Structural Description of Ion Hydrate Complexes in the Lutetium Nitrate–Water System Based on the X-Ray Diffraction Results

Abstract

Based on the experimental scattering intensities revealed by X-ray diffraction, radial distribution functions of the atomic electron density are calculated for the lutetium nitrate–water system in a wide concentration range. Models of structural environments around ions in the solvent matrix are made for each solution. Theoretical structural functions are computed for each model variant. Theoretical and experimental functions are compared. In the search for the best agreement of the functions the structural models are optimized. Numerical characteristics of the structures of lutetium nitrate solutions are determined: the number of solvent molecules coordinated by Lu³⁺ and \(\text{NO}_{3}^{-}\) ions, distances between the ions and water molecules, and types of ion associates. It is found that the structure of the saturated solution is characterized by ion pairs and contact ion triples with the monodentate orientation of anions to the cation. The Lu³⁺ ion coordinates six water molecules in the saturated solution. A transition to more diluted systems causes the transition to contact ion pairs. The cation coordination number increases to 8.