EFFECT OF DOPANTS ON ZIRCONIA STABILIZATION - AN X-RAY-ABSORPTION STUDY .2. TETRAVALENT DOPANTS

X-ray absorption spectra at Zr-K, Ce-L(III) and Ge-K edges in ZrO2-CeO2 and ZrO2-GeO2 solid solutions have been measured at 10 K. Both Ce and Ge substitute for Zr in the cation network, but CeO8 and GeO4 polyhedra are maintained around the dopants. The oversized CeO8 i compressed to a Ce-0 distance of 2.30 angstrom, which is smaller than the 2.35 angstrom seen in fluorite-type CeO2. Meanwhile, the distortion of the Zr-cation network is increased and the tetragonality is reduced. The undersized Ge has a Ge-0 distance of 1.80 angstrom, which is much shorter than the Zr-O(I) distance of 2.10 angstrom. A strong tendency for Ge-Zr ordering in tetragonal solid solutions into locally scheelite-like clusters. even within the solubility limit, is observed. This results in an increase in the tetragonality and a decrease in the distortion of the cation network. Despite the opposite effects on tetragonality, both Ce and Ge doping can stabilize tetragonal zirconia albeit via different mechanisms. The structural origin of the cubic phase is also elucidated.