EXPERIMENTAL AND THEORETICAL DETERMINATION OF THE ELECTRONIC-STRUCTURE AND OPTICAL-PROPERTIES OF 3 PHASES OF ZRO2

The addition of suitable dopants to ZrO2 can induce dramatic phase sta bilization, and this dopant-induced phase stabilization is the basis o f transformation toughening of zirconia-based structural ceramics. We have determined the electronic structure of three phases of ZrO2, cubi c, tetragonal, and monoclinic, using vacuum-ultraviolet and x-ray-phot oemission spectroscopies, coupled with ab initio band-structure and op tical property calculations using the orthogonalized linear-combinatio n-of-atomic-orbitals method, in an attempt to understand the complex i nteraction of the stabilizing dopants and associated atomic defects wi th the crystal structures of ZrO2 and their phase transitions. The exp erimental samples were single or polycrystalline stabilized materials which contain atomic defects, while the calculations were performed fo r undoped idealized ZrO2 structures without atomic defects. Reasonable agreement is found between experiment and theory at this level. The p rimary difference among the three phases of ZrO2 is the hybridization or mixing of the Zr 4d (x2-y2 and z2) and the Zr 4d (xy, yz, and zx) b ands, which form the conduction bands as the symmetry decreases from c ubic to monoclinic. This leads to a complex evolution of the O 2p to Z r 4d and the O 2s to Zr 4d interband transitions. In addition, in the real materials, the presence of yttrium stabilizer introduces addition al Y 4p valence bands and Y 4d conduction bands. The effective coordin ation of zirconium by oxygen is reduced from eight-fold to sevenfold b y the presence of the stabilizing ions and defects and this leads to t he introduction of an occupied Zr 4d valence band suggestive of the pr esence of Zr2+.