kristallilise struktuuriga materjali, sh puhta metalli ühe kristallilise vormi pöörduv muutus teiseks (nt raua jahtumisel γ-raua muutus α-rauaks) (vt polümorfism, α-raud, ϒ-raud), a reversible transition of a solid crystalline phase at a certain conditions (temperature, pressure) to a another phase of the same chemical composition with a different crystal structure e.g. a change in a pure metal from one form to another (e.g. the change from gamma iron to alpha iron) (see: polymorphism, alfa iron, gamma iron)