It was recalled that the reduction of rutile in the presence of a borate flux stabilized the phase, due to the incorporation of B from the flux. The B incorporation was accompanied by the reduction of Ti, according to the charge-compensation relationship: 3Ti4+ = 3Ti3+ + B3+ (interstitial). It was proposed that, at the temperatures used, local defects in B-doped rutile resulted from the displacement of Ti atoms to adjacent interstitial sites, coupled with the occupation by B of the triangular faces of the vacated octahedral sites. Annealing at lower temperatures resulted in an ordering of the local defects to form (101) planar inter-growths of rutile- and calcite-type structures.

I.E.Grey, C.Li, C.M.MacRae, L.A.Bursill: Journal of Solid State Chemistry, 1996, 127[2], 240-7