cristobalite-like structure of silica glass is proposed on the assumption of coincidence of peak frequencies in their Raman spectra at ~50 cm-1, whereas a-quartz has no vibrational modes below 100 cm-1. Thus, the approach proposed allows one to refine and concretise the universal nature of the BP, referring to the short and medium range structure and the vibrational properties of the corresponding crystal