ZIRCONIUM BROMIDE CLUSTER CHEMISTRY - A NEW TUNNEL STRUCTURE IN A(5)ZR(6)BR(15)BE (A=RB, CS)

Reactions in the ABr-ZrBr4-Be systems (A = Cs, Rb) at similar to 850 degrees C yield phases with a new tunnel structure constructed from a [Zr-6(Be)Br-12]Br-6/3 network. Crystal refinements are given for hexagonal Rb-5.0(1)Zr6Br15Be (P6(3)22, Z = 2, a = 13.009(1 Angstrom, c = 12.060(1) Angstrom, R/R(w) = 4.4/4.8%) and Cs-4.60(8)(Zr6Br15Be (P6(3), Z = 2, a = 13.105(1) Angstrom, c 12.156(1) Angstrom, R/R(w) = 4.9/3.5%). The boride also exists in the rubidium system. The alkalimetal 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(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(6)X(12)X(6/2) network.