Boron incorporation into rutile. Phase equilibria and structure considerations

Reduction of rutile in the presence of berate flux stabilizes the rutile phase relative to reduced rutiles due to incorporation of boron from the flux. In the presence of berates the rutile phase is stabilized to oxygen fugacities that are lower by almost two orders of magnitude compared with fugacities at the limit of the single-phase rutile phase field in the pure Ti-O system. Boron incorporation is accompanied by reduction of titanium to the trivalent state, according to the charge compensation relation: 3Ti(4+) = 3Ti(3+) + B3+ (interstitial). Results of powder X-ray diffraction and transmission electron microscopy studies on samples prepared in the temperature range from 1100 to 1300 degrees C have been used to establish a model for boron incorporation. It is proposed that at the temperatures studied, local defects in boron-doped rutile form from displacement of titanium atoms to adjacent interstitial sites coupled with occupation by boron of the triangular face of the vacated octahedral sites. This atomic grouping represents a small element of the TiBO3 (calcite-type) structure. Annealing at a lower temperature results in ordering of the local defects to form (101)(r) planar intergrowths of rutile and calcite-type structures. (C) 1996 Academic Press.