X-ray-absorption studies of zirconia polymorphs. I. Characteristic local structures

Abstract

Characteristic extended x-ray-absorption fine structure and x-ray-absorption near-edge structure spectra of zirconia polymorphs (monoclinic, orthorhombic, tetragonal, and cubic) have been identified at 10 K. For the host cations, these spectra are phase dependent but dopant independent because the dopant perturbation is short ranged. In the tetragonal structure, the first Zr-O shell is bifurcated into two tetrahedra, with outer O ions loosely bound and severely distorted. Although the tetragonal Zr cation network is nearly fcc and higher-order Zr-Zr coordinated shells are resolvable at distances up to 9 Å, other evidence points to incoherent-distortion reflecting phase instability. In monoclinic, orthorhombic, and cubic structures, a similar Zr-O polyhedron with sevenfold coordination and a comparable bond length is observed, along with a large distortion or splitting of the next-nearest-neighbor Zr-Zr shell. Progressive departure from the ideal fluorite structure by reducing the Zr-O coordination and dilating or distorting the local Zr-Zr network is seen to be correlated to increased phase stability but not to long-range crystal symmetry.