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 b2 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 Fe3+. A consistent analysis yields a maximum of the positive b2(R) of Cr3+ at Rmax=2.102+or-0.005 AA in MgO, whereas the negative b2(R) of Fe3+ has a minimum at Rmin=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.

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