Local structures and roles of Fe 3+ and Cr 3+ in p‐type semiconductor CuAlO 2

Single crystals of the p‐type semiconductor CuAlO 2 , synthesized from the CuO flux method, have been investigated by electron paramagnetic resonance (EPR) spectroscopy at temperatures from 3.5 to 294 K. EPR spectra reveal an axial Cr 3+ center and an axial Fe 3+ center, presumably arising from impurities in the starting materials. The Cr 3+ center at 294 K is characterized by g  = 1.979(1) and D  = − 7803(3) × 10 −4  cm −1 . The Fe 3+ center at 294 K has g //  = 1.9942(1), g ⟂  = 1.9964(1), D  = 261.9(3) × 10 −4  cm −1 , $B_{4}^{0} = 4.533(3) \times 10^{- 4} \,{\rm cm}^{- 1} $ , and $B_{4}^{3} = 104.23(3) \times 10^{- 4} \,{\rm cm}^{- 1} $ . These spin‐Hamiltonian parameters, evaluated by superposition model analyses and periodic density functional theory (DFT) calculations, suggest that both Cr 3+ and Fe 3+ occupy the Al 3+ site but the latter involves distortion related to perturbation from a substitutional Cu + ion at the nearest Al site. This difference in the local structure between Fe 3+ and Cr 3+ explains their contrasting effects on the p‐type conductivity in CuAlO 2 .