The low temperature heat capacities of ferrous bromide and ferrous iodide

The specific heats of ferrous bromide and ferrous iodide are reported in the approximate temperature range 6 to 295 K. The results are believed to be accurate to within 2 per cent at 295 K and 5 per cent at 6 K. The specific heats displayed anomalies at 11.4+or-0.1 K and 8.9+or-0.1 K respectively, and these have been associated with transitions to a state having ferromagnetic coupling between Fe ++ ions within layers, and antiferromagnetic coupling between adjacent layers. The approach to and departure from the peak in each curve of deduced magnetic heat capacity against temperature is logarithmic, and the entropy changes are consistent with values calculated assuming a spin 4 / 2 for the Fe ++ ions, that is 13.1+or-0.6 J mol -1 K -1 for both solids.