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Nickel-doped Al2O3 (sapphire) crystals have been studied using optical absorption and transmission electron microscopy. After reduction at elevated temperatures in a hydrogen atmosphere, these crystals exhibit a dark colouration. Our results indicate that the colouration is due to Mie scattering from precipitates resulting from the reduction process. Analytical transmission electron microscopy techniques revealed the presence of metallic precipitates and identified them as Al3Ni. The precipitate distribution is very inhomogeneous.
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Ballesteros et al. (1997) studied this question.
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