An AB Initio Investigation of a Grain Boundary in a Transition Metal Oxide
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IN A TRANSITION METAL OXIDE
I. DAWSON and P.D. BRISTOWE. Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, U.K. M.C. PAYNE and M-H. LEE. TCM Group, Cavendish Laboratory, Madingley Road, Cambridge, CB3 OHE, U.K. ABSTRACT We have used ab initio total energy plane wave pseudopotential methods to perform the first completely ab initio investigation of the atomic and electronic structure of a grain boundary in a transition metal oxide. The E = 15 (210)[001] tilt boundary in rutile TiG 2 is studied using the conjugate gradients iterative minimisation technique for performing total energy calculations within the LDA and pseudopotential approximations. The stability of the experimentally observed translation state of the boundary is confirmed, and some insight is gained into its electronic structure. INTRODUCTION The rutile phase of TiO 2 exhibits some unique dielectric and optical properties which can be utilised in various technological applications such as thin film coatings, sensor devices, and electrochemistry. It has also been used in catalysis, and as a paint pigment. The material is frequently used in the form of thin polycrystalline films, which contain grain boundaries and other structural and chemical imperfections. The presence of these defects has a direct effect on the physical properties of the material and consequently on the performance of the electronic device concerned. It is clear that as a prerequisite to understanding how such imperfections affect the properties of the material, it is necessary to obtain microscopic information concerning the defects at both the atomic and electronic level. In this paper, we describe a microscopic investigation of a specific planar interface in TiO 2 which has been studied previously using high resolution electron microscopy [1,2], and also computationally using a classical shell model potential [1]. This interface is the E = 15 36.90 (210)[001] tilt boundary. Previous work has focussed on determining the atomiý structure of the boundary which is characterised both experimentally and theoretically by an in-plane translation of approximately 1 [120]. Experiment suggests a small volume contraction at the boundary which may be due to a loss of oxygen or an excess of titanium ions. The classical 271 Mat. Res. Soc. Symp. Proc. Vol. 408 e1996 Materials Research Society
calculations, which were performed under stoichiometric conditions, suggested a small volume expansion at the boundary. The advantage of the present work, which uses a first principles methodology, is that it provides important information concerning the distribution of charge density at the boundary, the density of electronic states, the atomic structure and grain boundary energy. The only input data are the atomic number and mass of the elements concerned, i.e., titanium and oxygen. Hence there is no concern about free fitting of potentials to experimental data, as in the tight binding method. We assume that the boundary is perfectly stoichiomet
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