Molecular dynamics simulations of some amorphous and crystalline photonic materials
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INTRODUCTION
Development of new materials for photonic applications is the theme of many current activities in materials science and technology. Barium titanate (BaTiO3) with a large electro-optic coeffficient1 and TiO2 containing sodium silicate glasses2 with relatively high nonlinear refractive indices (n2 = 1-10 x 10"19m2/W) are considered to be some of the promising photonic materials. While experimentation continues to be the major source for obtaining information regarding the structure and properties of these and other materials, recent advances in development of atomistic computational procedures such as molecular dynamics (MD) may prove valuable in circumstances in which the appropriate experiments are difficult to perform. MD also allows structural insights that may explain experimental observations and provide predictive tools for the properties of these emerging materials. Experimental difficulties exist in preparation of large, defect-free, optical quality BaTiO3 single crystals, although amorphous BaTiO3 gels and crystalline powders have been prepared3 5; however, it is difficult to prepare pure BaTiO3 glasses in large quantities.6 In the case of TiO2 containing glasses, relatively large nonlinear refractive indices are observed and are believed to a)Now
with the Department of Materials Science and Engineering, 848 Benedum Hall, University of Pittsburgh, Pittsburgh, Pennsylvania 15261. b)Now with the Department of Mechanical, Aeronautical, and Materials Engineering, University of California, Davis, California 95616. 1104
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 5, May 1990
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be due to the presence of tetrahedrally coordinated titanium ions.2 This argument has also been used to explain the ultra low expansivity of the TiO2-SiO2 glasses.7 In sputtered amorphous films the coordination number of titanium may be 4 or 6, depending upon the type and concentration of other cations.8 With a view to developing a better understanding of the structure-property relationships for BaTiO3 and titania containing glasses, the MD technique was utilized in the present work to simulate crystalline and amorphous forms of BaTiO3 and TiO2. A rigid ion potential model was used in all the MD simulations which were also performed onNaCl, a highly ionic material, to provide an internal check for the accuracy of the model. II. MOLECULAR DYNAMICS SIMULATIONS TECHNIQUE
Molecular dynamics (MD) simulations represent a powerful tool for deriving the local structure of a variety of amorphous and crystalline materials. MD essentially consists of solving the classical equations of motion of an assembly of particles interacting through realistic interatomic potentials. The solution of these equations provides important quantitative information regarding the structure of materials and thus predict properties. The time resolution of MD technique is on the order of 10"15 s, which makes it particularly attractive for a study of optical properties in the IR region. We © 1990 Materials Research Socie
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