An investigation of 12 polycrystalline rare-earth oxysulfides has found long-range antiferromagnetic order in all but the europium, erbium and thulium compounds. The authors also report that cerium, praseodymium, neodymium and samarium compounds show this order in what they identify as previously unreported cases.
The compounds share a triangular-bilayer slab structure. Their magnetism was tested through temperature-dependent DC susceptibility and field-dependent DC magnetization, allowing the researchers to compare the family’s response as temperature and applied field changed.
One family, many magnetic outcomes
The series did not settle into one magnetic pattern. It showed diverse ground states and field-induced metamagnetic transitions, meaning that an applied field produced a change in magnetic state. The authors also describe three broad single-ion-anisotropy classes—Ising-like, XY-like and more isotropic—across the family.
Specific-heat measurements were limited to four compositions—Ce, Gd, Dy and Yb—from 2 to 300 K. The magnetic contribution was isolated using scaled La2O2S or Lu2O2S lattice standards. The study’s conclusion says that Gd and Yb retained significant magnetic entropy below 1.8 K.
The team also calculated crystal-field energy splittings with a point-charge model in the J basis for all RE2O2S compositions except Eu and Gd. The calculation accompanied the reported comparisons of single-ion-anisotropy classes and field-induced transitions.
The same Nd formula, different readings
The study compared three differently synthesized Nd2O2S samples, labeled A, B and C. High-temperature Curie-Weiss fits gave effective moments of 3.62(1), 4.04(1) and 4.41(1) µB/Nd3+ for A, B and C, respectively. The fitted Curie-Weiss temperatures became progressively more negative: -22.8(6), -58.6(6) and -77.6(6) K.
The study paired those magnetic differences with synchrotron pair distribution function analysis on the three samples to probe the structural origin of magnetic disorder. PDF refinement estimated oxygen substitution on the sulfur site at 37 ± 16% in A, 58 ± 17% in B and 78 ± 15% in C. The paper cautions that the percentages may overstate the true level of disorder.
After prolonged air exposure, nearly all compositions changed magnetically. Re-annealing with excess sulfur generally recovered the as-synthesized response, except for Ce2O2S. The authors link this pattern to sulfur loss during air exposure.
A changing lattice, unresolved questions
Rietveld refinement of measured powder X-ray diffraction patterns was used to derive lattice-parameter and bond-length trends. Across the rare-earth series from La to Lu, the refined lattice parameter a decreased by 8.5(1)% and c by 6.6(1)%.
These results map bulk behavior in polycrystalline powders, but they do not determine the microscopic magnetic structures or exchange constants for the full series. Specific heat was measured for only Ce, Gd, Dy and Yb, and the authors caution that the PDF-derived disorder percentages may be exaggerated relative to the true level.
The findings leave several pieces unresolved. The available evidence does not show that sulfur-site disorder is the only source of magnetic variability, and it does not settle whether the low-temperature entropy in Gd2O2S and Yb2O2S lies below the measured range or reflects additional dynamics. The microscopic nature of the field-induced phases also remains open.
The manuscript is an arXiv preprint identified as arXiv:2608.26023v1 and dated 26 August 2026.
Paper data and sources
Original title: Rare-earth oxysulfides RE$_2$O$_2$S as model mixed-anion frustrated magnets
Authors: Austin M. Ferrenti, Ksenia Khoroshun, Mohamed Oudah et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
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