Why are there no metastable structures in binary rocksalts?

It is of note that those binary systems that have Rocksalt as their ground state structure appear to only exhibit polymorphism under high pressures or at the nanoscale, and as soon as those conditions are released, the systems will relax back to rocksalt, irrespective of whether the compound is mostly ionic, covalent or metallic. This observation prompted us to do an in-depth theoretical study of the potential energy surface of compounds with the rocksalt ground state. Through ab-initio calculations, we find that ~80 % of all structure-types found for A₁B₁ compounds in the ICSD database will relax to the rocksalt structure in each of the investigated systems: ionic MgO, covalent PbTe and metallic TaC. In addition, using a previously validated heuristic we examine the remaining prototypes (~20% of them) and their polymorphic transformations into the rocksalt structure, and find that large fraction of them are also expected to transform rapidly into the rocksalt structure. We contrast this with a case with a well-known abundance of metastable allotropes of elemental carbon, or group-IV carbides, in which many low-energy structures exhibit slow transitions into their respective ground states. We therefore propose that the lack of metastable polymorphs in binary rocksalts is mainly a consequence of the rapid nature of the transformation mechanisms between the relevant structure-types and lower energy rocksalt ground state.