Martensitic phase transition in pure zirconia: a crystal chemistry viewpoint

A crystal chemistry approach was carried out in order to decipher the mechanisms involved in phase transitions of zirconia. A detailed analysis of structures, based on structural sheets of half fluorite cell thick and described in terms of SDF and HDF (for slightly and highly distorted fluorite) sheets is proposed for the first time. This approach allowed clarifying the relationships between tetragonal, orthorhombic and monoclinic forms. This concept permits to propose simple structural relationships between the structures of the different polymorphs. These relationships and their correlated transformation pathways were confirmed using symmetry group relations. The results were compared to those obtained by the phenomenological approach of the tetragonal -> monoclinic (T -> M) martensitic phase transition. The different lattice correspondences are revisited and discussed. For the first time, some fine structural models were proposed to describe the pathways of atomic displacements taking place during each possible phase transitions. The T -> M phase transition is the main transformation pathway. It may solely gives rise to the C-type lattice correspondence, the (100)(M)//(100)(T) and [001](M)//[001](T) relationship allowing the minimum structural changes during the transformation. The B-type variant with (100)(M)//(100)(T) and [010],(M)//[001](T) may be developed from an indirect pathway involving either a ferroelastic phase transition by domain switching taking place in the T phase before the T -> M phase transition operates or a two stage process involving a combined T -> O and O -> M successive and independent transitions. The orthorhombic phase is an alternative form to the monoclinic one and is in no way an intermediate phase.