Molecular Dynamics Studies of Superionic Conductors
Structural and dynamical properties of superionic conductors AgI and CuI are studied using molecular dynamics (MD) techniques. Our model of these superionic conductors is based on the use of effective pair potentials. To determine the constants in these potentials, cohesive energy and bulk modulus are used as input; in addition one uses notions of ionic size based on the known crystal structure.
Salient features of the MD technique are outlined. Methods of treating long range Coulomb forces are discussed in detail. This includes the manner of doing Ewald sum for MD cells of arbitrary shape. Features which can be incorporated to expedite the MD calculations are also discussed.
A novel MD technique which allows for a dynamically controlled variation of the shape and size of the MD cell is described briefly. The development of this novel technique has made it possible to study structural phase transitions in superionic conductors. For α-AgI, among the structural properties we have studied are: partial pair correlation functions, mean square displacements of iodines, cation density maps, Havens ratio, etc. The dynamical properties examined include cation self-diffusion, nature of cation jumps, bias in successive jumps, velocity auto correlation functions, current-current correlation functions.
In CuI, we have examined the microscopic nature of γ→α transition. It is found that at about 700 K the copper ions undergo an order-disorder transformation leading to a specific heat anomaly. The nature of the first-order transition and its precursor effects are also analyzed. Results for a number of other structural and dynamical properties for α-CuI are presented.
In AgI the α⇆β transition is studied using the new MD technique. In our model, upon heating β-AgI, the iodines undergo hcp→bcc transformation and the silver ions become mobile, whereas the reverse transformation is observed on cooling α-AgI.
KeywordsMolecular Dynamics Monte Carlo Structural Phase Transition Pair Correlation Function Molecular Dynamics Calculation
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