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

Abstract

The atomic scale structure of network glass forming systems with theAX2 stoichiometry is considered and particular attention is paid to the systemsGeSe2,ZnCl2 and GeO2 for which the partial-pair correlation functions have been measured by using the method ofisotopic substitution in neutron diffraction. The basic structural motif in all of these systems is theA(X1/2)4 tetrahedron, although homopolar bonds are also a significant feature in the case of liquid and glassyGeSe2. The structural motifs link to form a network in which ordering occurs on two differentlength scales at distances greater than the nearest-neighbour. One of these length scales isassociated with an intermediate range and manifests itself by the appearance of a so-calledfirst sharp diffraction peak in the measured diffraction patterns at a scattering vectorkFSDP where and rAX is the nearest-neighbour distance for unlike chemical species. The other is associated withan extended range, which has a periodicity given by , where kPP denotes the scattering vector of the principal peak and . The nature and interplay between the ordering on the intermediate and extended lengthscales is characterized and discussed.