Magnetic properties of the copper trimer chains 4-picolinium2 Cu3Cl8 and 4-picolinium2 Cu3Br8

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Abstract
(4‐pic)2Cu3Cl8 and (4‐pic)2Cu3Br8 contain planar, bibridged copper halide trimers. The trimers are then stacked with longer semicoordinate Cu–halide–Cu bonds between adjacent trimers. The magnetic susceptibilities show that trimers are in a spin‐1/2 ground state at low temperatures with coupling constants J/k=30 and −100 K, respectively, determined by fitting the data to a new high‐temperature expansion with only exchange constants and external field as expansion parameters. We show that even though the spin‐1/2 trimers are antiferromagnetically exchange coupled along the chain, they order ferromagnetically at low temperature providing examples of pseudo‐one‐dimensional, S=1/2, Heisenberg ferromagnets. Electron paramagnetic resonance measurements at the X band were also performed and confirm the susceptibility measurements.