Magnetic order and phase transition in the iron oxysulfide La 2 O 2 Fe 2 OS 2

Abstract

The Mott-insulating iron oxychalcogenides exhibit complex magnetic behaviour and we report here a neutron diffraction investigation into the magnetic ordering in La 2 O 2 Fe 2 OS 2 . This quaternary oxysulfide adopts the anti-Sr 2 MnO 2 Mn 2 Sb 2 -type structure (described by space group I4/mmm) and orders antiferromagnetically below T N = 105 K. We consider both its long-range magnetic structure and its magnetic microstructure, and the onset of magnetic order. It adopts the multi-k vector "2k" magnetic structure (k = (½ 0 ½) and k = (0 ½ ½) and has similarities with related iron oxychalcogenides, illustrating the robust nature of the "2k" magnetic structure.

Local structure of Mott insulating iron oxychalcogenides La2O2Fe2OM2(M=Karki1, Alfailakawi2, Frandsen3 et al. 2021Phys. Rev. B

“…4). However, no discontinuity was observed in the case of M = S. Such a finding is consistent with previous reports of c-lattice discontinuities in oxyselenides [19,53,54] and the less prominent occurrence of this behavior in the oxysulfides [18]. The discontinuities were attributed to magnetostrictive effects in the oxyselenides.…”

“…The discontinuities were attributed to magnetostrictive effects in the oxyselenides. In the case of oxysulfides, a relatively reduced amount of c-lattice discontinuity was argued to be due to shorter c lattices [18]. According to Horigane et al [40], deviations in the c-lattice parameter from the average crystallographic structure are due to the fact that the thermal displacement parameter Our analysis of the local structure of La 2 O 2 Fe 2 O(S, Se) 2 reveals the presence of short-range orthorhombic distortions on a length scale of 1-2 nm over a temperature range from 2 to 300 K. These distortions are very similar to those found in FeSe [34,38] and (Sr, Na)Fe 2 As 2 [30,31], which have been attributed to short-range, structural nematic fluctuations that persist up to temperatures well above the magnetic and/or structural transition temperatures.…”

“…This layered mixed anion material consists of a body-centered tetragonal crystal structure (I4/mmm) with fluoritelike [La 2 O 2 ] 2+ layers and [Fe 2 O] 2+ sheets separated by M 2− anions [16]. In this structure, [Fe 2 O] 2+ consists of an anti-CuO 2 arrangement with an Fe 2+ cation coordinated by four M 2− (above and below the plane) and two in-plane oxygen atoms, forming a tilted Fe-centered FeO 2 Se 4 octahedron [12,15,[17][18][19][20]. Figure 1 [16], structural studies using x-ray [19] and neutron [16] powder diffraction studies did not observe a structural phase transition in La 2 O 2 Fe 2 O(S, Se) 2 .…”

Local Structure of Mott Insulating Iron Oxychalcogenides La$_{2}$O$_{2}$Fe$_{2}$O$M$$_{2}$ ($M$ = S, Se)

“…Neutron powder diffraction experiments confirmed the average crystal structure of La 2 O 2 Fe 2 O(S, Se) 2 as tetragonal systems with space group (I4/mmm) (see Fig. 2) in agreement with the previous studies on these materials [13][14][15][16][17][18]. Unlike many iron pnictides which undergo a structural phase transition from tetragonal to orthorhombic symmetries near T s , La 2 O 2 Fe 2 O(S, Se) 2 lattice symmetries are not observed to change with temperature.…”

“…This layered mixed anion material consists of a body-centered tetragonal crystal structure (I4/mmm) with fluorite-like [La 2 O 2 ] 2+ layers and [Fe 2 O] 2+ sheets separated by M 2− anions [15]. In this structure, [Fe 2 O] 2+ consists of an anti-CuO 2 arrangement with an Fe 2+ cation coordinated by four M 2− (above and below the plane) and two in-plane oxygen atoms, forming a tilted Fe-centered FeO 2 Se 4 octahedron [13,14,[16][17][18]. Fig.…”