Ionic glasses: Structure, properties and classification

Abstract

Due to the absence of microstructure and virtually infinite compositional versatility, glassy materials are perfect candidates for rational methods to predict structure-property relationships. In the most common models of glass structure, a three-dimensional network backbone and less-localized interstitial species are taken as the fundamental constituents of a glass. Such theoretical frameworks break down for the wide range of glass-forming compositions in which covalent bonding does not percolate and, therefore, there is no network in the classical sense. Most prominently, this applies to the class of ionic glasses, presently emerging as a separate class of amorphous materials, which includes classical invert glasses, geometrically frustrated compounds, mixtures of simple salts and ionic liquids, and organic–inorganic hybrids. Here, we will critically review ionic glasses as a distinct group of materials in which structural predictions are complicated by the dominance of long-range bonding interactions which lack directionality. First, we will reassess Zachariasen’s and Hägg’s rules for conventional glass formation in an attempt to broaden the current understanding of glass formation to include ionic glasses. By reviewing the compositions and properties of many simple and complex glass-forming salts, we discern the commonalities shared by ionic glasses and their unique characteristics as compared to other classes of glass. Specifically, we discuss the importance of non-percolating topology to dynamic properties such as mechanical behavior, ionic conductivity, relaxation processes and crystallization. We also demonstrate the relevance of the adjacent fields of ionic liquids and amorphous metal organic frameworks, and highlight possibilities for fruitful interdisciplinary exchange. The structure of ionic glasses, being dominated by Coulombic interactions, lends itself to high compositional flexibility and therefore, creates significant opportunities in the search for optical materials with enhanced transmission windows, fast ion conductors, phase-change materials, or, e.g ., adapted solders, hermetic seals and dielectric insulators. • The field of ionic glasses is reviewed with a focus on structure and properties. • Classification criteria are proposed. • Invert, halide, oxoanionic, halide-oxide and superionic glasses are included. • Links are drawn to the adjacent fields of ionic liquids and metal organic framework glasses.