CORRELATION BETWEEN IONIC-COVALENT PARAMETERS AND INFRARED SPECTROSCOPIC DATA IN II-III TRANSITION-METAL SPINEL-TYPE OXIDES

The infrared reflection spectra of II-III transition metal spinel-type oxides AB(2)O(4) (A = Mg, Mn, Fe, Co, Ni, Cu and Zn; B = Al, Cr, Fe, Co, Ga, Rh) are presented. The spectra are discussed in terms of catio n distribution and ionicity of these compounds. The (TO-LO)(1) splitti ng of the two high energy bands supplies a criterion for the ionic cha racter. A linear relationship between the ionic-covalent parameter (IC P) of the octahedral trivalent cation and the (TO-LO)(1) splitting is observed for normal spinels, and another one between the parameter S-S p = Sigma ICPtetra + Sigma ICPocta and the (TO-LO)(1) splitting is obs erved for inverse and partially inverse spinels. (ICP is a new paramet er related to acid strength, taking into account the ionic-covalent ch aracter of the bond [Portier et al., J. Alloys Comp. 209, 285-289 (199 4)]. This study shows that the ionicity of AB(2)O(4) spinels is mainly determined by ICP of the trivalent cation, the nature and coordinatio n of the divalent cation, and the existence of a Jahn-Teller effect. A ssignments of high-energy bands are proposed. (C) 1997 Elsevier Scienc e Ltd.