Phase transitions in vitreous and amorphous SiO 1

As a rule, glassy and amorphous solids are characterized by random, irregular structures. More recent examinations, however, indicated that even this group of materials possesses certain structural laws. Glassy and amorphous SiO 2 samples were investigated using X‐ray diffraction. The results of this investigation can be interpreted as structural phase transitions. When subjected to a pressure‐temperature treatment ( p = 7 GPa, T = 700 °C), a 15 per cent irreversible densification of vitreous silica without crystallization could be achieved. This can be interpreted as a reconstructive transformation of a certain topological structure (vitreous silica having a cristobalite topology) into another topological structure (densified vitreous silica having a quartz topology). Temperature‐dependent measurements of disperse SiO 2 (porous glass, silica gels) led to the conclusion that a displacive phase transition (analogous to the α‐β‐transition in cristobalite) had taken place in the amorphous network.