Microstructure of mixed-nitrate melts and glasses

Abstract

We evaluate the stability of various structures for a Ca-2(NO3)(7) core unit compensated by K or Rb ions, as a basic constituent of the glass-forming melts of CKN and CRbN compounds [3ANO(3)center dot 2Ca(NO3)(2) with A = K or Rb]. We find that three alternative structures are stable for the core unit and lie at approximately the same energy, provided the Ca-NO3 overlap repulsion is stiff enough as suggested by the planar structure of the NO3 group, but independent of the amount of electronic charge transfer to these groups. In these structures, each Ca ion can lie in a fourfold or a fivefold coordination by NO3 groups, so that the two Ca ions can be bridged by one, two, or three NO3 groups. This picture is compatible with the accepted view of CKN as a prototype fragile glass-former and with the stability of mixed-nitrate glasses over a broad range of composition.