Application of the Tikhonov regularisation method to the EXAFS analysis of UO2(H2AsO4)2 x H2O

We consider the possibility of determining structural parameters from partial pair correlation functions (PCFs) for UO2(H2AsO4)2 x H2O assuming that its crystal structure is unknown. UO2(HAsO4) x 4H2O being chemically similar to UO2(H2AsO4)2 x H2O, we wanted to investigate whether the scattering characteristics calculated by FEFF-8 for the uranium hydrogen arsenate with a known crystal structure are transferable for the analysis of the uranium dihydrogen arsenate. The Tikhonov regularisation method was applied to solve the inverse ill-posed problem of finding up to three partial PCFs from one EXAFS spectrum of UO2(H2AsO4)2 x H2O. From the U LIII-edge EXAFS spectrum, we determined the U-U, U-As and U-O PCFs. The As-As, As-U and As-O PCFs were received from the As K-edge EXAFS spectrum. The interatomic distances obtained for the first and in several cases also for the second co-ordination shell of all pairs are in agreement with recently published crystallographic values. This demonstrates that the Tikhonov regularisation method is a powerful tool for the structural analysis of samples for which only chemical information (oxidation state, elemental composition, type of ligands) is known. (authors)