Phenomenology of Polymorphism: IV. The Trimorphism of Ferrocene and the Overall Metastability of Its Triclinic Phase

Structural and thermodynamic data reported in the literature for the three crystalline phases of ferrocene are used to build a topological representation of its (p, T) phase diagram. Two phases (orthorhombic and monoclinic) exhibit stable phase regions whose temperature ranges increase with increasing pressure. The triclinic phase is found to be metastable at any temperature and pressure. In fact, in the (p, T) diagram, it is not associated with any state of lowest energy although it transforms into the monoclinic phase according to a transition which is reversible. This transition occurs in the phase region where the orthorhombic phase is the stable one.