Absorption Spectrum of Ni2+ Ions in Spinel

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.