La₄₈Br₈₁Os₈: Isolated clusters in an unusual superstructure with significantly greater intercluster bonding

Exploration of reactions in the La-Br-Z system for Z = Fe, Ru, and Os in welded Nb containers at 900-950 °C resulted in only the title phase. The La₄₈Br₈₁Os₈ stoichiometry is very close to that of known triclinic Pr₆Br₁₀Os but with an ~32-times larger cell, 138 independent atoms, and completely different intercluster connectivities in a complex monoclinic superstructure (a = 33.076(5) Å, b = 23.466(3) Å, c = 23.537(2) Å, β = 110.701(4)°, P2₁/c (No. 14), Z = 4, 23 °C). Tetragonally compressed, ~16 e^- lanthanum octahedra centered by Os are heavily interbridged by Br, including Br(f-a) (f = face) and Br(i-a-a) functions, to increase coordination numbers about some Br (to 4) and La (to 6) and to give an average of 19.63 bonded Br/La₆Os vs the usual 18. These result in a cell volume 10% less than for an equivalent (hypothetical) La₆Br₁₀Os and Br-Br contacts as short as 3.30 Å. Increased polar La-Br interactions presumably drive these changes. Optimal atom sizes for this structure have been found so far only in this novel compound.