We report the first experimental evidence of the cyclic form of ozone, found in three air stable surface reconstructions of MgO (111) annealed above 1450 ± C. The MgO ͑111͒-͑ p 3 3p 3 ͒R30 ± surface consists of equilateral oxygen trimers while the MgO ͑111͒-͑2 3 2͒ and MgO ͑111͒-͑2 p 3 3 2 p 3 ͒R30 ± surfaces are periodic arrangements of trimers and single oxygen atoms. The oxygen trimers appear to be centered over underlying Mg atoms. The structures fit transmission electron diffraction data better than neutral plane faceting based models proposed for the polar MgO (111) surface. [S0031-9007(98)07807-7] PACS numbers: 61.14.Rq, 61.16.Bg, 68.35.BsThe stability of polar oxide surfaces has long been a problematic question in surface science. A bulk terminated polar surface has an infinite surface energy because alternating layers of oppositely charged ions produce a large dipole moment perpendicular to the surface [1]. For the model MgO (111) polar oxide surface theoretical results have pointed to two similar solutions for this problem: microscopic faceting into neutral ͕100͖ planes upon annealing [2-5] and surface reconstructions, which are essentially faceting to neutral planes but on an atomic scale [5][6][7].The microscopic faceting model has long been supported by low energy electron diffraction (LEED) and scanning electron microscopy (SEM) of 1200 ± C annealed MgO (111) surfaces [8,9] which revealed micron-sized triangular facets. These facets were interpreted to be neutral ͕100͖ planes. It has been recently shown, however, that these facets are much shallower vicinal ͕111͖ planes introduced by acid etching in sample preparation [10]

It has long been thought that (111) surfaces of rock salt oxides microfacet to neutral surfaces upon annealing because of the very large energies involved in bulk terminating a layer of like ions. However in a recent reflection electron microscopy (REM) study Gajdardziska-Josifovska et al. found that MgO(lll) surfaces annealed in flowing oxygen furnaces at 1500°C not only did not microfacet, but displayed a √3×√3R30° surface periodicity that was stable in air. To determine the structure of this unusually stable surface MgO (111) transmission electron microscopy (TEM) samples were annealed in a vacuum furnace in the present study and their transmission electron diffraction (TED) patterns were analyzed with direct phasing methods.The TEM samples were prepared by orienting a MgO single crystal and sawing lmm wafers along a (111) plane. Disk samples were then ultrasonically drilled, dimpled, mechanically polished and/or hot nitric acid etched, and milled with 5 KeV Ar+ ions.