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, Li2Ti03-Mg0 and LizTi03-NazTi03, are presented. It is proposed that positive departures from Vegard's law in non-metzllic 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. Both show immiscibility domes and positive departures from Vegard's law.

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Two mechanisms of doping Li(3)NbO(4), which has an ordered, rock salt superstructure, have been established. In the "stoichiometric mechanism", the overall cation-to-anion ratio is maintained at 1:1 by means of the substitution 3Li(+) + Nb(5+) --> 4Ni(2+). In the "vacancy mechanism", Li(+) ion vacancies are created by means of the substitution 2Li(+) --> Ni(2+). Solid solution ranges have been determined for both mechanisms and a partial phase diagram constructed for the stoichiometric join. On the vacancy join, the substitution mechanism has been confirmed by powder neutron diffraction; associated with lithium vacancy creation, a dramatic increase in Li(+) ion conductivity occurs with increasing Ni content, reaching a value of 5 x 10(-4) Omega(-1) cm(-1) at 300 degrees C for composition x= 0.1 in the formula Li(3-2x)Ni(x)NbO(4). This is the first example of high Li(+) ion conductivity in complex oxides with rock salt-related structures.

The new phase Li,Ni2Ta06 has a novel rock salt superstructure, a = 8.4259(3) A, b = 5.9073(3) A, c = 17.7329 A, Fddd. Within the cubic-close-packed oxide array, Ta occupies one set of octahedral sites giving isolated TaO, octahedra which edge-share with Li/NiO, octahedra. Li and Ni are distributed non-randomly over three other sets of octahedral sites with Li : Ni occupancy ratios of 0.73 : 0.27, 0.59 : 0.41 and 0.55 : 0.45, respectively. The higher Li : Ni ratio of the first set, 0.73:0.27 may be associated with cation-cation repulsion effects since this site is closest to Ta. Li,Ni,Ta06 is a very modest semiconductor with conductivity of ca. 4 x l o p 6 R-' cm-' at 300 "C and activation energy 0.77 eV; the disorder in the Li' site occupancy is, therefore, static and does not yield significant levels of Li+ ion conductivity.

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