Hydrothermal synthesis of ordered corkite, PbFe3(PO4)(SO4)(OH)6, a S = 5/2 kagom\'e antiferromagnet

TL;DR

This study reports the synthesis and magnetic characterization of corkite, a kagomé antiferromagnet, revealing its ordered structure, magnetic transition at 48 K, and insights into magnetic frustration in jarosite-related materials.

Contribution

First synthesis and magnetic analysis of corkite, demonstrating increased polyanion order and magnetic frustration compared to natural and other jarosite phases.

Findings

Corkite exhibits a magnetic transition at 48 K indicating long-range antiferromagnetic order.

Magnetic susceptibility suggests strong antiferromagnetic coupling with a Curie-Weiss temperature of -526 K.

Magnetic entropy change aligns with theoretical expectations for S=5/2 systems.

Abstract

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.