Compositional and physical changes on perovskite crystal surfaces
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Compositional and physical changes on perovskite crystal surfaces S. P. Chen Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 23 April 1997; accepted 14 October 1997)
The surface composition of BaTiO3 , SrTiO3 , and CaTiO3 perovskite (100) surface is determined by shell-model calculations. The TiO2 -terminated surface is energetically favorable for BaTiO3 and SrTiO3 , which is consistent with experimental observations on SrTiO3 . On the other hand, the CaO-terminated surface is preferred for CaTiO3 where Ca21 is the smallest 21 cation in these titanates. Ions on (100) surface rumple and induce surface dipoles. The surface ferroelectric polarization stabilizes the surface and changes its sign as the surface composition changes from TiO2 to CaO. This phenomenon is expected to affect the stability and properties of epitaxial films on perovskite substrates.
I. INTRODUCTION
Perovskites such as BaTiO3 and SrTiO3 have been used extensively as ferroelectrics as well as substrates to grow epitaxial films of oxides, metals, high-temperature superconductors, and organic polymers.1–3 The surface chemistry, structure, defects, energetics, and kinetics are all important in determining the stability and properties of these films and interfaces. Most of the surfaces used for the growth of films and contacts are (100) surfaces of perovskites.1,2,4,5 There have been several experimental studies of the surface structure and symmetry by surface techniques that indicate the (100) surface of SrTiO3 has a 1 3 1 pattern with steps.2 Additionally, recent studies show that by using a buffered NH4 F-HF (BHF) solution, one can prepare a clean 100% TiO2 -terminated surface as compared to 75 to 95% TiO2 -terminated surface mixed with 25 to 5% SrO-terminated surface.6–9 However, there are no theoretical studies on perovskite surface structure or chemistry. In this report, we determine the surface structure and chemistry of three titanates by using shellmodel potentials10 and compare the results with available experiments. II. CALCULATIONAL PROCEDURE
The shell-model potentials10 for BaTiO3 , SrTiO3 , and CaTiO3 are taken from Bush et al.11 These potentials have a short-range part that has Buckingham form11 and a long-range Coulomb interaction for the charge effects. The ions are treated as polarizable.10 The charges of the ions are separated into core charges, q(core), and shell charges, q(shell). The shell charges are attached to the core charges by springs. The parameters of the shell-model potentials are fitted to experimental crystal structures and properties of many oxides to maximize the portability of the potentials. The details are described in Ref. 11. We have used the MARVIN program12 to 1848
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calculate the surface energies, structures, relaxation, and dipoles.13 These methods have been very successful in studying the surface properties of ionic surfaces.13,14 The calculated lattice parameters for the cubic or pseu
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