Perovskite layers stoichiometry determination

Table 9.15 Comparison of determinations of stoichiometry in thin Ba0,7Sr0 3Ti-, 03 perovskite layer measured by ICP-MS using different instruments and XRF.

The SrTiOa (111) [211-214] and (110) polar surfaces [215-217] and the BaTi03(lll) surface [218] have been produced and studied. At variance with rocksalt polar surfaces, many of these investigations suggest that one can obtain non-reconstructed quasi-planar polar surfaces. It should be realized that the perovskite structure is such that there exist ordered configurations of vacancies in the surface layers compatible with (1x1) diffraction patterns. In addition, SrTiO3(110) displays a variety of reconstructions, such as c(2 x 6) [215,217], under reducing conditions. No precise determination of the layer stoichiometry has been performed. 

The 100 face of a perovskite ABO3 crystal is the most stable surface, although it cannot be prepared by cleavage. There exist two non-equivalent terminations with respective stoichiometries AO and BO2 - for example, SrO and Ti02 for SrTiOs. In the surface layer, the transition metal cations B are five-fold coordinated, instead of six-fold in the bulk, and the cations A are surrounded by eight oxygens, instead of twelve in the bulk. The 100 surfaces of both SrTiOs and BaTiOs have been studied, but a quantitative surface structure determination has only been performed on the first compound a contraction of about 10% on the SrO face and a weak expansion of the order of 2%, at the limit of the error bar, on the Ti02 face, have been found by Bickel et al (1989 1990). Hikita et al (1993), on the other hand, deduce an outward relaxation on both faces. 

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