Phenomenological theory of lattice dynamics and polymorphism of ZrO2 -: art. no. 104106

The cubic-tetragonal-monoclinic structural evolution of zirconia (ZrO2) is studied within a lattice dynamical treatment using a model approach taking into consideration the variability of the oxygen ion charge. The relevant calculations reveal the two physical factors driving that evolution. A specific Zr/O ionic radii ratio is one of them: the radius of Zr4+ is found to be too small to ensure stability of the ZrO8 cube, but not sufficiently small to ensure the stability of the ZrO6 octahedron. Thus, the ZrO7 coordination polyhedron arises as a basic structural fragment specifying the monoclinic (baddeleyite) lattice. Another factor is the dependence of an ionic charge on its local environment in the crystal. This results in the charge redistribution between nonequivalent oxygen ions during the structural transformations, which is found to be indispensable to stabilize the baddeleyite lattice as a ground-state structure of zirconia. Special attention is paid to the elastic anomalies accompanying the tetragonal-monoclinic transition, which were never considered in the preceding studies. According to the present work, those anomalies are related to the intermediate orthorhombic structure (D-2h(8),No. 54) which is characterized as essentially unstable.