Chromium luminescence as a probe of site effects in the alum lattice

The ²E_g → ⁴A_2g transition of chromium(III) doped in CsM^III(XO₄)₂·12H₂O (M^III=Cr, Al, Ga, In, Co, Rh or Ir, X = S; M^III = Cr, Al, Ga, In or Rh, X = Se) has been measured. The emission spectra are interpreted in terms of the α and β alum structures. The energy and splitting of the electronic origin are determined by the nature and magnitude of a trigonal field. The trigonality is attributed to a combination of polarisation effects arising from groups oriented along the three-fold axis, and π overlap between the co-ordinated water lone pair and chromium t_2g orbitals. The polarisation effect is strongly dependent on the counter ion and chromium site size. The degree of π overlap is determined by the water co-ordination geometry, defined by both electronic stabilisation factors and hydrogen-bonding interactions with the host lattice. π Overlap is favoured by the trigonal-planar water co-ordination to chromium in the β lattice. This results in a large origin splitting and low transition energy relative to the α alums where π overlap is reduced by trigonal-pyramidal water co-ordination. Variations in the emission, within an alum class, are the result of the polarisation strength of the counter ion, and distortions to the chromium co-ordination environment imposed by the host lattice.