New Polymorphs of RE₂FeSe₂O₂ (RE = La, Ce)

Abstract

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 (C2/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 in-

Crystal structure and magnetic modulation in β−Ce2O2FeSe2

“…Extra reflections (relative intensity ∼0.4% of the strongest reflection) are observed compared to the A-centered Amam (No. 63) space group reported by Nitsche et al [13], which indicates that β-Ce 2 O 2 FeSe 2 034403-2 ; P n2 1 a in same cell setting as P nma). Room temperature second-harmonic generation measurements were performed on a modified Kurtz-nonlinear optical (NLO) system using a previously published methodology [22].…”

“…McCabe et al reported the nuclear and magnetic structure of bulk layered α-Ce 2 O 2 FeSe 2 [4,6]; the space group of the nuclear structure is Imcb (72). Nitsche et al observed all three polymorphs (which they label as oI -, oA-and mC-Ce 2 O 2 FeSe 2 according to their symmetry) in single crystals grown at different temperatures [13]. The space groups of β-and γ -Ce 2 O 2 FeSe 2 they reported were Amam (No.…”

“…One important family of compounds is those with general composition Ln 2 O 2 MSe 2 (Ln = La, Ce; M = Mn, Fe, Zn, Cd) [3][4][5][6][7][8][9][10][11] and closely related composition such as La 2 O 2 Cu 2−4x Cd 2x Se 2 [12]. These materials are semiconductors with band gaps varying from ∼0.3 to ∼3.3 eV [1,[3][4][5]8,10,11,[13][14][15][16]. For example, layered La 2 O 2 CdSe 2 was reported as a wide-gap (3.3 eV) semiconductor and investigated as an optoelectronic device component [14,15].…”

“…Most adopt a layered structure (α phase) which can be modulated in either a commensurate or incommensurate manner within individual layers to accommodate different transition metal arrangements [8,10,11]. There are also two nonlayered structures reported (orthorhombic β phase and monoclinic Pb 2 HgCl 2 O 2 -type γ phase) [3,13]. All three structures can be adopted by Ce 2 O 2 FeSe 2 by modifying the synthesis conditions [4,13].…”

“…There are also two nonlayered structures reported (orthorhombic β phase and monoclinic Pb 2 HgCl 2 O 2 -type γ phase) [3,13]. All three structures can be adopted by Ce 2 O 2 FeSe 2 by modifying the synthesis conditions [4,13]. McCabe et al reported the nuclear and magnetic structure of bulk layered α-Ce 2 O 2 FeSe 2 [4,6]; the space group of the nuclear structure is Imcb (72).…”