Local structures and roles of Fe3+ and Cr3+ in p-type semiconductor CuAlO2

Single crystals of the p-type semiconductor CuAlO2, synthesized from the CuO flux method, have been investigated by electron paramagnetic resonance (EPR) spectroscopy at temperatures from 3.5 to 294K. EPR spectra reveal an axial Cr3+ center and an axial Fe3+ center, presumably arising from impurities in the starting materials. The Cr3+ center at 294K is characterized by g=1.979(1) and D=-7803(3)x10(-4)cm(-1). The Fe3+ center at 294K has g(//)=1.9942(1), g(-)=1.9964(1), D=261.9(3)x10(-4)cm(-1), B-4(0)=4.533(3)x10(-4)cm(-1), and B-4(3)=104.23(3)x10(-4)cm(-1). These spin-Hamiltonian parameters, evaluated by superposition model analyses and periodic density functional theory (DFT) calculations, suggest that both Cr3+ and Fe3+ occupy the Al3+ 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 Fe3+ and Cr3+ explains their contrasting effects on the p-type conductivity in CuAlO2.