ESR study of transition metal ions in magnesium titanate

Abstract

Trigonally symmetric electron spin resonance spectra have been observed for Co²⁺, Ni²⁺ and Mn⁴⁺ ions occupying single sites, and Cr³⁺ ions occupying two sites in ilmenite-structure MgTiO₃. The fine-structure parameters at 295K for Cr³⁺(1), Cr³⁺(2), Mn⁴⁺ and Ni²⁺ ions are D=-1672, +364, -1769 and 1370*10^-4 cm^-1 respectively, whilst for Co²⁺ ions g/sub ///=2.867 and g_{perpendicular to} =4.99. Previous results for Fe³⁺ and Mn²⁺ ions are substantially confirmed. Oxidation and reduction experiments show that the valence state of iron in MgTiO₃ may be readily changed; the observed effects are consistent with the earlier proposal that Fe³⁺ ions are located at both the Mg²⁺ and Ti⁴⁺ sites. A comparison of experimental and calculated fine-structure splittings for Cr³⁺ ions suggests that Cr³⁺ (1) and Cr³⁺ (2) ions are located at sites which involve Cr³⁺-Ti⁴⁺ and Cr³⁺-Mg²⁺ ion pairs in nearest-neighbour cation sites respectively, although whether the Cr³⁺ ion in each pair occupies the Mg²⁺ or Ti⁴⁺ host lattice site is indeterminate. The calculations are based on the superposition model of crystal fields and existing data for ruby. The ESR data for MgTiO₃ and Al₂O₃ containing Mn²⁺, Co²⁺ and Ni²⁺ ions suggests that the single-ion anisotropy in the layered antiferromagnets MnTiO₃, CoTiO₃ and NiTiO₃ may be comparable with the dipolar anisotropy.