Hydrothermal synthesis of ordered corkite, PbFe3(PO4)

Austin M. Ferrenti,Vanessa Meschke, Susmita Ghosh,Jackson Davis,Natalia Drichko,Eric S. Toberer,Tyrel M. McQueen

Corkite, PbFe3(PO4)(SO4)(OH)6, an understudied relative of the jarosite family of Heisenberg antiferromagnets, has been synthesized and its magnetic properties characterized for the first time. Relative to natural samples, synthetic corkite displays signatures in both infrared and Raman spectra of a more ordered arrangement of polyanion groups about the kagomé sublattice that retains inversion symmetry. Magnetic susceptibility measurements reveal that corkite undergoes a transition to a long-range, antiferromagnetically-ordered state below TN ​= ​48 ​K, lower than that observed in the majority of jarosite phases, and indicative of further spin frustration. Curie-Weiss fitting of the measured magnetic susceptibility yields an effective magnetic moment of peff ​= ​5.87(3) μB/Fe3+ and θCW ​= ​−526.6(4.4) K, analogous to that observed in similar high-spin Fe3+ systems, and indicative of strong antiferromagnetic coupling. Estimation of the change in magnetic entropy as a function of temperature from T ​= ​0 ​K to T ​= ​195 ​K, ΔSmag ​= ​14.86 J⋅molFe3+−1K−1, is also in good agreement with the ΔSmag ​= ​Rln(2S ​+ ​1) ​= ​14.9 ​J ⋅ mol−1K−1 expected for a S ​= ​5/2 system. Annealing of synthetic corkite at 200 ​°C results in changes to both the measured IR and Raman spectra, indicative of partial removal of bridging hydroxyl groups from the kagomé sublattice. In comparison to the pure jarosites, where both structure and magnetism remain largely invariant upon a variety of chemical substitutions, the replacement of one sulfate group per formula unit with a higher-valent phosphate group applies additional steric and electronic pressure on the kagomé lattice in corkite, further frustrating the magnetic ground state of the material. Corkite thus represents both an outlier in the known body of jarosite-type materials, and an illustration of how existing structures may be further strained in the development of highly frustrated magnetic systems.