Whenever a compound crystal is cut normal to a randomly chosen direction, there is an overwhelming probability that the resulting surface corresponds to a polar termination and is highly unstable. Indeed, polar oxide surfaces are subject to complex stabilization processes that ultimately determine their physical and chemical properties. However, owing to recent advances in their preparation under controlled conditions and to improvements in the experimental techniques for their characterization, an impressive variety of structures have been investigated in the last few years. Recent progress in the fabrication of oxide nano-objects, which have been largely stimulated by a growing demand for new materials for applications ranging from micro-electronics to heterogeneous catalysis, also offer interesting examples of exotic polar structures. At odds with polar orientations of macroscopic samples, some smaller size polar nano-structures turn out to be perfectly stable. Others are subject to unusual processes of stabilization, which are absent or not effective in their extended counterparts. In this context, a thorough and comprehensive reflexion on the role that polarity plays at oxide surfaces, interfaces and in nano-objects seems timely.This review includes a first section which presents the theoretical concepts at the root of the polar electrostatic instability and its compensation and introduces a rigorous definition of polar terminations that encompasses previous theoretical treatments; a second section devoted to a summary of all experimental and theoretical results obtained since the first review paper by Noguera (2000 J. Phys.: Condens. Matter 12 R367); and finally a discussion section focusing on the relative strength of the stabilization mechanisms, with special emphasis on ternary compound surfaces and on polarity effects in ultra-thin films.

The electronic and atomic structure of several (1 × 1) terminations of the (110) polar orientation of SrTiO3 surface are systematically studied by first-principles calculations. The electronic structure of the two stoichiometric SrTiO-and O2-terminations are characterized by marked differences with respect to the bulk, as a consequence of the polarity compensation. In the former, the Fermi level is located at the bottom of the conduction band, while in the latter the formation of a peroxo bond between the two surface oxygens results in a small-gap insulating surface with states in the gap of the bulk projected band structure. We also consider three non stoichiometric terminations with TiO, Sr and O compositions, respectively, in the outermost atomic layer, which automatically allows the surface to be free from any macroscopic polarization. They are all insulating. The cleavage and surface energies of the five terminations are computed and compared, taking into account the influence of the chemical environment as a function of the relative richness in O and Sr. From our calculations it appears that some (110) faces can even compete with the TiO2 and SrO terminations of the (100) cleavage surface: in particular, the (110)-TiO termination is stable in Sr-poor conditions, the (110)-Sr one in simultaneously O-and Sr-rich environments. The available experimental data are compared to the outcomes of our calculations and discussed.

We propose a detailed description of the structural and electronic properties of neutral and charged TinO2n+m clusters (n=1–3 and m=0,1), through simulations based on the density functional theory in the local spin density approximation. In all the isomers studied, strongly bound titanyl groups are found. The order of stability of the low-energy stoichiometric clusters may change considerably from that found by the approaches based on classical electrostatics. The most stable isomers of the oxygen-rich neutral clusters show characteristic peroxide groups. All these facts stress the importance of the covalent contribution to the cohesion of the clusters. Large atomic relaxations, accompanying the change from a closed-shell to an open-shell electronic configuration when an electron is added or removed, can often induce reversals of stability among the isomers. A careful discussion of the computed electron affinities and excitation energies as a function of the size and the atomic conformation of the clusters is performed, in relation to recent experimental data.

At the onset of dissolution in water, cubic MgO smoke crystals present (110) cuts of the edges of the cubes. Next, (111) facets progressively dominate the shape of the crystallites, which finally transform into truncated octahedra. The morphology of the crystallites that are derived from surface energies computed within the density functional theory (DFT), only involve (100) and (111) facets. We explain the unexpected (110) cuts via a "constrained" Wulff equilibrium shape that arises from a slower kinetics of formation of (111) facets than (110) ones. Experiment and theory fully agree on the hierarchy of hydroxylated surface energies: Gamma(111) < Gamma(100) < Gamma(110), both supporting the partial dissociation of water on MgO(100). Finally, from low to high P(H2O) (high to low T), DFT-based calculations predict a switch from Wulff shapes involving dry (100) facets, in which the (100)/(111) area ratio decreases upon increasing P(H2O), to shapes involving hydroxylated (100) surfaces, in which the above ratio increases with P(H2O).

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