RECENT PROGRESS IN TETRAVALENT TERBIUM CHEMISTRY

The stability of the tetravalent state in Ln compounds has been the su bject of numerous investigations, if tetravalent cerium compounds are among the most stable and can be handled with minor risk of decomposit ion, the existence of tetravalent praseodymium fluorides is of no doub t, but they cannot give rise to reliable experimental data. With regar d to the terbium, it has now been established that the tetravalent oxi dation state may be stabilized under certain circumstances. However, t he terbium tetrafluoride itself exhibits a domain of thermal instabili ty which has turned out to synthesize mixed-valence terbium fluorides, such as Tb4F15 or KTb3F12. Up to now, there were no available crystal lographic data from single-crystal works for the Tb4+ ion. Obtaining s ingle-crystals of Li2TbF6, KTbF5, CsTbF5, Li2CaTbF8, KTb3F12, both alp ha and beta-BaTbF6, represents a quantum leap in the capability of cry stal structure determination and refinement. These results have brough t a revival of the search for new Tb-IV compounds which has lead to th e discovery of a large family of LiLnTbF(8) fluorides, the structural relationships of which, with LiNaY2F8 and BaY2F8, are discussed. (C) 1 998 Elsevier Science S.A.