  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 kagom\'e sublattice that
retains inversion symmetry. Magnetic susceptibility measurements reveal that
dried 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 = 6.29(1) muB/Fe^3+ and theta_CW = -526.0(1.1) K,
analogous to that observed in similar high-spin Fe^3+ 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, dS_mag = 14.86
J/mol_Fe^3+ K, is also in good agreement with the dS_mag = Rln(2S+1) = 14.9
J/mol K expected for a S = 5/2 system. 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 kagom\'e 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.
