Calculation of the Crystal Field Phonon Interaction using a Point Charge Model

If crystal field - phonon coupling is to be applied in running mcphas and mcdisp, magnetic ions may be added to a phonon set-up of mcphas.j. These magnetic ions should not be placed at exactly the same position as the phonon atoms - i.e. da db dc should be chosen (slightly) different to enable the mcphas.j loader to identify which is which.

The coupling between phononic and crystal field degrees of freedom can be calculated from equations (108) and (109). This can be done using the program makenn with option -cfph applying small differential changes to the atomic positions in the unit cell in order to evaluate the gradient of the crystal field parameters. makenn will do this calculation for every magnetic ion in the unit cell, i.e. for all ions which have the so1ion or ic1ion module in the sipf file. It will create for these ions a new position in makenn.j with the required shift and also store a designated sipf file. After running makenn with option -cfph the user should only take care to change the module so1ion or ic1ion to or phonon in the original sipf files of the magnetic ions so that there is always one phononic and one magnetic sipf file for each magnetic atom in the unit cell.

For examples see several models on crystal field phonon interaction in examples, for starting a linear chain of Ce$^{3+}$ ions including a theory manuscript can be found in examples/Ce3p_chain_cfphonon.

Note makenn makes use of the program pointc to calculate crystal field parameters $B_{\gamma}(i)=B_l^m$ ($\gamma$ is a short hand notation for $lm$) at site $i$ with Stevens convention (or alternatively also $L_l^m$ with Wybourne normalisation, see appendix E for details and definition of symbols) using the well known expressions of the point charge model (103).