Mobility of Ag Atoms in the Binary System Ag 2 Se-Ag 2 S: A Molecular Dynamics Study
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toms in the Binary System Ag2SeAg2S: A Molecular Dynamics Study A. Smith, R. Ravelo and N. Pingitore MRS Proceedings / Volume 481 / 1997 DOI: 10.1557/PROC481279
Link to this article: http://journals.cambridge.org/ abstract_S1946427400243923 How to cite this article: A. Smith, R. Ravelo and N. Pingitore (1997). Mobility of Ag Atoms in the Binary System Ag2SeAg2S: A Molecular Dynamics Study. MRS Proceedings,481, 279 doi:10.1557/PROC481279 Request Permissions : Click here
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MOBILITY OF Ag ATOMS IN THE BINARY SYSTEM Ag 2Se-Ag2S: A MOLECULAR DYNAMICS STUDY' A. Smith%, R. Ravelo*, N. Pingitore** *Department of Physics and Materials Research Institute, University of Texas, El Paso, TX 79968 "**Departmentof Geological Sciences and Materials Research Institute, University of Texas, El Paso, TX 79968 ABSTRACT The compounds in the binary system Ag2Se-Ag 2S are known to undergo a phase transition to a superionic phase at a temperature which varies with concentration. The diffusion mechanism of Ag ions have been studied in a Molecular Dynamics formalism. We have calculated the diffusion coefficient as function of temperature and concentration and have calculated the activation energy for diffusion from the Arrhenius plot of DAg vs temperature. Results obtained for the end points Ag2Se and Ag2 S agree with experimental measurements. INTRODUCTION The silver chalcogenides, Ag2X (where X = Se, S, or Te), have been of considerable interest due to the electrical, thermoelectrical, and optoelectronic properties. These materials, called superionic conductors (SIC) or fast-ion conductors, are crystalline structures which exhibit extremely high values of ionic conductivity comparable to those of liquid electrolytes at relatively low temperatures [1]. These structures go from non-conducting a-phase to superconducting #-phase at a transition temperature of 406K for Ag 2Se and 450K for Ag 2 S [2]. In the superionic or high temperature f-phase, the basic crystallographic structure of these materials, consist of two interpenetrating sublattices where the anions form a stable but anharmonic bcc structure through which cations are more or less free to move [3]. The diffusion of superionic conductors have attracted attention from the viewpoint of a many-particle diffusion. The mobility p and the diffusion coefficient D of Ag ions in Ag 2Se, Ag2S , and solid solutions have been measured using tracer diffusion technique by Okazaki [4]. The values of D for the solid solutions measured at 453, 503, and 553 K were around 0.03, 0.43, and 1.58 xl0-1cm2/sec respectively, and did not vary much as function of composition. The experimental f-value defined as f = -I-. was found to deviate remarkably from the Einstein relation D -kT" kTThe hopping of the mobile ions is strongly correlated and influenced by the interactions between the anions and cations. In order to understand the diffusion mechanism in these systems, we have performed MD calculations of sol
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