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

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 b 2 reported here are interpreted as resulting from the three occupied t2g orbitals of the 4A2g ground state of Cr3+ as compared with the 6A1g of Fe 3+. A consistent analysis yields a maximum of the positive b 2(R) of Cr3+ at Rmax=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.