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Available data for the first sharp diffraction peak (FSDP), which is observed in many amorphous substances including glasses, is analysed from the point of view of the glass-forming ability (GFA) of a substance. To clarify the subject, the ways of defining GFA and numerical evaluation are discussed first of all. It is shown that, in contrast to intuition, there is no direct connection between FSDP and GFA. Moreover, a non-glass-forming melt or a glass at the boundary of the glass-forming region may demonstrate strong and narrow FSDP. On the other hand, the obtained correlations between FSDP characteristics (position, intensity, halfwidth) and sample parameters, both internal (chemical composition, short-range order) and external (temperature, pressure), need explanations in terms of models that try to understand the origin of FSDP and corresponding medium-range order in the amorphous state, the glassy state being a particular case.

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Chalcogenide glasses of the Se-S system (0-10 at % S) crystallized under the influence of a cav itating ultrasonic field at temperatures of 65 and 73°C. The process was evaluated with respect to spectra of optical transmission in the interval of 400-5000 cm -1 . Layer by layer removal of the surface of a sample with subsequent evaluation of its transparence was performed for assessment of the heterogeneity of crystalliza tion. It is shown that the crystallization sharply intensifies under the simultaneous impact of temperature and a variable field of elastic stresses, in comparison with the crystallization under their separate action. Optical effects exhibit nonlinearity by glass composition and an anisotropy concerning the position of a US source. A complicated hierarchical pattern of the nucleation stage is shown by electron microscopy. The results are construed in the context of the synergetic approach, and hypervalent bonds based on S.A. Dembovskii are viewed as a microscopic base of self organization.

It is shown tha ¶ there exists a numerical relalionship between the pasition of the 6rsl sharp diffraction peak. Q j . and the fin1 intemiomic distance. n. which obeys a simple equalion d = o r ] . t h where d = ZnlQl is Ihe equivalent dislance and the cwfficients (a. b) specify the p u p of amorphous subsmces-@asses. amorphous m e a and semiconductors. lrquids of a definite nalure. The scale of medium-range order = djri is introduced and it is pmposed that the S i ! plot could be used for classification of amorphous suuctures.