EPR of V4+ and Fe3+ in titanites

EPR investigations of yellow-green titanites from four localities showed presence of V⁴⁺ and Fe³⁺. They were investigated at X-band and room temperature. For V⁴⁺ the principal values of the g matrix and ⁵¹V hyperfine splitting and the directions of their principal axes indicate presence of a VO²⁺ ion substituting for Ti. Due to a high zero-field splitting only approximate values of the fine structure parameters of Fe³⁺ could be determined. With directions of their principal axes very similar to those of V⁴⁺ this ion must also substitute for Ti. Unusually large linewidths for both ions with little variation for samples from different localities are ascribed to the reported domain structure of titanites and accumulation of impurities in the domain boundaries. While for Fe³⁺ a small variation of the fine structure parameters explains this broadening, for V⁴⁺ a distribution of g-factors equal to its total anisotropy must be responsible whereas the V=0 bond length is remarkably constant. Due to preferential incorporation of impurities in the grain boundaries a contribution of dipolar broadening cannot be excluded.