A structural model for mixed-nitrate ionic glasses

Abstract

We propose and evaluate a structural model for mixed K-Ca nitrate melts and glasses, which invokes the presence of strongly bound Ca2(NO 3)7 groups surrounded by K ions that can transport charge by relatively free jumps. To this purpose we first construct ionic models for the pure ANO3 compounds (with A = alkali metal ion), by searching for suitable A+Z·(NO3)-Z interionic potentials in the space of interionic-law parameters with the help of known models for alkali chlorides. The available evidence mainly consists of infrared-active mode frequencies, and is compatible with two alternative types of interionic parameters. A choice between them is made on the basis of the fact that the NO3 group has a flat triangular shape and can execute torsional oscillations giving rise to a low-frequency Raman-active mode in the melt. This interionic force model is then combined with a similar model for Ca(NO3)2 to evaluate the structural properties of a 3KNO3·2Ca(NO3)2 unit, which is the constituent of the well-known glassformer CKN. We expect that the present model should be representative of other mixed alkali-divalent ion nitrate systems in the range of concentration where glass formation is observed.