The effect of alkali content and temperature on the microstructure of lithium borate glasses, xLi 2 O-(1 Ϫx)B 2 O 3 , has been investigated for glass compositions xϭ0.2-0.5 and temperatures up to 1250 K. The molecular dynamics technique has been applied, with Ewald summation and periodic boundary conditions, to a collection of ca. 256 particles confined in a primitive cubic cell and interacting through a Born-Mayer-Huggins-type potential augmented with three-body angular terms. The short-range order ͑SRO͒ structure was found to consist of boron-oxygen tetrahedral, BO " 4 Ϫ ͓O " ϭbridging oxygen atom ͑BO͔͒, and triangular units with variable number of nonbridging oxygen ͑NBO͒ atoms, BO " 3 , BO " 2 O Ϫ , and BO " O 2 2Ϫ (O Ϫ ϭNBO). The relative abundance of SRO units was determined and found to depend on both glass composition and temperature. Increasing Li 2 O content at constant temperature or increasing temperature at a fixed composition was shown to cause rearrangements of the SRO structure and to lead towards BO " O 2 2Ϫ units in the range of compositions and temperatures investigated. Such changes were expressed in terms of chemical equilibria involving the SRO units. The local environments hosting the Li ions were investigated and distinguished in two main types: the first type of site is formed by BO's, while the second type involves the participation of NBO's. The vibrational response of Li ions in the two types of site was computed and found to correlate very well with the experimental far-infrared profiles. Calculation of diffusion coefficients of Li ions showed that diffusion is carried out predominantly through NBO sites. In addition, glass regions rich in Li/NBO were found to develop with increasing lithium oxide content and to percolate eventually into microchannels suitable for ion migration.

The reflectance spectra of bulk superionic glasses xAgI-(1Ϫx)͓Ag 2 O-nB 2 O 3 ͔ with nϭ2, 0рxр0.65 ͑diborate͒ and nϭ0.5, 0.40рxр0.60 ͑pyroborate͒ have been measured in the infrared to investigate the structure of the boron-oxygen network and the nature of sites hosting silver ions. The analysis of the midinfrared spectra showed that the diborate network consists of borate triangles BØ 3 and BØ 2 O Ϫ , and borate tetrahedra BØ 4 Ϫ ͑Øϭoxygen atom bridging two boron centers͒. Similarly, it was shown that pyroborate dimers B 2 O 5 4Ϫ , orthoborate monomers BO 3 3Ϫ , and borate tetrahedra constitute the short-range order of pyroborate glasses. The relative abundance of these borate units was found to be affected by AgI doping in a way that can be described by the isomerization reaction BØ 2 O Ϫ BØ 4Ϫ for nϭ2 and the disproportionation reactionBoth reactions shift to the right upon increasing the amount of AgI. This influence of the doping salt on the glass structure causes the lowering of the glass transition and fictive temperature at which the structure of the supercooled liquid is frozen into the glassy state. A parallel study of infrared transmission spectra of thin films in the diborate family showed the presence of a background interference wave that affects strongly the relative intensity of bands due to borate tetrahedra and triangular units. This finding suggests that conclusions based on direct comparison of infrared spectra of thin film and bulk samples of the same composition should be drawn with caution. The study of the far-infrared profiles of glasses in the pyroborate series suggested that the majority of silver ions exist in two distributions of coordination environments; one is formed primarily by oxygen atoms provided by the borate network and the other is made mainly by iodide ions, without excluding the presence of mixed oxyiodide sites. The spectroscopic characteristics of silver iodide sites were found to change progressively with AgI addition and to point towards sites of tetrahedral coordination such as those found in crystalline AgI. However, for diborate glasses the far-infrared results suggest the presence of oxide, iodide, and mixed O/I environments for silver ions. Therefore, this study shows that the formation and organization of separate silver iodide sites in xAgI-(1Ϫx)͓Ag 2 O-nB 2 O 3 ͔ glasses depends on both Ag 2 O and AgI content. ͓S0163-1829͑99͒02030-5͔

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The reflectance spectra of the superionic glasses xCuI·(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 polyhedra:  MoO4 2- monomers (the majority units), Mo2O7 2- dimers (minority units), and MoO6 6- isolated octahedra (minority units). The presence of MoO4/2O2 2- type octahedra, which contain four bridging and two nonbridging oxygen atoms, was excluded on the basis of the spectroscopic results. The relative abundance of the three types of molybdate structural units was found to depend on the glass transition temperature, T g; the equilibrium between these molybdate units shifts toward dimers and octahedra upon decreasing T g. This is effected by increasing the mole fraction of CuI in the glass. Thus, CuI affects indirectly the glass 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 three coordination environments; two of these are formed by oxygen atoms provided by the oxyanion matrix and the third is formed primarily by iodide anions. The vibrational characteristics of such CuI-like environments suggest their disordered nature with respect to crystalline CuI. The findings of this work are in agreement with the results of our previous investigations on analogous AgI-containing glasses and lends support to the conduction pathway model proposed for ion transport in such superionic glasses. Calculation of effective charges on the basis of the far-infrared data indicated that copper is present in the +1 oxidation state in these glasses.

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