Structure of fast-ion-conducting AgI-doped borate glasses in bulk and thin film forms

The reflectance spectra of bulk superionic glasses xAgI-(1-x)[Ag₂O-nB₂O₃] with $n=2,$ $0<~x<~0.65$ (diborate) and $n=0.5,$ $0.40<~x<~0.60$ (pyroborate) 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 B₃ and B₂O^-, and borate tetrahedra B₄^- ({}=oxygen atom bridging two boron centers). Similarly, it was shown that pyroborate dimers B₂O₅^4-, orthoborate monomers BO₃^3-, 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 B₂O^-B₄^- for $n=2$ and the disproportionation reaction B₂O₅^4-BO₃^3-+B₄^- for $n=0.5.$ Both 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-(1-x)[Ag₂O-nB₂O₃] glasses depends on both Ag₂O and AgI content.