What is a solid to solid phase transition?

Matter that undergoes a solid-to-solid phase transition changes from one solid phase to another solid phase. Simply put, a phase transition occurs when matter moves from one phase to another. The phases of matter are gas, solid, and liquid (and in some cases, plasma). Some elements may have more than one solid phase, and those elements can undergo solid-to-solid phase transitions. To transition from one phase to another requires changes in external factors such as pressure or temperature.

Depending on the element, there are different geometric atom arrangements, or crystal structures, that are possible. These arrangements result in different kinds of solids. Carbon, for example, has multiple solid phases, including both graphite and diamond. These two solids are both entirely carbon in composition. However, these solids are different due to the way their carbon atoms are arranged in space - their crystal structure.

Scientists have observed solid-to-solid phase transitions in which matter transitions from one crystal phase to another. However, the mechanism that results in a solid-to-solid transition is still not well understood.