Vibrational spectra of hexaaqua complexes VIII. The antisymmetric SO4 stretching bands in alums: LO-TO superior to correlation-field and site-group splitting

FT-IR spectra of RbAl(SO4)(2). 12H(2)O and KAl(SO4)(2). 12H(2)O alums were investigated in the region of the nu(3)(SO4) mode. The spectra were analyze d at room (approximate to 293 K, RT) and at low temperature (approximate to 100 K, LT). The doublet arising from this mode, evident in the LT spectrum , could be attributed to either site-group or correlation-field splitting. In order to reveal its exact origin, isomorphous isolation of SO42- ions in the corresponding selenate alums was performed. The spectra of such sample s show that in the case of RbAl(SO4)(2). 12H(2)O the correlation-field spli tting is dominant over the site-group splitting, while in the case of KAl(S O4)(2). 12H(2)O, both correlation and site-group splitting coexist. By mean s of specular reflectance spectroscopy at near normal incidence, reflection spectra were acquired and the frequencies of the LO and TO phonons were ca lculated. It was shown that, in terms of the corresponding, the following s equence holds in the case of RbAl(SO4)(2). 12H(2)O: LO-TO > correlation-fie ld > site-group splitting. (C) 1999 Elsevier Science B.V. All rights reserv ed.