ErF 3 ‐reiche ternäre Erbiumfluoride mit den schweren Alkalimetallen: I. KEr 3 F 10 und RbEr 3 F 10

ErF 3 ‐Rich Ternary Erbium Fluorides with the Heavy Alkali Metals: I. KEr 3 F 10 and RbEr 3 F 10 The conversion of erbium trifluoride (ErF 3 ) with chlorides of the heavy alkali metals (ACl; A = K, Rb and Cs) at 700–800 °C in tantalum capsules sealed by arc‐welding surprisingly results in the formation of ErF 3 ‐rich ternary alkali‐metal erbium(III) fluorides with the compositions AEr 3 F 10 (A = K and Rb) or CsEr 2 F 7 , respectively. The first‐mentioned compounds are characterized by high coordination numbers at the alkali‐metal cation (CN(A + ) = 15 and 16) as well as by a uniform surrounding of the Er 3+ cation (CN = 8, square antiprism). In KEr 3 F 10 (cubic, $Fm{\bar 3}m$ Fm 3 m ; a = 1154.06(7) pm, Z = 8) eight (F1) − anions always arrange as a cube, whose six faces are each capped by an Er 3+ cation. These [(F1) 8 Er 6 ] 10+ groups constitute a cubic closest sphere‐packing with K + cations in all of the tetrahedral interstices. The [Er 6 ( μ 3 ‐F1) 8 ] 10+ units are interconnected via the remainder fluoride anions (F2) to build up a three‐dimensional framework so that the characteristic [ErF 8 ] 5− polyhedra (d(Er 3+ −F − ) = 220 – 235 pm) emerge. In RbEr 3 F 10 (hexagonal, P 6 3 mc ; a = 818.43(5), c = 1336.54(8) pm, Z = 4) the analogous [ErF 8 ] 5− polyhedra (d(Er 3+ −F − ) = 219 – 237 pm) initially convene to triple groups [Er 3 F 19 ] 10− through cis‐ edge condensation, which are then further connected via F − corners to arrange as a two‐dimensional network perpendicular to the c axis. Finally, the cross‐linking of these layers is achieved by common F − vertices again, such that large cavities apt to take up the Rb + cations are formed. A second part of these series will report on the syntheses and crystal structures of the ErF 3 ‐poorer AEr 2 F 7 ‐type representatives with A = K, Rb and Cs.