Experimental and theoretical determination of the electronic structure and optical properties of three phases of ZrO2
The addition of suitable dopants to ZrO2 can induce dramatic phase stabilization, and this dopant-induced phase stabilization is the basis of transformation toughening of zirconia-based structural ceramics. We have determined the electronic structure of three phases of ZrO2, cubic, tetragonal, and monoclinic, using vacuum-ultraviolet and x-ray-photoemission spectroscopies, coupled with ab initio band-structure and optical property calculations using the orthogonalized linear-combination-of-atomic-orbitals method, in an attempt to understand the complex interaction of the stabilizing dopants and associated atomic defects with the crystal structures of ZrO2 and their phase transitions. The experimental samples were single or polycrystalline stabilized materials which contain atomic defects, while the calculations were performed for undoped idealized ZrO2 structures without atomic defects. Reasonable agreement is found between experiment and theory at this level. The primary 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) bands, which form the conduction bands as the symmetry decreases from cubic to monoclinic. This leads to a complex evolution of the O 2p to Zr 4d and the O 2s to Zr 4d interband transitions. In addition, in the real materials, the presence of yttrium stabilizer introduces additional Y 4p valence bands and Y 4d conduction bands. The effective coordination of zirconium by oxygen is reduced from eightfold to sevenfold by the presence of the stabilizing ions and defects and this leads to the introduction of an occupied Zr 4d valence band suggestive of the presence of Zr2+.