Polarized Electronic Absorption Spectrum of Tetragonal NiCl42− Cs3(MgCl4)Cl

The optical absorption spectrum (4000–17 000 Å) of crystals of Cs₂(Mg_0.996Ni_0.004Cl₄)Cl was measured at approximately 5 and 80°K. The host material was found by x‐ray diffraction to have the Cs₃(CoCl₄)Cl structure $\text{(I4\hspace{0.17em}/\hspace{0.17em}mcm)}$ at temperatures down to at least 10°K. In the solid solution the spectra showed that nickel(II) substituted for magnesium, which lies at sites of $D_{\text{2d}}$ symmetry with the principal axis aligned with the c axis of the tetragonal crystal. Spectra were measured in both E ⊥ c and E ‖ c polarizations, and the results were in accord with a ligand‐field model having ${\mathrm{B,\; C,\; \xi ,\; B}}_0^2{\mathrm{,\; B}}_0^4,\mathrm{and}{B}_4^4$ as parameters. Numerous phonon‐creation bands were observed that provided information about excited state potential wells and nuclear motions. In some band systems the individual bands were unusually narrow and well resolved.