Tailoring Chemical Bonds to Design Phase Change Materials

In applications of phase change materials, one usually switches between the crystalline and the glassy state. In our group, we have been trying to understand and tailor phase change materials for various applications. One challenge we have encountered was a deeper understanding of atomic arrangement and properties of the glassy state of these phase change materials. Glasses are commonly described as disordered counterparts of the corresponding crystals; both usually share the same short-range order, but glasses lack long-range order. Here a quantification of chemical bonding in a series of glasses and their corresponding crystals is performed, employing two quantum-chemical bonding descriptors, the number of electrons transferred and shared between adjacent atoms. For popular glasses like SiO₂, GeSe<i>₂</i> and GeSe, the quantum-chemical bonding descriptors of the glass and the corresponding crystal hardly differ. This explains why these glasses possess a similar short-range order as their crystals. Unconventional glasses, which differ significantly in their short-range order and optical properties from the corresponding crystals are only found in a distinct region of the map spanned by the two bonding descriptors. This region contains crystals of GeTe, Sb₂Te₃ and GeSb₂Te₄, which employ metavalent bonding. Hence unconventional glasses are only obtained for solids, whose crystals employ theses peculiar bonds. We can thus employ the map to identify crystals which possess glasses with rather different properties. The map even predicts systematic trends for the property change and speed of crystallization, which helps to tailor the properties of the glasses.<br/>Subsequently, it will be demonstrated that we can also use this design concept to design other classes of functional materials such as thermoelectrics. In particular, we can show how to design optimum dopants for a given thermoelectric material.