The rules of metastability: detailed transformation mechanisms in chemical elements by means of molecular dynamics techniques

The existence of polymorphs or chemical element allotropes is a fact of nature that remains surprising. On the one hand, it is unevenly distributed, with only some compounds or elements distinguished by a large number of polymorphs or allotropes. On the other hand, many crystal structure are predicted, which can exist in principle. We argue that this imbalance may derive form an imperfect knowledge of transformation mechanisms, which solely determine the formation of a certain product. Especially in a scenario of nucleation and growth, reasoning on the mechanical stability of the final product only may mislead the overall judgement on the accessibility of a particular compound. We illustrate mechanistic analysis of selected reconstructive phase transitions by state of the art accelerated molecular dynamics techniques. We emphasize the role of nucleation in phase selection, and stress the necessary inclusion of details on intermediate steps for a more capable crystal structure prediction activity