THE VALENCE OF CU AND CR IONS IN ZN1-XCUXCR2SE4 SPINEL SYSTEM

The results of extensive experimental and theoretical studies of magnetic properties for CuCr2X4 (X = S, Se) spinel compounds are summarized. They concern the magnetometric, neutron and X-ray diffraction, X-ray photoelectron spectra and nuclear magnetic resonance studies, made in the different ranges of temperature. All the experimental data were interpreted within the framework of an ionic model (however, the chemical bonds between anions and cations have there the mixed ionic-covalent character, the ionic contribution is predominant). This work demonstrates the temperature dependence of the valence of the Cr and Cu ions in CuCr2Se4 compound. It has been proved that the Lotgering and the Goodenough models, assuming a valence stability of these ions, fail. Evidence for the temperature dependence of the valence for these cations has been presented also in the case of the Zn(1-x)Cu(x)Cr2Se4 solid solutions. These systems may be classified as "inter-atomic" intermediate valent to be distinguished from valence fluctuating "intra-atomic" mixed valent systems.