Study of point defects in alkaline-earth sulfides
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JohnM. Vail Physics Department, University ofManitoba, Winnipeg, Manitoba R3T2N2, Canada (Received 26 May 1988; accepted4 August 1988) The results of a computer simulation study of point defects including vacancy, interstitial, and F + center in alkaline-earth sulfides are presented. The study is based on ICECAP/HADES simulation procedures and uses empirical interionic potentials obtained from the analysis of macroscopic data for these materials. The results predict the dominance of Schottky disorder and suggest that vacancy migration predominates in alkaline-earth sulfides. Furthermore, the calculated F + center absorption energy is in good agreement with the experimental data deduced from the optical stimulated studies in these materials.
I. INTRODUCTION Alkaline earth sulfides have long been known as luminescent materials; calcium sulfide (CaS) has been considered as an excellent host material for efficient cathode-ray tube phosphors.1 Recent investigations have revealed applications of the other sulfides, namely strontium sulfide (SrS) and barium sulfide (BaS) in alloy semiconductors,2 radiation dosimetry,3 multi-color thin-film electroluminescent devices,4 and magnetooptical devices5. The physical properties of these materials, which make them useful for device applications, are determined to a large extent by the interaction of the host lattice and the free charge carries with point defects such as impurity ions or lattice vacancies. Impurity ions in sulfides have been studied using luminescence and magnetic resonance techniques but little has been reported about intrinsic point defects, namely vacancies and interstitials in these materials. In an earlier report6 we have investigated the intrinsic point-defect structure of calcium sulfide using the Harwell HADES program. We now extend the previous investigation to study the point defects in strontium and barium sulfides in an attempt to expand our physical understanding of the defect properties of these materials.
II. TECHNIQUES A. Simulation methods
B. Potentials
In the present work, we use HADES/ICECAP simulation procedures to study point defects in alkalineearth sulfides. Simulation of the defective lattice and the evaluation of suitable interionic potentials are the essential features of these procedures. HADES 7 divides the crystalline lattice into two regions: an inner region I immediately surrounding the defect, where the lattice configuration is evaluated ex1362
J. Mater. Res. 3 (6), Nov/Dec 1988
http://journals.cambridge.org
plicitly using pairwise potentials that represent the ionic interactions, and an outer region II which can be treated by the Mott-Littleton approximation using information on macroscopic response functions, namely the elastic and dielectric constants of the lattice. The ionic polarization is simulated by the shell model, originally proposed by Dick and Overhauser.8 In this model, the ionic polarization is described by the displacement of a massless charged shell from a massive charged core; the two being connected by a har
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