Determination of uranium(IV) crystal field parameters from RIXS

Understanding the magnetic properties of the actinides is non-trivial as spin-orbit and crystal field couplings are of the same magnitude so Russell-Saunders and jj models do not fully account for the data. Whilst the ligand field can, to an extent, be changed there is no consensus on how this can be modelled. We have used the template effect of the cations in A4[U(UNCS)8] complexes where moving from A = Cs, Me4N, nPr4N and Et4N the geometry changes from Square antiprismatic to cubic and the magnetism chnages across the series. Using SQUID and MCD measurements the modelling gives very diverging results that do not match the experiment. We believe this is because the crystal field parameters are not fully known. Modelling of RIXS at the U M4 or M5 edges can give direct access to these and this proposal aims to measure the RIXS spectra of A4[U(NCS)8 (A = Cs, Me, Et) and Cs5[U(NCS)9].HNCS so that the geometry can be fully understood.