All “Roads” Lead to Rocksalt Structure

Rocksalt is the most common crystal structure among binary
compounds.
Moreover, no long-lived, metastable polymorphs are observed in compounds
with the rocksalt ground state. We investigate the absence of polymorphism
via first-principles random structure sampling and transformation
kinetics modeling of three rocksalt compounds: MgO, TaC, and PbTe.
Random structure sampling reveals that for all three systems the rocksalt
basin of attraction covers much more of configuration space than any
other, making it statistically the most significant structure. The
kinetics modeling shows that virtually all other relevant structures,
including most of the 457 known A1B1 prototypes,
transform rapidly to rocksalt, making it the only option at ambient
conditions. These results explain the absence of polymorphism in binary
rocksalts, answer why rocksalt is the structure of choice for high-entropy
ceramics and disordered ternary nitrides, and help understand/predict
metastable states in crystalline solids more generally.