Local atomic structure and phase stability of zirconia polymorphs: An x-ray absorption spectroscopy study.

Abstract

Synchrotron x-ray absorption spectra of four zirconia polymorphs (monoclinic, orthorhombic, tetragonal and cubic) with dopants of various sizes and charges (Ga$\sp{3+}$, Fe$\sp{3+}$, Y$\sp{3+}$, Gd$\sp{3+}$, Ge$\sp{4+}$, Ce$\sp{4+}$ and Nb$\sp{5+}$) have been obtained from 10K to room temperature. EXAFS and XANES have been analyzed to determine radial distribution functions over 10A, static and thermal distortion of atomic pairs, site occupancy around dopant and oxygen vacancy. These local atomic structures have provided new insight into the atomic origin of the phase stability of zirconia polymorphs and solid solutions. Each zirconia polymorphs has a unique, characteristic Zr-centered radial distribution functions, which is dopant independent. The cation network in tetragonal zirconia is very nearly the fcc type, despite a bifurcation of the Zr-O shell. The local Zr environment in the stabilized cubic zirconia, however, deviates severely from its long range fluorite-like order. The intrinsic instability of the Zr cation network in tetragonal zirconia is manifested by its softer vibration modes and weaker coherent scattering between the central Zr ion and distant cations. The addition of Y$\sp{3+}$, a stabilizing dopant, results in a significant decrease in the thermal distortion of its local environment. All of the dopant cations studied here form a substitutional solid solution with zirconia. Nevertheless, three remain severe local distortions, necessitated by the large difference in dopant-oxygen distance and Zr-oxygen distance. Dopant ionic size also determines the preferred locations of oxygen vacancies. Vacancies introduced by oversized dopants are located as nearest-neighbors to Zr atoms, whereas vacancies introduced by undersized dopants are shared between Zr and dopant atoms. Short-range cation order has been found in tetragonal structures of the ZrO$\sb2$-GeO$\sb2$ and ZrO$\sb2$-YNbO$\sb4$ systems. Locally, the ordering produces a sheelite-like structure and enhances tetragonal stability by reducing network distortion around the undersized dopants. Meanwhile, these dopants (Ge$\sp{4+}$ and Nb$\sp{5+}$) increase the bonding disparity between the two Zr-O subshells causing an increase in layer tetragonality. In contrast, doping in all other cases leads to decreased tetragonality through the randomization effect of oxygen vacancy and dopant oversize on the layer structure of tetragonal zirconia.