CHROMIUM LUMINESCENCE AS A PROBE OF SITE EFFECTS IN THE ALUM LATTICE

The (2)E(g) --> (4)A(2g) transition of chromium(III) doped in CsM(III) (XO(4))(2) . 12H(2)O (M(III) = Cr, Al, Ga, In, Co, Rh or Ir, X = S; M( III) = Cr, AI, Ga, In or Rh, X = Se) has been measured. The emission s pectra are interpreted in terms of the alpha and beta alum structures. The energy and splitting of the electronic origin are determined by t he nature and magnitude of a trigonal field. The trigonality is attrib uted to a combination of polarisation effects arising from groups orie nted along the three-fold axis, and pi overlap between the co-ordinate d 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 pi overlap is determined by the water co-ordination geometry , defined by both electronic stabilisation factors and hydrogen-bondin g interactions with the host lattice. pi Overlap is favoured by the tr igonal-planar water co-ordination to chromium in the beta lattice. Thi s results in a large origin splitting and low transition energy relati ve to the alpha alums where pi overlap is reduced by trigonal-pyramida l water co-ordination. Variations in the emission, within an alum clas s, are the result of the polarisation strength of the counter ion, and distortions to the chromium co-ordination environment imposed by the host lattice.