Exploitable results
The existence of inhomogeneities on supermolecular scale in liquids and amorphous solids is one of the immanent consequences of disorder. Two classes of local deviations of the properties of a medium from those averaged over macroscopic volumes are to be distinguished: first, «equilibrium» fluctuations of thermodynamic variables (functions of state) which reflect the thermal motion and as a consequence depend not only on coordinates but on time as well; second, structural inhomogeneities which in glasses are static («frozen-in»). These latter, which sometimes are said to form the structure of intermediate, or medium, are here called microinhomogeneities (MIH)
The investigation of MIH has drawn considerable attention in the last years The interest in them is stimulated by a wide range of features where MIH play an important role. As examples it is possible to mention the composition and production of optical glasses with minimal light losses, and the influence of the MIH on damping processes of the vibrational frequencies. Of no lesser importance is the necessity of developing the general understanding of the nature of structural disorder both in liquid and glassy state. The possibility to study important features of disorder in the frozen, static state gives special advantage to the investigations of glasses as a means for achieving this fundamental goal.
Details of the supermolecular microinhomogeneous structure are individual features of each glassforming system. Establishing general laws and clarifying the character of their manifestation in specific cases are possible by comparing different systems with systematically varied chemical structures and compositions. The present work aims at filling up substantial gaps in this general line. Its centre is a comprehensive study of MIH in binary glassforming systems containing lead oxide as a network modifier. Unlike more common, alkali and alkaliearth modifiers, lead oxide has its own network forming tendency which results, as demonstrated in the first stage of the work with silicium oxide as the main network former (project INTAS 93-1316), in peculiarities of MIH behavior. The study of a similar system with germanium oxide as the main network former has been set as the central direction of the present, extended project. Related investigations of some other hitherto unstudied systems has been also performed as well as some related theoretical work of more general character.
A set of. samples of lead oxide-germanium oxide glasses of varying compositions from pure GeO2 up to 70 mol.% PbO have been synthesized in the Inst. of Silicate Chemistry, Russian Academy of Sciences, S-Pb, by G.A. Sycheva and of V.V. Golubkov and used by all S-Pb groups for studies of small angle X-ray scattering in glasses and melts, ultrasonic velocity measurements in melts, Raman light scattering in glasses. Glasses have been prepared by melting in platinum crucible at 1400-1600 C (depending on the composition) with permanent stirring. In the case of low lead content glasses periodical handle stirring was used. The glasses containing from 3 to 70 mol.% PbO (as compared to the higher limit 50 mol.% found in literature) were obtained
In spite of all attempts the synthesized samples had not sufficient optical quality for measurements of Rayleigh-MB scattering of light; thus these measurements were restricted to one-component GeO2 and extended to some alkali-germanate systems also studied ultrasonically
Inelastic neutron scattering made on PbO-SiO2 glasses gave the g(w) for different concentrations of PbO and different temperatures in the glass and the supercooled liquid. These values were used for the evaluation of the coupling coefficient C(w) by comparison with Raman scattering.
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