Structure and magnetism of hydronium jarosite, a model Kagomé antiferromagnet

We have determined the structure of a synthetic sample of the mineral hydronium jarosite by powder neutron diffraction, locating all the hydrogen atoms. Octahedrally coordinated Fe³⁺ ions are linked through their vertices to produce Kagomé layers of moments with a coverage of ca. 95 ± 4%, depending on the synthesis conditions. Magnetic susceptibility measurements indicate strong coupling of these moments, with Curie–Weiss constant of –1200 K, but there is no evidence for long-range magnetic order in neutron powder diffraction patterns taken down to 1.5 K. Instead, we observe an anomaly in the magnetic susceptibility at 17.2 K which is a characteristic of spin-glass freezing and similar to experimental work and theory for other Kagomé antiferromagnets.