Studies of the temperature dependence of the short range structure and local dynamics in silver borate glasses

EXAFS temperature dependent measurements have been carried out on the borate glass doped with silver iodide, (AgI)_0.55(Ag ₂O.4B₂O₃)_0.45, at the K-edges of Ag and I, in the range 39-500 K. Local negative thermal expansion has been measured in the explored temperature range; in the same range, crystalline AgI shows a positive trend. This behaviour, already found for another silver borate glass containing a higher content of silver iodide, was interpreted in terms of a relaxing of the AgI amorphous network within the silver borate host matrix. In the light of the results obtained for the glass (AgI)_0.55(Ag ₂O.4B₂O₃)_0.45, having a much lower AgI concentration and showing an almost equal thermal expansion, a different interpretation is now necessary. The Ag-I first shell distance contraction is a specific characteristic of the AgI amorphous network within the silver borate host matrix: temperature increase favours shorter average distance of the mobile Ag⁺ cations from I ions. The host matrix influences, however, the local thermal disorder around Ag and I species.