Tetracyanidoborates with triply charged rare earth metal cations and their optical properties

The aim of this thesis is the description of the synthesis and characterization of a new group of tetracyanidoborates: tetracyanidoborates with trivalent rare earth metal cations. Their optical properties in the ultraviolet and visible range are also discussed. Common synthetic routes for tetracyanidoborates are adapted and applied to the preparation of the rare earth tetracyanidoborate hydrates. They are accessible with high yields and high purity through a reaction between the tetracyanidoboronic acid and rare earth hydroxides. It is shown that the rare earth tetracyanidoborates form isostructural groups, like the [LRE(H₂O)₅][B(CN)₄]₃.0.5 H₂O, where LRE³⁺ is La, Ce, Pr, Nd, Sm, Eu and Gd, the [HRE(H₂O)₇][B(CN)₄]₃ and the [HRE(H₂O)₈][B(CN)₄]₃.3 H₂O, where HRE³⁺ is Tb, Dy, Ho, Er, Tm, Yb, Lu and Y. Furthermore, the coordination number 9 is noticed to be common among the light rare earth cations, whereas the minor coordination number 8 is prevalent for the heavy rare earth cations in their tetracyanidoborates. This different construction of the coordination spheres between light and heavy rare earth cations leads to different structures depending on the energetic efficiency of the structural arrangement. Generally, the rare earth tetracyanidoborate hydrates are found to crystallize in the monoclinic crystal system. Moreover, other different crystal structures are observed depending on the crystallization temperature and the type of coordinated ligands and co-crystallized solvent molecules. The tetracyanidoborate hydrates with triply charged rare earth cations are characterized comprehensively by X-ray diffraction, vibrational spectroscopy, NMR-spectroscopy as well as by thermal analysis. Furthermore, the optical properties of some dehydrated rare earth tetracyanidoborates are investigated by UV-spectroscopy and luminescence measurements. The results of the optical measurements indicate that the tetracyanidoborates with rare earth metal cations exhibit rare earth cation specific luminescence properties which are apparently improved by the tetracyanidoborate anion. Therefore, the tetracyanidoborates with trivalent rare earth metal cations could have possible applications in phosphors.