Flux Synthesis, Crystal Structures, and Magnetic Ordering of the Rare-Earth Chromium(II) Oxyselenides RE2CrSe2O2 (RE = La–Nd)

The rare-earth chromium(II) oxyselenides RE2CrSe2O2 (RE = La–Nd) were synthesized in eutectic NaI/KI fluxes, and their crystal structures were determined by single-crystal and powder X-ray diffraction (Pb2HgCl2O2-type, C2/m, Z = 2). The magnetic structure of La2CrSe2O2 was solved and refined from neutron powder diffraction data. Main building blocks are chains of edge-sharing CrSe4O2 octahedra linked together by two edge-sharing ORE3Cr tetrahedra forming infinite ribbons. The Jahn–Teller instability of divalent Cr2+ (d4) leads to structural phase transitions at 200 and 130 K in La2CrSe2O2 and Ce2CrSe2O2, respectively. RE2CrSe2O2 are Curie–Weiss paramagnetic above TN ≈ 14–17 K. Neutron powder diffraction reveals anti-ferromagnetic ordering of the Cr2+ moments in La2CrSe2O2 below TN = 12.7(3) K with an average ordered moment of 3.40(4) μB/Cr2+ at 4 K, which was confirmed by muon spin rotation experiments.