Self-diffusion in ceria

Ceria (CeO2) is an oxygen storage material vital to the proper functioning of automobile three-way catalysts and is typically viewed as an anion condu ctor. Prior experimental work using temperature programmed static secondary ion mass spectrometry (TPSSIMS) has indicated that for rutile TiO2, a prot otypical oxide, the mobile species are Ti cations rather than O anions. To further expand on the mobile species in CeO2 we have investigated the diffu sion of both cerium and oxygen ions by TPSSIMS. The CeO2( 1 1 1) film was h eteroepitaxially grown by molecular beam epitaxy on a yttria stabilized zir conia substrate. Although high quality low-energy electron diffraction patt erns and Auger electron spectroscopy spectra free of impurity signals were obtained after just a few sputtering and annealing cycles, further cleaning was necessary to remove intense alkali and alkaline earth signals observed in SSIMS. The CeO2(1 1 1) surface was slightly enriched in O-18 by first a nnealing the film in UHV at 830 K and then exposing the 130 K crystal to O- 18(2). TPSSIMS data in conjunction with temperature programmed desorption d ata demonstrate that surface oxygen atoms begin to exchange with the bulk a round 550 K. Physical deposition of submonolayer amounts of isotopically en riched cerium (Ce-136) in an O-18(2) background allowed the simultaneous st udy of the diffusion of both cerium and oxygen ions. Surface cerium cations were found to be immobile with no diffusion into the bulk for temperatures up to 900 K, the highest temperature studied. (C) 2001 American Vacuum Soc iety.