The stabilisation of Cr5+in Al2O3

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Abstract
The thermal conductivity of Al2O3 doped with approximately 8 at. ppm Cr and approximately 18 at. ppm Mg was measured in the temperature range 1.4K to 100K. The absence of any resonant phonon scattering at approximately 5.5K indicated that little or no Cr5+ was present in the sample. In the context of earlier measurements of Al2O3 samples doped with Mg, Cr and Fe in various combinations, it is concluded that Cr5+ is only stabilised by relatively large concentrations of Mg being present to produce the appropriate charge compensation conditions. A Mg/Cr concentration ratio of approximately 30 will stabilise Cr5+ in appreciable quantities whereas a ratio of approximately 2 will not.