Fast Ion Conduction

Summary This exercise provides an introduction to a class of materials known as fast ion or superionic conductors. The objective is to explore the properties of such materials and characterise their behaviour.

Background Many inorganic materials exhibit the phenomenon of fast ion conduction - they conduct an electric current in the solid sate. This happens because one or more of the ions is capable of diffusing rapidly through the system while it is still in the solid sate. An example of such a material is silver iodide (AgI) in the so-called α -phase, which exists between 400 K and 800 K approximately. In this phase the iodine anions form a body centred cubic lattice and the silver cations formally occupy pseudo-tetrahedral sites between the iodide ions. With increasing temperature however, the structure shows increasing disorder. The nature of this disorder is the object of this exercise -investigate and characterise it.

Step 1 Copy or download the following files: CONFIG FIELD CONTROL

Step 2 Run DLPOLY.Z, in a constant volume simulation at high temperature for about 2000 time steps and examine the OUTPUT file. Try to determine from the information provided what is going on. Display the REVCON file and examine the structure.

Step 3

If you are satisfied that you are seeing the essential phenomenon, run the program again and this time generate a HISTORY file, sampling the data at convenient intervals (see the DL_POLY-4 User Manual to find out how to do this). Now you may try a number of things such as:

  1. Calculate the mean square displacements of the ions as accurately as possible. Repeat the procedure at some other temperature (preferably more than one) and obtain the diffusion coefficients over a range of temperatures. Determine the activation energy with an Arrhenius plot.
  2. Calculate the van Hove function (Gs(r,t)) - nicely described in “Correlations in the motion of atoms in liquid argon”. Examine the results of this and decide if the ion diffusion is solid-like or liquid-like.