Abstract
For pt.I see ibid., vol.9, no.10, p.1955 (1976). A strong EPR absorption, excited by the radiofrequency electric field, is found in magnesium-doped Al2O3 when the concentration of magnesium exceeds that of all other impurities. The strength of this EPR absorption correlates with a strong resonant phonon absorption reported by Brown. From the detailed behaviour of the absorption and in particular its strong similarity to the known behaviour of the 3d1 ions Ti3+ and V4+ in Al2O3, it is deduced to be due to the isoelectronic 3d1 ion Cr5+ in the Al3+ site. A cluster model of the Jahn-Teller effect is used both to identify the ion and to deduce that the Jahn-Teller reduction factor is gamma =0.025. There is strong evidence that the Cr5+ site axes do not coincide with the crystallographic C3 axis but are each inclined to it at an angle of 0.3 degrees in the bc plane, an effect probably due to the details of charge compensation.
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