The antiferromagnetic mineral jarosite, KFe₃(OH)₆(SO₄)₂, has been intensely studied for several decades, in part because its high-spin Fe³⁺ ions are arrayed on a corner-sharing triangular (kagomé) lattice, which ensures magnetic frustration. In fact, jarosite has one of the most highly frustrated two-dimensional lattices known to exist. Frustrated magnets are materials whose localized magnetic moments, or spins, interact through competing interactions that cannot be simultaneously satisfied, creating the possibility that lattice manipulation can produce exotic magnetic phases of commercial value. Extensive theoretical work has hinted at a rich phase diagram for frustrated kagomé antiferromagnets like jarosite, but experimental evidence remained lacking until very recently. Since applied high pressure shortens interatomic distances, the technique offers a route to accessing magnetic variants of frustrated parent lattices that may be difficult to access synthetically at ambient conditions. For this reason, researchers working at the U.S. Department of Energy’s (DOE’s) Advanced Photon Source (APS) and National Synchrotron Light Source II (NSLS II) investigated the pressure -temperature phase diagram of jarosite at pressures up to 121 GPa. Upon finding that jarosite’s antiferromagnetic magnetic order unexpectedly disappeared at ~45 GPa (Fig. 1), they went on to probe for an explanation of this surprising behavior by characterizing the structural, electronic, and magnetic changes in jarosite through the phase transition using a suite of in situ diamond anvil cell techniques. Their results were published in Physical Review Letters.

The APS-based research was conducted at several x-ray beamlines: HPCAT-XSD 16-ID-B, 16-ID-D, and 16-BM-D; X-ray Science Division 3-ID; and   GeoSoilEnviroCARS 13-BM-C. Fourier-transform infrared spectroscopy (FTIR) experiments were conducted at beamline 22-IR-1 of the NSLS II. The APS and NSLS II are Office of Science user facilities at Argonne National Laboratory and Brookhaven National Laboratory, respectively.

The researchers searched for pressure-driven phase transitions in jarosite by collecting powder x-ray diffraction (PXRD) patterns at ambient temperature up to 78.6 GPa and extracting unit cell lattice parameters at each pressure. To better understand the complex behavior they encountered at 43.7 GPa, they performed density-functional theory (DFT) calculations that suggested a new high-pressure structure (R¯3c space group) emerged from the previous antiferromagnetically ordered R¯3m structure.  FTIR measurements at high pressures supported the DFT-predicted conclusion that the high-pressure phase of jarosite was accurately described by the R¯3c structure.

Seeking further confirmation as to the nature of the high-pressure phase, the researchers examined the local electronic structure of the Fe³⁺ ions in the high-pressure phase using non-resonant x-ray emission spectroscopy (XES) and ambient-temperature, variable-pressure synchrotron Mössbauer spectroscopy (SMS). These data in combination again supported the calculated structural transition from the R¯3m to the R3c phase.

Use of SMS data in characterizing the magnetic ordering temperature in jarosite indicated that the collapse of magnetic order was coincident in pressure with the structural phase transition that was observed in the PXRD and FTIR data, predicted by the DFT calculations, and inferred from the combined XES and SMS data.

The extensive characterization allowed the researchers to exclude certain possible magnetic ground states in the measured region of study. These included a spin glass state and a conventional quantum spin liquid state or a valence bond solid state. They also concluded that the pressure-induced phase was distinct from the field-induced phase in jarosite. Lastly, the results of the DFT calculations suggested that an insulator-to-metal Mott transition did not occur, and that jarosite remained insulating across the phase transition. Based on symmetry arguments, the researchers hypothesized that the resulting structural changes altered the magnetic interactions to favor a possible quantum paramagnetic phase at high pressure.  ― Vic Comello

See: Ryan A. Klein,¹ James P. S. Walsh,¹ Samantha M. Clarke,² Zhenxian Liu,³ E. Ercan Alp,⁴ Wenli Bi,⁵ Yue Meng,⁴ Alison B. Altman,¹ Paul Chow,⁴ Yuming Xiao,⁴ M. R. Norman,⁴** James M. Rondinelli¹*** Steven D. Jacobsen,¹**** Danilo Puggioni,¹***** and Danna E. Freedman¹*, “Pressure-Induced Collapse of Magnetic Order in jarosite,” Phys. Rev. Lett 125, 077202 (2020). DOI: 10.1103/PhysRevLett.125.077202

Author affiliations: ¹Northwestern University, ²Lawrence Livermore National Laboratory, ³University of Illinois at Chicago, ⁴Argonne National Laboratory, ⁵University of Alabama at Birmingham

Correspondence: ^* [email protected], ** [email protected], *** [email protected], **** [email protected], ***** [email protected]

 Creating and understanding magnetic frustration in synthetically derived materials was supported by the Army Research Office (W911NF1810006). Initial work to develop high-pressure methodologies was supported by the Air Force Office of Scientific Research (FA9550-17-1-0247). S. D. J. acknowledges support from the Capital/U.S. Department of Energy (DOE) Alliance Center (CDAC) for beamtime at HPCAT-XSD, the National Science Foundation (NSF) (EAR-1452344), and the David and Lucile Packard Foundation. D. P. and J. M. R. acknowledge the Army Research Office under Grant No. W911NF-15-1-0017 for financial support and the Department of Defense High Performance Computing Modernization Program (DOD-HPCMP) for computational resources. W. B. was partially supported by the Consortium for Materials Properties Research in Earth Sciences (COMPRES). M. R. N. is supported by the Materials Sciences and Engineering Division, Basic Energy Sciences, Office of Science, U.S. DOE. Part of this work was performed under the auspices of the DOE by Lawrence Livermore National Security, LLC, under Contract No. DE-AC52-07NA27344. The gas loading was also partially supported by COMPRES under the NSF Cooperative Agreement EAR-1634415. HPCAT-XSD operations are supported by the DOE-National Nuclear Security Administration’s Office of Experimental Sciences. The National Science Foundation—Earth Sciences (EAR-1634415) and Department of Energy—Geosciences (DE-FG02-94ER14466) support GeoSoilEnviroCARS. Experiments at GeoSoilEnviroCARS beamline 13-BM-C were conducted under the Partnership for Extreme Crystallography (PX^2), which is also supported by COMPRES. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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