Two-phonon excitations in IR and NIR spectra of alunites

Binary gerade-ungerade combinations of the fundamentals nu(OH/OD), delta(pe rpendicular to)(OH/OD), delta(parallel to)(OH/OD), and nu(SO4), observed in IR and NIR spectra of alunite KAl3(OH)(6)(SO4)(2), of its isotopomer aluni te-d(6) and of related compounds are discussed. With the nu(OH/OD) frequenc ies an anharmonicity analysis has been carried out. Calculation of overtone frequencies with consideration of anharmonicity and comparison of these va lues with the observed three quasi-overtones of nu(OH/OD) in the NIR spectr a (actually combinations of die Raman- and IR-active fundamentals nu(OH/OD) ) suggest that these quasi-overtones are strongly perturbed by resonances. (C) 1999 Elsevier Science B.V. All rights reserved.