High-temperature treatment in a reducing atmosphere of Ni-doped MgO crystals resulted in the formation of colloids with an average size of about 50 nm. The colloids are coherent nickel precipitates having a fcc structure ($a=0.352\mathrm{}$ nm), and occasionally a bcc structure ($a=0.288\mathrm{}$ nm). The ratio of magnetic to nuclear scattering amplitudes for the (111) precipitate reflection was about 17% below the value for nickel. Optical absorption measurements indicate that a broad optical band centered at ∼ 2.2 eV is associated with the colloidal precipitates.

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