Formation of contact and multiple cyclic cassiterite twins in SnO 2 -based ceramics co-doped with cobalt and niobium oxides.

Contact and multiple cyclic twins of cassiterite commonly form in SnO 2 -based ceramics when SnO 2 is sintered with small additions of cobalt and niobium oxides (dual doping). In this work, it is shown that the formation of twins is a two-stage process that starts with epitaxial growth of SnO 2 on CoNb 2 O 6 and Co 4 Nb 2 O 9 seeds (twin nucleation stage) and continues with the fast growth of (101) twin contacts (twin growth stage). Both secondary phases form below the temperature of enhanced densification and SnO 2 grain growth; CoNb 2 O 6 forms at ∼700°C and Co 4 Nb 2 O 9 at ∼900°C. They are structurally related to the rutile-type cassiterite and can thus trigger oriented (epitaxial) growth (local recrystallization) of SnO 2 domains in different orientations on a single seed particle. While oriented growth of cassiterite on columbite-type CoNb 2 O 6 grains can only result in the formation of contact twins, the Co 4 Nb 2 O 9 grains with a structure comparable with that of corundum represent suitable sites for the nucleation of contact and multiple cyclic twins with coplanar or alternating morphology. The twin nucleation stage is followed by fast densification accompanied by significant SnO 2 grain growth above 1300°C. The twin nuclei coarsen to large twinned grains as a result of the preferential and fast growth of the low-energy (101) twin contacts. The solid-state diffusion processes during densification and SnO 2 grain growth are controlled by the formation of point defects and result in the dissolution of the twin nuclei and the incorporation of Nb 5+ and Co 2+ ions into the SnO 2 matrix in the form of a solid solution. In this process, the twin nuclei are erased and their role in the formation of twins is shown only by irregular segregation of Co and Nb to the twin boundaries and inside the cassiterite grains, and Co,Nb-enrichment in the cyclic twin cores.