Reduced ternary rare earth halides: State of the art

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Since the first discovery of Gd 2Cl 3 in 1964, a plethora of reduced binary rare earth halides with formal oxidation states less than 2 + has been obtained. Their common structural feature is an M 6 metal cluster surrounded by halide ions and, whenever necessary, edge connected. At present this appears to have no analogy in reduced ternary rare earth halide chemistry. Compounds with the oxidation state 2 + have been obtained for europium, ytterbium, samarium, thulium, dysprosium and scandium. The formula types AMX 3, A 2MX 4 and AM 2X 5 have been observed. AMX 3 compounds are either of the perovskite type ( e.g. cubic CsTmCl 3 or RbTmCl 3 with three modifications and two transitions near 185 and 375 °C) or isotypic with CsNiCl 3 (scandium(II) halides AScX 3 with A ≡ Cs, Rb, X ≡ Cl, Br and CsScI 3). A 2MX 4 compounds (examples are Cs 2YbCl 4 and Cs 2TmCl 4) are isostructural with K 2NiF 4 which is related to the perovskite structure. Samarium(II) and europium(II) halides of the AM 2X 5 type ( e.g. KSm 2Cl 5) are structurally closely related to K 2SmCl 5. In KSm 2Cl 5 (equivalent to KSm[SmCl 5]) samarium(II) has coordination numbers of 8 and 7. The successful synthesis of ternary scandium(II) halides gave a first indication that there is a possibility of producing ternary reduced halides with other than the four classical divalent rare earths. The example LiDy 2Cl 5 (with coordination numbers of 8 for dysprosium(II) and 6 for lithium) which was obtained via lithium metal (metallothermic) reduction of DyCl 3 clearly shows that the production of further compounds is possible.