Zirconium Bromide Cluster Chemistry. A New Tunnel Structure in A5Zr6Br15Be (A = Rb, Cs)

Abstract

Reactions in the ABr-ZrBr{sub 4}-Be systems (A = Cs, Rb) at {approximately}850 {degree}C yield phases with a new tunnel structure constructed from a [Zr{sub 6}(Be)Br{sub 12}]Br{sub 6/3} network. Crystal refinements are given for hexagonal Rb{sub 5.0(1)}Zr{sub 6}Br{sub 15}Be (P6{sub 3}22, Z = 2, a = 13.009(1{angstrom}, c = 12.060(1) {angstrom}, R/R{sub w} = 4.4/4.8%)) and Cs{sub 4.60(8)}Zr{sub 6}Br{sub 15}Be (P6{sub 3}, Z = 2, a = 13.105(1) {angstrom}, c = 12.156(1) {angstrom}, R/R{sub w} = 4.93/3.5%). The boride also exists in the rubidium system. The alkali-metal cations all exhibit fractional occupancies, most being located within or on the walls of the tunnels defined by the bridged cluster array, some with only three close bromine neighbors. Some two-site disorder appears in the rubidium structure (and persists in a marginal refinement of it in P6{sub 3}), while this problem is absent in the cesium compound refinement in the lower symmetry. These characteristics are common in cluster network structures containing larger cations. The formation of 16-e clusters is achieved even with the limited binding of some cations, the cesium salt showing only temperature-independent paramagnetism. This structure type defines a new, fifth type of a M{sub 6}X{sub 12}X{sub 6/2} network.