Superposition model for sixfold-coordinated Cr3+in oxide crystals (EPR study)

Abstract

The effect of uniaxial (100), (111) and (110) stresses on the EPR fine-structure lines of Cr³⁺ on octahedral Ti⁴⁺ sites in SrTiO₃ and Mg²⁺ sites in MgO have been measured at 300K. The spin-lattice strain coefficients G₁₁ and G₄₄ were determined. From them, the authors deduced the intrinsic superposition-model parameters b₂=2.37+or-0.04 cm^-1 and t₂=-0.36+or-0.01 in SrTiO₃, and b₂=2.34+or-0.01 cm^-1 and t₂=-0.12+or-0.11 in MgO, respectively. A substantial dependence of G₁₁ strain coefficients on charge misfit between Cr³⁺ and the substituted ion was found as has been calculated theoretically by Sangster (1981). The small negative t₂ exponents and positive b₂ reported here are interpreted as resulting from the three occupied t_2g orbitals of the ⁴A_2g ground state of Cr³⁺ as compared with the ⁶A_1g of Fe³⁺. A consistent analysis yields a maximum of the positive b₂(R) of Cr³⁺ at R_max=2.102+or-0.005 AA in MgO, whereas the negative b₂(R) of Fe³⁺ has a minimum at R_min=1.7+or-0.2 AA. The deduced b₂(R) dependence is confirmed for trigonal LaAlO₃ and Al₂O₃, where the known b₂⁰(Fe³⁺)/b₂⁰(Cr³⁺) ratio and sign are quantitatively accounted for.