Simulating Vacancy, Impurity And Electronic Defect States In Mgo, Lici And La 2 Cuo 4 Using Quantum Cluster And Classica
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SIMULATING VACANCY, IMPURITY AND ELECTRONIC DEFECT STATES IN MgO, LiCI AND La2CuO4 USING QUANTUM CLUSTER AND CLASSICAL LATTICE SIMULATION TECHNIQUES IN A CONSISTENT MANNER. ROBIN W. GRIMES and C. R. A. CATLOW The Royal Institution of Great Britain, 21 Albemarle Street, London, W IX 4BS, U.K., A. L. SHLUGER Department of Solid State Physics, Latvia University, Riga, U.S.S.R. R. PANDEY Department of Physics, Michigan Technological University, Houghton, MI.49931, U.S.A. R. BAETZOLD Corporate Research Laboratory, Eastman Kodak Company, RochesterNY 14650-2001, U.S.A. A. H. HARKER Theoretical Studies Department, AEA Industrial Technology, Harwell, Oxon, OXi 1 ORA, U.K. ABSTRACT We calculated defect energies by using a Hartree-Fock method to model an inner region that includes the defect site and one or two shells of lattice ions. This is surrounded by an embedding region that is described by a classical Mott-Littleton calculation. Electrostatic multipole consistency between the twojegions is maintained throughout. The results of defect calculations on MgO are examined as a function of basis set. We find that a sophisticated basis set is required before a reliable defect formation energy can be guaranteed. This experience has allowed us to develop a new model for the off-center relaxation of the exciton in LiC1. These calculations have required the use of pseudo potential cores for anions and floating functions to model the diffuse electron. Lastly, we report on recent calculations concerning the stability of hole states in La2CuO4. We find considerable delocalization of the hole over both the Cu2 + ions and the 02- ions that form the Cu-O planes. INTRODUCTION The use of quantum cluster techniques to study defect states in ceramics is limited by the small number of ions that can realistically be included in calculations. As such, these methods cannot be expected to model the long-range Coulombic interactions that govern the response of the lattice to the inclusion of a charged defect. Conversely, classical Born-like ionic models of ceramics can easily include many hundreds of ions. Unfortunately, such methods are also limited in their usefulness since they cannot model electronic processes. The incentive for a combination of the two techniques is obvious. This has been realised in a new code, ICECAP [1-4] which incorporates a central HartreeFock SCF cluster calculation embedded in a Mott-Littleton environment. The idea of embedding the quantum cluster to account for the effect of the remaining lattice is not new and a number of alterantive "embedded quantum cluster" codes exist (see for example references 5, 6 and 7). However, the important feature that the ICECAP code includes and which many other methods neglect, is that the embedding region will respond to the incorporation of a defect center and that this response is self-consistent with the quantum cluser calculation. The purpose of this paper is to present the results of some recent studies carried out using the ICECAP code and thereby to illustrate the diversity of
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