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The surface structures and reactions on the single domained surfaces of SrTi03(001) are investigated using MBE and various surface characterization techniques, such as RHEED, UPS and XPS. The selective formation of the two types of SrTi03(001) surfaces, the TiO, terminated and the SrO terminated surfaces, was carried out by Ri deposition/desorption cleaning method and Sr deposition, followed by oxidation in the NO, atmosphere. It is shown that both surfaces have increased atomic distance between the first and the second layers, and the outward displacement of the first layer oxygen atoms. Surface reaction of H,O was investigated in relation to the Ti3+ defects formed by Ar+ bombardment, with the aim of obtaining knowledge on the reaction between surface functional groups and organometallic compounds at the well-ordered surfaces.

Three kinds of SrTiO3(001) surfaces, TiO2 terminated, SrO terminated, and a mixture of these surfaces, are selectively formed and their surface structures are examined by means of reflection high energy electron diffraction, x-ray photoelectron spectroscopy, and x-ray photoelectron diffraction. The TiO2 terminated surface is obtained under a moderate temperature (<700 K) cleaning by the Bi deposition/desorption method and subsequent oxidation, where the distance between the first TiO2 plane and the SrO plane underneath is 1.95+0.07 Å and the oxygen of the first TiO2 plane is pulled out 0.10 Å. The SrO terminated surface is obtained by growing one monolayer of SrO on the TiO2 terminated surface, where the distance between the first SrO plane and TiO2 plane underneath is 1.95+0.10 Å and the oxygen of the first SrO plane is pulled out 0.16 Å. These two surfaces are different from the surface obtained by Ar+ sputtering and annealing, which is found to be the mixture of TiO2 and SrO terminated surfaces.

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In a wider empirical context, the result adds to an awareness that the behavior of the formally closo, formally 24-electron, 11-vertex cluster system may in fact quite regularly deviate from that expected on the basis of the straightforward Williams-Wade6,7 formalism. Thus, of structurally characterized species, the present compound, the related ruthenadicarbaborane [2,4-Me2-l-(ij6-MeC6H4-p-iPr)-l,2,4-RuC2B8H8],17 and the substituted dicarbaborane [2,3-Me2C2B9H6-4,7-(OH)2-10-Br]20 all exhibit very significant deviations from straightforward closo structure, the osmadicarbaborane [(rj6-C6Me6)OsC2Me2B8H8] can adopt a conventional nido structure,21 and the high fluxionalities of the closo-structured [CB10HU]" anion22 and of [BuHn]2" itself23 show that transition states of presumably open structure must be energetically very near to the closed deltahedral configuration for these simpler species also. The open structure reported here in fact resembles that proposed as the intermediate for the series of diamond-square-diamond steps in the "pentagonal belt rotation" mechanism suggested22 for these fluxionalities.Acknowledgment. We thank the Royal Society (London), the British Council, and the SERC (United Kingdom) for support.