New Polymorphs of RE 2 FeSe 2 O 2 ( RE = La, Ce)

New monoclinic polymorphs of RE 2 FeSe 2 O 2 ( RE = La, Ce) were synthesized at 800 °C in a NaI/KI flux, and characterized by single‐crystal X‐ray diffraction ( C 2/ m , Z = 2). mC ‐La 2 FeSe 2 O 2 and mC ‐Ce 2 FeSe 2 O 2 represent new structures in the RE 2 FeSe 2 O 2 family, and contain iron atoms exclusively in a distorted octahedral coordination of four selenium and two oxygen atoms. Such FeSe 4 O 2 octahedra occur together with FeSe 4 ‐tetrahedra if the synthesis temperature increases to 1000 °C, while at 1100 °C all iron atoms are tetrahedrally coordinated by selenium. Even though the polymorph formation is controlled by the synthesis temperature, no phase transformations could be observed. La 2 FeSe 2 O 2 is a semiconductor with a band gap of approximately 0.3 eV and shows antiferromagnetic ordering at T N ≈ 20 K with an effective magnetic moment of 4.35(2) μ B .