Medium range order in glass and the `germanate anomalyʹ effect

The origin of extrema in the property-composition curves of germanate glasses has been a much debated issue. Using density and structural data for K- and Rb-germanate glasses we find that the short-range order structure alone cannot reproduce the maximum in density. By combining short- and medium-range order structures we calculate the dependence of density on composition in remarkably good agreement with experiment. The results show that the maximum in density can be traced to the coexistence of germanate octahedral units with compact three-membered rings of tetrahedral units.