Seven-coordinate trivalent rare earths: the phyllochlorides ARE2Cl7 (A  K, Rb, Cs; RE  Sm  Lu, Y) and the crystal structure of InY2Cl7

Abstract

Abstract 30 ternary rare earth chlorides ARE 2 Cl 7 (RE  Sm  Yb, Y) were synthesized, most for the first time, and were crystallographically characterized; CsLu 2 Cl 7 (below −30 °C) and InY 2 Cl 7 are additional examples. At ambient temperature they belong either to the KDy 2 Cl 7 type (KRE 2 Cl 7 (monoclinic)) or to the RbDy 2 Cl 7 type (RbRE 2 Cl 7 and CsRE 2 Cl 7 (orthorhombic)). KY 2 C1 7 , for example, undergoes a second-order phase transition from the monoclinic form I to the orthorhombic high temperature form II. The connection of monocapped trigonal prisms [RECl 7 ] via a common face to [RE 2 Cl 11 ] groups and further via common edges to an [RE 2 Cl 7 ] layer (phyllochloride) is characteristic of this extended group of ternary chlorides. The layers are stacked in the [100] direction and are held together by A + cations whose coordination polyhedra are essentially bicapped pentagonal prisms. The crystal structure of InY 2 Cl 7 ( a = 1271.6(2) pm; b = 689.9(1) pm; c = 1264.2(2) pm; Z = 4; space group, Pnma ) is reported. The thermal behaviour of AY 2 Cl 7 (A  K, In, Rb, Cs) compounds was studied in detail. Structural details rather than the lone (5s) 2 pair of indium(I) are responsible for the observation that the length of the a axis of InY 2 Cl 7 is shorter than that of KY 2 Cl 7 . The [Y 2 Cl 11 ] group of InY 2 Cl 7 is compared with the confacial bioctahedron [Y 2 Cl 9 ] in Cs 3 Y 2 Cl 9 . The caesium compounds CsRE 2 Cl 7 are trimorphic, and the temperatures at which form I transforms to form II decrease rapidly with decreasing RE 3+ size whereas form III is stable from about 400 °C up to the melting point.