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 Cr 3+ on octahedral Ti 4+ sites in SrTiO 3 and Mg 2+ sites in MgO have been measured at 300K. The spin-lattice strain coefficients G 11 and G 44 were determined. From them, the authors deduced the intrinsic superposition-model parameters b 2 =2.37+or-0.04 cm -1 and t 2 =-0.36+or-0.01 in SrTiO 3 , and b 2 =2.34+or-0.01 cm -1 and t 2 =-0.12+or-0.11 in MgO, respectively. A substantial dependence of G 11 strain coefficients on charge misfit between Cr 3+ and the substituted ion was found as has been calculated theoretically by Sangster (1981). The small negative t 2 exponents and positive b 2 reported here are interpreted as resulting from the three occupied t 2g orbitals of the 4 A 2g ground state of Cr 3+ as compared with the 6 A 1g of Fe 3+ . A consistent analysis yields a maximum of the positive b 2 (R) of Cr 3+ at R max =2.102+or-0.005 AA in MgO, whereas the negative b 2 (R) of Fe 3+ has a minimum at R min =1.7+or-0.2 AA. The deduced b 2 (R) dependence is confirmed for trigonal LaAlO 3 and Al 2 O 3 , where the known b 2 0 (Fe 3+ )/b 2 0 (Cr 3+ ) ratio and sign are quantitatively accounted for.