COORDINATION STATES OF MOLYBDENUM AND THE NATURE OF COPPER-ION SITES IN THE SUPERIONIC GLASSES XCUI-CENTER-DOT(1-X)CU2MOO4 (X=0.4, 0.5) STUDIED BY INFRARED REFLECTANCE SPECTROSCOPY

The reflectance spectra of the superionic glasses uCuI .(1 - x)Cu2MoO4 (x = 0.4, 0.5) have been measured in the mid- and far-infrared range to investigate the structure of the oxyanion matrix and the nature of sites hosting the charge carrier copper ions. The analysis of the mid- infrared spectra revealed the presence of three types of molybdate pol yhedra: MoO42- monomers (the majority units), Mo2O72- dimers (minority units), and MoO66- isolated octahedra (minority units). The presence of MoO4/2O22- type octahedra, which contain four bridging and two nonb ridging oxygen atoms, was excluded on the basis of the spectroscopic r esults. The relative abundance of the three types of molybdate structu ral units was found to depend on the glass transition temperature, T-g ; the equilibrium between those molybdate units shifts toward dimers a nd octahedra upon decreasing T-g. This is effected by increasing the m ole fraction of CuI in the glass. Thus, CuI affects indirectly the gla ss structure by lowering the temperature at which the structure of the supercooled liquid is frozen into the glassy state. The study of the far-infrared profiles has suggested that the cuprous ions exist in thr ee coordination environments; two of these are formed by oxygen atoms provided by the oxyanion matrix and the third is formed primarily by i odide anions. The vibrational characteristics of such CuI-like environ ments suggest their disordered nature with respect to crystalline CuI. The findings of this work are in agreement with the results of our pr evious investigations on analogous AgI-containing glasses and lends su pport to the conduction pathway model proposed for ion transport in su ch superionic glasses. Calculation of effective charges on the basis o f the far-infrared data indicated that copper is present in the +1 oxi dation state in these glasses.