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 we lded Nb containers at 900-950 degrees C resulted in only the title phase. T he Li48BT81Os8 stoichiometry is very close to that of known triclinic Pr6Br 10Os but with an similar to 32-times larger cell, 138 independent atoms, an d completely different intercluster connectivities in a complex monoclinic superstructure (a = 33.076(5) Angstrom, beta = 23.466(3) Angstrom, c = 23.5 37(2) Angstrom, B = 110.701(4)degrees, P2(1)/c (No. 14), Z = 4, 23 degrees C). Tetragonally compressed, similar to 16 e(-) lanthanum octahedra centere d by Os are heavily interbridged by Br, including Brf-a (f = face) and Bri- 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. Th ese result in a cell volume 10% less than for an equivalent (hypothetical) La6Br10Os and Br-Br contacts as short as 3.30 Angstrom. 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.