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

Abstract

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.

References

1. J. K. Beattie, S. P. Best, B. W. Skelton and A. H. White, J. Chem. Soc., Dalton Trans., 1981, 2105.
2. R. S. Armstrong, J. K. Beattie, S. P. Best, G. P. Braithwaite, P. Del Favero, B. W. Skelton and A. H. White, Aust. J. Chem., 1990, 43, 393.
3. R. S. Armstrong, J. K. Beattie, S. P. Best, B. W. Skelton and A. H. White, J. Chem. Soc., Dalton Trans., 1983, 1973.
4. S. P. Best and J. B. Forsyth, J. Chem. Soc., Dalton Trans., 1990, 395.
5. S. P. Best and J. B. Forsyth, J. Chem. Soc., Dalton Trans., 1991, 1721.
6. S. P. Best and J. B. Forsyth, J. Chem. Soc., Dalton Trans., 1990, 3507.
7. G. J. Goldsmith, F. V. Shallcross and D. S. McClure, J. Mol. Spectrosc., 1965, 16, 296.
8. R. L. Carlin and I. M. Walker, J. Chem. Phys., 1967, 46, 3921.
9. F. D. Camassei and L. S. Forster, J. Mol. Spectrosc., 1969, 31, 129.
10. C. D. Flint and P. Greenough, J. Chem. Soc., Faraday Trans. 2, 1972, 897.
11. C. D. Flint, P. Greenough and A. P. Matthews, J. Chem. Soc., Faraday Trans. 2, 1973, 23.
12. C. D. Flint and P. Greenough, J. Chem. Soc., Faraday Trans. 2, 1974, 815.
13. C. D. Flint and A. P. Matthews, J. Chem. Soc., Faraday Trans. 2, 1974, 1301.
14. P. Greenough and A. G. Paulusz, J. Chem. Phys., 1979, 70, 1967.
15. D. L. Kraus and G. C. Nutting, J. Chem. Phys., 1941, 9, 133.
16. L. Grabner, R. A. Forman and E. Y. Wong, Phys. Rev. B, 1972, 6, 797.
17. R. S. Armstrong, S. P. Best, B. D. Cole and K. W. Nugent, Proceedings of the Twelth International Conference on Raman Spectroscopy, eds. J. R. Durig and J. F. Sullivan, Wiley, New York, 1990, p. 414.
18. D. A. Johnson and A. G. Sharpe, J. Chem. Soc. A, 1966, 798.
19. S. P. Best, R. S. Armstrong and J. K. Beattie, J. Chem. Soc., Dalton Trans., 1982, 1655.
20. S. P. Best, R. S. Armstrong and J. K. Beattie, Inorg. Chem., 1980, 19, 1958.
21. S. P. Best, J. K. Beattie, R. S. Armstrong and G. P. Braithwaite, J. Chem. Soc., Dalton Trans., 1989, 1771.
22. S. P. Best, J. K. Beattie and R. S. Armstrong, J. Chem. Soc., Dalton Trans., 1984, 2611.
23. R. S. Armstrong, J. K. Beattie, S. P. Best, B. D. Cole and P. L. W. Tregenna-Piggott, J. Raman Spectrosc., 1995, 26, 921.
24. L. Gelato, J. Appl. Crystallogr., 1981, 14, 151.
25. S. Sugano and Y. Tanabe, J. Phys. Soc. Jpn., 1958, 13, 880.
26. S. Sugano and I. Tsujikawa, J. Phys. Soc. Jpn., 1958, 13, 899.
27. B. Bleaney, Proc. R. Soc. London, Ser. A, 1950, 204, 203.
28. S. P. Best, B. N. Figgis, J. B. Forsyth, P. A. Reynolds and P. L. W. Tregenna-Piggott, Inorg. Chem., 1995, 34, 4605.
29. J. L. Wood, J. Chem. Phys., 1965, 42, 3404.