The structure of tetrahedral network glass forming systems at intermediate and extended length scales

The atomic scale structure of network glass forming systems with the AX 2 stoichiometry is considered and particular attention is paid to the systems GeSe 2 , ZnCl 2 and GeO 2 for which the partial-pair correlation functions have been measured by using the method of isotopic substitution in neutron diffraction. The basic structural motif in all of these systems is the A(X 1/2 ) 4 tetrahedron, although homopolar bonds are also a significant feature in the case of liquid and glassy GeSe 2 . The structural motifs link to form a network in which ordering occurs on two different length scales at distances greater than the nearest-neighbour. One of these length scales is associated with an intermediate range and manifests itself by the appearance of a so-called first sharp diffraction peak in the measured diffraction patterns at a scattering vector k FSDP where and r AX is the nearest-neighbour distance for unlike chemical species. The other is associated with an extended range, which has a periodicity given by , where k PP denotes the scattering vector of the principal peak and . The nature and interplay between the ordering on the intermediate and extended length scales is characterized and discussed.