Medium-range order in amorphous materials

Abstract

Specification of medium-range order (MRO) based on different definitions of MRO is given. The extent of MRO and the significance of dihedral angle distributions is discussed. X-ray scattering was calculated (in the region s=0.5 to 9 Å−1) for GeS2, As2S3 and X-ray and neutron scattering for GeO2 in order to find dominant configurations and-or processes of their ordering which lead to the development of the first sharp diffraction peak (FSDP). In GeS2 we studied the effect of the intercluster distance and of mutual desorientation of dominant configurations (part of ribbons of a H.T. GeS2 layer) on the position and intensity of the FSDP. Calculations are accompanied by intercluster atomic distance distributions which document the complex character of MRO. Calculation of H.P. dependent X-ray scattering shows that a variation of H.P. from O to 90 kbar corresponds to a decrease of the intercluster distance of about 1.5 Å. In As2S3 dominant configurations influencing the development of the FSDP are ordered band-like configurations which align short sections of neighbouring helices. In GeO2 the dominant configurations are parts of helices. Simulation of H.P. X-ray scattering shows that H.P. increases the MRO in the direction of helical axis and at the same time distorts the short-range order.