Inorganic glasses: old and new structures on the eve of the 21st century

History has shown that glass is a remarkable noncrystalline substance, usually made naturally or artificially from the simplest raw materials. Mimicking evolution however, mankind has been responsible for the creation of new families of a wide variety of glasses which gradually appeared through creative-thinking particularly during last hundred years. The most discussed issue is the thermodynamic stability of the glassy state as a special form of matter with its low-dimensional organisational structure, as well as its classification within the hierarchy level of noncrystalline solids. In this respect the most important is entropy. We can say that the major part of the entropy under Vogels temperature, T_v, has its origin in the thermal entropy contribution, W_th. When the temperature becomes higher than T_v, the configurational part of the entropy, W _cf, starts to play a role. This W_cf part is mainly connected to the micro-configurational displacements of particles. At, and above, the glass transition temperature, T_g, the conformational part of entropy, W_conf, which is connected to the displacements of particles through diffusion in the macro-sample is involved. It seems that liquids above the T_g transition are formed by two mechanically distinct “species”. Under the T _g temperature, a matrix system is formed, in vast majority, by particles excited just to the lower level of the amplitude of an anharmonic oscillator. Above T_g, the second “species” starts to appear which is formed by thermally-excited particles able to overcome viscous and elastic forces of the matrix in their vicinity and bring the particles, through thermal excitement and interactions within their vicinity, to the upper amplitude levels of a nonlinear oscillator. The thermally excited particles thus form the active and ephemeral vacancy spaces. These vacancies have very high expansion coefficient and are responsible for high expansion coefficient of liquids in general