Deviations from Vegard's law in oxide solid solutions. The systems Li2TiO3–MgO and Li2TiO3–Na2TiO3

Several oxide solid-solution systems are known that show both (a) positive departures from Vegard's law in the plots of lattice parameter against composition for quenched, macroscopically homogeneous samples and (b) depressed immiscibility domes within the T–x field of the solid solutions on the phase diagrams. Experimental results on two systems, Li2TiO3–MgO and Li2TiO3–Na2TiO3, are presented. Both show immiscibility domes and positive departures from Vegard's law. It is proposed that positive departures from Vegard's law in non-metallic solid solutions may indicate the presence of clustering, due to incipient unmixing, within apparently homogeneous, single phase solid solutions. Non-metallic solid solutions appear to behave more simply than alloys and possible reasons for this are discussed.