Atomistic and ab initio DFT modelling of the defect structures in Al3+/Cr3+-doped and co-doped Y3Fe5O12

Abstract

The defect structures when Y3Fe5O12 is doped with either Al3+ or Cr3+, and evenly co-doped with both, which have been a matter of controversy in the literature, are modeled using atomistic and ab initio DFT methods. When Y3Fe5O12 is doped with Al3+, the defect reaction energy obtained marginally favors the preferential substitution of Al3+ for Fe3+ at the tetrahedral sites as opposed to octahedral ones. This is indicative that for Al3+-doped samples processed at elevated temperatures, or containing undetected impurities, the substitution of Al3+ for octahedral Fe3+ is likely. To model the defect structure of the Cr3+ -doped Y3Fe5O12, it was essential that the Cr3+ ions crystal field stabilization energy (CFSE) and the Fe3+-O2-- Cr3+ spin-spin coupling derived from the ab initio DFT calculations ,be taken into account. The results show the substitution of the Cr3+ ion for an octahedral Fe3+ ion to be energetically favorable relative to its substitution for a tetrahedral Fe3+ one. It is also shown that the antisite defect, where the Cr3+ ion substitutes for Y3+ at a dodecahedral site with the expelled Y3+ ion substituting for an octahedral Fe3+ ion, is possible under certain processing conditions. For the Al3+ /Cr3+ co-doped Y3Fe5O12, the Al3+ and Cr3+ ions were found to, respectively, substitute for the tetrahedral and octahedral Fe3+ ions. The energy values obtained suggest this defect structure to be insensitive to the processing conditions and/or the presence of undetected impurities. The structural and magnetic implications of these defect structures are discussed.