Fast Ion Conduction¶
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.
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 ref [Vashishta1978] 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 what you are asked to investigate and characterise.
Download Exercise2.tar.xz to $DL_POLY/data directory and unpack:
cd $DL_POLY/data wget https://ccp5.gitlab.io/dlpoly/Exercise2.tar.xz
Now go to the $DL_POLY/execute directory and start up the GUI:
cd $DL_POLY/execute java -jar ../java/GUI.jar &
Copy the contents of the subdirectory Exercise2 into the execute subdirectory. To do this using the GUI, select from the main menu Execute > Store/Fetch Data and in the Data Archive window type Exercise2 in the Fetch box and click Fetch. You will obtain the files CONTROL, FIELD, TABLE and CONFIG. The last of these is a high temperature crystal of silver iodide in the alpha phase. Your task is as follows.
Run DLPOLY.Z, see , 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.
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 a convenient intervals (see the DL_POLY manual to find out how to do this). Now you may try a number of things:
Run the MSD option from the Analysis menu and 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.
Run the Gs(r,t) option from the Analysis menu and calculate the van Hove self correlation function (which is nicely described in ref [Rahman1964]). Examine the results of this and decide if the ion diffusion is solid like or liquid like.