Breaking a Paradigm: Observation of Magnetic Order in the Purple U(IV) Phosphite: U(HPO3)2.

The hydrothermal crystal growth of a new uranium phosphite, U(HPO3)2, is reported. This material was found to exhibit optical and magnetic properties not commonly observed in U(IV) containing materials, specifically purple coloration and low temperature magnetism. We discuss the synthesis, structure determination, and characterization of the title compound and comment on its optical, thermal, and magnetic properties. The magnetic behavior is consistent with frustrated antiferromagnetism that arises from the hexagonal honeycomb lattice of U(IV) ions. This material gives rare evidence for U(IV) ions participating in magnetic order.