Ni(II)-doped CsCdBrCl2: variation of spectral and structural properties via mixed-halide coordination

The mixed-halide compound CsCdBrCl2 is studied by X-ray diffraction and by using the Ni(II) ion as an optical probe. Low-temperature absorption, luminescence, and EXFAS spectra of the Ni(II) impurity are recorded. The structure of CsCdBrCl2 is shown to consist of corner-sharing [Cd3X12]6- trimers. Each trimer has a structure of three face-sharing octahedra Cl3CdX3CdX3CdCl3 where X has an equal probability of being a Br- or a Cl- ion. This equal occupancy on the bridging halide positions occurs over the whole crystal rather than within each [Cd3X12]6- trimer unit. Optical spectroscopy shows that the Ni(II) ion exists in all possible [NiBrxCl6 - x]4- isomeric forms (x = 0, 1, ..., 6). The energy of the 3A2g-->1A1g transition is a linear function of x, due to the change in inter-electron repulsion through the differing covalencies of the ligand compositions. The energy of this transition can be varied over 2750 cm-1. The inhomogeneous broadening that results from the halide disorder is discussed from the point of view of the variation of ligand-field strength and inter-electron repulsion. A model including a differential nephelauxetic effect is required to explain the energies of the ligand-field states.