Recent progress in tetravalent terbium chemistry

Abstract

The stability of the tetravalent state in Ln compounds has been the subject of numerous investigations. If tetravalent cerium compounds are among the most stable and can be handled with minor risk of decomposition, the existence of tetravalent praseodymium fluorides is of no doubt, but they cannot give rise to reliable experimental data. With regard to the terbium, it has now been established that the tetravalent oxidation state may be stabilized under certain circumstances. However, the terbium tetrafluoride itself exhibits a domain of thermal instability which has turned out to synthesize mixed-valence terbium fluorides, such as Tb4F15 or KTb3F12. Up to now, there were no available crystallographic data from single-crystal works for the Tb4+ ion. Obtaining single-crystals of Li2TbF6, KTbF5, CsTbF5, Li2CaTbF8, KTb3F12, both α and β-BaTbF6, represents a quantum leap in the capability of crystal structure determination and refinement. These results have brought a revival of the search for new TbIV compounds which has lead to the discovery of a large family of LiLnTbF8 fluorides, the structural relationships of which, with LiNaY2F8 and BaY2F8, are discussed.