Glass formation in alkali molybdate systems

Glass formation in alkali molybdate systems is investigated through measurements of the critical cooling rate, i.e. the cooling rate necessary to prevent crystallization entirely. Similarities and differences with alkali tungstate systems are noted, and the latter are explained by postulating a more continuous transition of the molybdenum ion from tetrahedral to octahedral coordination with decreasing alkali oxide content of the systems as compared with the behavior of tungstates, where tetrahedral coordination is found in all glasses. In addition, disproportionation of dimolybdate ions into monomers and higher polymeric units is assumed to be less pronounced in molybdate systems than the disproportionation of ditungstate ions in the corresponding tungstate systems. Density and thermal expansion data on molten alkali molybdate systems indicate that glass forming melts have a relatively spacious structure, in accordance with the idea that glass formation is facilitated by the formation of chains of (distorted) tetrahedra.