DENSITY-FLUCTUATIONS IN THE NETWORK STRUCTURE OF AGI DOPED AGPO3 GLASS - THE ORIGIN OF THE PHONON-FRACTON CROSSOVER

Neutron diffraction studies of AgI doped AgPO3 glasses show the growth of a sharp diffraction peak at Q approximate to 0.7 Angstrom(-1) with increasing salt content, indicative of a change in intermediate range order. We show that this peak is due to local density fluctuations in the network of PO4 tetrahedra caused by the requirement to maintain c onnectivity while decreasing the average density. The phosphate backbo ne shows an inhomogeneous density distribution over regions of diamete r approximate to 16 Angstrom within which the number of atoms in a sph ere of radius R increases at a rate lower than R(3). It is proposed th at this local inhomogeneity is related to the observation of a ''phono n-fracton crossover'' in the dynamics of similar glasses.