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

The defect structures when Y 3 Fe 5 O 12 is doped with either Al 3+ or Cr 3+ , and evenly co-doped with both, which have been a matter of controversy in the literature, are modelled using atomistic and ab initio DFT methods. When Y 3 Fe 5 O 12 is doped with Al 3+ , the defect reaction energy obtained marginally favors the preferential substitution of Al 3+ for Fe 3+ at the tetrahedral sites as opposed to octahedral ones. This is indicative that for Al 3+ -doped samples processed at elevated temperatures, or containing undetected impurities, the substitution of Al 3+ for octahedral Fe 3+ is likely. To model the defect structure of the Cr 3+ -doped Y 3 Fe 5 O 12 , it was essential that the Cr 3+ ions crystal field stabilization energy (CFSE) and the Fe 3+ -O 2- - Cr 3+ spin-spin coupling derived from the ab initio DFT calculations ,be taken into account. The results show the substitution of the Cr 3+ ion for an octahedral Fe 3+ ion to be energetically favorable relative to its substitution for a tetrahedral Fe 3+ one. It is also shown that the antisite defect, where the Cr 3+ ion substitutes for Y 3+ at a dodecahedral site with the expelled Y 3+ ion substituting for an octahedral Fe 3+ ion, is possible under certain processing conditions. For the Al 3+ /Cr 3+ co-doped Y 3 Fe 5 O 12 , the Al 3+ and Cr 3+ ions were found to, respectively, substitute for the tetrahedral and octahedral Fe 3+ 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