Optical properties of 3d transition-metal-doped Mg Al2 O4 spinels

Strong emission bands in the visible region are observed in Mg Al2 O4 crystals doped with transition-metal ions under excitation at the band-to-band transitions. We report optical responses of Cr-, Co-, and Ni-doped Mg Al2 O4 and present optical models for M -doped Mg Al2 O4 (M=Ti, V, Cr, Mn, Co, and Ni) to describe the charge-transfer transitions and the transitions between multiplet levels of 3d electrons, which are observed competitively or coexisting, depending on the number of 3d electrons. While the optical responses of Cr- and Ni-doped Mg Al2 O4 are dominated by the multiplet-multiplet transitions, those of Ti- and V-doped Mg Al2 O4 are governed by the charge-transfer transitions. The two kinds of transitions coexist in the Mn- and Co-doped Mg Al2 O4. These behaviors are well understood based on the numerical results of unrestricted Hartree-Fock approximation.