Absorption Spectrum of Ni2+ Ions in Spinel

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Abstract
Single crystals of Ni2+‐doped spinel were made by using a large solar furnace in our laboratory, and their absorption spectra were measured in near‐infrared, visible, and ultraviolet regions. The observed bands were analyzed according to the crystal‐field theory in which the spin–orbit coupling was taken into account, and all of them were identified with the bands of Ni2+ ions not only in octahedral but also in tetrahedral site symmetry. The crystalline‐field parameter Dq in both symmetries are − 1015 and 452 cm−1, respectively. Their ratio Dq( tetra ) / Dq( octa ) amounts to − 4.008 / 9 which shows an excellent accordance with the theoretical value − 4 / 9. By adjusting the MgO concentration, the crystals with slightly different Ni–O distance r could be obtained. From the measurement of r and Dq of these crystals, it was found that Dq varies linearly as r −5 . These results are considered to show the validity of applying a point‐charge model in the treatment of the problem of this kind.