Exchange-Enhanced Optical Absorption of Fe3+ in Al2O3

The polarized optical absorption of isolated and exchange-coupled Fe3+ ions in Al2O3 has been measured at room, liquid-nitrogen, and liquid-helium temperatures. Single crystals were grown from a PbO–PbF2 flux and contained Fe3+ concentrations c ranging from 0.1 to 2 at.%. The single ion optical spectrum of Fe3+ in the distorted octahedral surroundings of Al2O3 is characterized by the following parameters: B=660 cm−1, C/B=4.75, and 10Dq=1510 cm−1, in reasonable agreement with the recent work of Lehmann and Harder on natural iron-bearing corundum crystals. At sufficiently low temperatures, the spectrum is due only to single ion transitions for all concentrations. However, as the temperature is raised, there is a striking increase in the intensities of the 4A1, 4Ea, and the 4Eb bands in the higher concentration samples. At room temperature, the intensity of the 4A1, 4Ea band increases more rapidly than c. Furthermore, the 4Eb band contains a partially resolved low-frequency component which represents the first observation of an optical transition due to exchange-coupled Fe3+ pairs. For c≥0.5 at.%, the room temperature spectrum in the visible region is dominated by exchange-enhanced transitions for which ΔS=0. Similar spectra have been reported previously for the isoelectronic Mn2+ ion.