I would like to understand the phonon spectrum using Raman and IR active modes. How would you determine the modes in spectrum?
From the Raman and IR data you can only get the k = 0 phonon modes. For determining he complete phonon dispersion you must do inelastic neutron scattering on a large single crystal. If that is not possible then you must learn how to do ab-initio phonon calculations. You can download programs for doing this. However to use it properly you need fast computers and also perhaps some guidance from experts. Once you have calculations you can then check from the agreement of k = 0 modes with the Raman and IR data. If there is reasonable agreement then you calculations are properly. If not then you must find the find the mistake and redo the calculations. From these ab-initio calculations you will be able to assign the Raman and IR modes. You can calculate phonon dispersion and density of states (PDOS) as well. PDOS will enable you to calculate any thermodynamic property.
Before you do all these you must first study some text book like:
1. Dynamics of Perfect Crystals, G. Venkataraman, L.A. Feldkamp, V.C. Sahni, MIT Press (1975)
and have a good fundamental understanding. Otherwise you will never succeed in using the programs properly. Good luck and have fun. If ou do not understand anything then do come back to me.
You can also read a more understandable book:
Introduction to Lattice Dynamics (Hardcover) by A. K. Ghatak, L. S. Kothari
Dear Tapan Chatterji,
Thank you for giving valuable suggestion.
Currently I am working on phonon modes at Gamma point with PwScf.
Now the calculation is going on. I have see whether It will work or not.
Thank you.
What is the substance that you are investigating? How many atoms has it in the unit cell?
That is a bit complicated but with a high speed computer you should be able to calculate the phonons. Only let us see whether you end up with negative frequencies. This often happens with strongly correlated electron systems like transition metal oxides. Hope your system is better behaved.
The most important thing to remember is that for phonon calculations you usually need to do a significantly more accurate calculation than usual. You should expect to need several orders of magnitude tighter convergence criteria for the relaxation of the ionic positions than for a regular calculation where you just want to find the structure, and also the other standard convergence parameters, such as basis set (plane wave cutoff in pwscf) and k-point sampling need to be handled with care. If you end up with negative modes, gradually tightening up all these things is the first thing to do.
@TB and TC Please elaborate a bit more on k-point sampling. For example consider a material with space group Pnma (LaCrO3) with Z=4. I use quantum espresso for calculations. I manually put create different k-paths i.e. G-X-Z-U-R-T-G-Y and in each interval I took 10 points for my electronic structure calculations. What I found was more number of points in some interval (Z-U). why it is so? second point I could not understand how to determine or get "weight" for these different points. If I am generating automatic k-points the number of points in each path is same but path length are different which makes graph looks so bad. Though density of states are good. Please explain.
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