Low-Temperature Magnetic Susceptibility of FeCl3

We have measured the magnetic susceptibility of carefully purified FeCl₃ and FeCl₃·6H₂O over the temperature range from 4.2° to 30°K. An antiferromagnetic transition with a Néel temperature $T_N\text{\hspace{0.17em}=\hspace{0.17em}9.75°\hspace{0.17em}±\hspace{0.17em}0.2°}\mathrm{K}$ was observed in FeCl₃. This is significantly lower than previously reported values determined by Mössbauer effect and neutron diffraction. Mössbauer resonance was found to be an excellent test for small amounts of water decomposition products of anhydrous FeCl₃. Ferric chloride hexahydrate was found to be paramagnetic and obeyed the Curie law down to 4.2°K.