View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by UCL Discovery PHYSICAL REVIEW B 90, 165111 (2014) Structural and magnetic characterization of iron oxyselenides Ce2O2Fe2OSe2 and Nd2O2Fe2OSe2 E. E. McCabe,1,2,* A. S. Wills,3 L. Chapon,4,5 P. Manuel,5 and J. S. O. Evans2,† 1School of Physical Sciences, University of Kent, Canterbury, CT2 7NH, United Kingdom 2Department of Chemistry, Durham University, Durham DH1 3LE, United Kingdom 3Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, United Kingdom 4Diffraction Group, Institute Laue Langevin, 71 Avenue des Martyrs, 38000 Grenoble, France 5ISIS Facility, Rutherford Appleton Laboratory, Chilton Didcot, OX11 0QX, United Kingdom (Received 4 August 2014; published 9 October 2014) We present here an investigation of the magnetic ordering in the Mott insulating oxyselenide materials Ln2O2Fe2OSe2 (Ln = Ce, Nd). Neutron powder diffraction data are consistent with a noncollinear multi-k ordering on the iron sublattice structure and analysis indicates a reduced magnetic correlation length perpendicular 2+ 3+ 3+ to the [Fe2O] layers. The magnetic role of the Ln cations is investigated and Ce moments are found to order at T 16 K. DOI: 10.1103/PhysRevB.90.165111 PACS number(s): 75.40.−s, 75.25.−j I. INTRODUCTION studies on La2O2Fe2OSe2 indicated the presence of antiphase boundaries or stacking faults perpendicular to the stacking di- The discovery of iron-based superconductivity [1] in several rection and gave evidence for 2D-Ising-like magnetism within mixed anion systems has driven research to understand these sheets [25]. In the light of this work, it is timely to report the magnetism of these materials which is related to their a similar analysis on Ln O Fe OSe (Ln = Ce, Nd) analogs to superconducting behavior [2]. The 1111 family of iron- 2 2 2 2 test the robustness of the 2-k model to changes in Fe–O bond based superconductors derive from LnFeAsO (Ln = trivalent lengths and to a magnetic moment on the Ln site. We show that lanthanide) and adopt the ZrCuSiAs- structure [3], composed neutron powder diffraction (NPD) data are consistent with the of alternating layers of antifluorite-like edge-linked FeAs 4 proposed 2-k magnetic structure and that much larger magnetic tetrahedra alternating with fluorite-like edge-linked OLn 4 stacking domains are formed with these smaller magnetic tetrahedra. Although the superconductivity arises in the iron lanthanides. In contrast with previous work [10], we show arsenide layers, the Ln3+ cation has a significant role in both that Ce moments order at low temperatures and that unlike tuning the superconducting T in the doped materials [1,4] and c many related oxyarsenide systems, this Ce moment ordering in defining the magnetism of the parent nonsuperconducting + does not cause a reorientation of the Fe2 moments. materials (such as the spin reorientations in NdFeAsO [5] and PrFeAsO [6]). This has led to much research into the interplay II. EXPERIMENTAL DETAILS between transition metal and lanthanide magnetic sublattices in these and related mixed-anion systems [7–19]. 5 g each of Ce2O2Fe2OSe2 and Nd2O2Fe2OSe2 were La2O2Fe2OSe2 was the first member of the “M2O” (M = prepared by solid state reaction of CeO2 (Alfa-Aesar, 99.8%) transition metal) family of oxyselenide materials reported and or Nd2O3 (Electronic Materials, 99%), Fe (Aldrich, 99.9%) 2+ adopts a layered structure with fluorite-like [La2O2] slabs and Se (Alfa Aesar, 99.999%) powders. For Nd2O2Fe2OSe2, (analogous to those in 1111 iron oxyarsenides) separated stoichiometric quantities of these reagents were intimately 2− 2+ by Se anions from [Fe2O] sheets (Fig. 1)[20]. The ground by hand using an agate pestle and mortar. The resulting Fe2+ cations are coordinated by two O(2) ions within these gray powder was pressed to form several pellets of 5 mm 2− anti-CuO2 sheets, and by four Se ions above and below diameter. These were placed inside a quartz tube which was ◦ the sheets, forming a network of face-shared FeO2Se4 trans- evacuated and sealed. This was then heated slowly to 1000 C, octahedra. Much research has been carried out to understand held at this temperature for 12 h, and then cooled in the furnace. the exchange interactions and magnetic ordering present in For Ce2O2Fe2OSe2, a stoichiometric mixture of reagents was 2+ these [Fe2O] sheets [21]. La2O2Fe2OSe2 orders antiferro- ground together and pressed into pellets which were placed magnetically (AFM) below ∼90 K [22] and two magnetic in a quartz tube with an oxygen-getter (Al powder in 10% structures have been discussed: a 2-k model first proposed for excess which was physically isolated from the reagents). This ◦ Nd2O2Fe2OSe2 [23] and a collinear model [10,22]. Recent tube was evacuated, sealed, slowly heated to 1000 C, and work suggests that the 2-k model is the more likely [24–26]. held at this temperature for 12 h. The reaction tube was then In this magnetic structure (Fig. 1), both next-nearest-neighbor quenched into a bucket of iced water. Both Ce2O2Fe2OSe2 ◦ (NNN) interactions (ferromagnetic J2 ∼ 95 Fe–Se–Fe inter- and Nd2O2Fe2OSe2 were formed as black, polycrystalline ◦ actions, and AFM J2 180 Fe–O–Fe interactions) are fully powders. Preliminary structural characterization was carried satisfied. Interestingly, a similar 2-k model, composed of out using a Bruker D8 Advance diffractometer fitted with a ◦ perpendicular AFM chains, has been predicted for the FeAs LynxEye silicon strip detector (step size 0.021 ) operating sheets in iron arsenides and oxyarsenides [27,28]. The recent in reflection mode with Cu Kα radiation. NPD data were collected on the time of flight (TOF) diffractometer WISH [29] on target station 2 at the ISIS spallation neutron source. The *Corresponding author: [email protected] powders were placed in cylindrical 6 mm diameter vanadium † Corresponding author: [email protected] cans (to heights of ∼6cm).ForCe2O2Fe2OSe2,16min 1098-0121/2014/90(16)/165111(7) 165111-1 ©2014 American Physical Society MCCABE, WILLS, CHAPON, MANUEL, AND EVANS PHYSICAL REVIEW B 90, 165111 (2014) (a) (b) (e) J O Se a 2’ Fe J La 2 O(1) Se J 1 Fe O(2) b (c) 4.10 La 4.08 Ce 4.06 / Å Pr a 4.04 Nd 4.02 4.00 1.10 1.12 1.14 1.16 Ln3+ caon radius (d) 18.60 La 18.50 Ce / Å Nd Pr c 18.40 18.30 1.10 1.12 1.14 1.16 Ln3+ caon radius 2− 2− 2+ 3+ FIG. 1. (Color online) (a) Nuclear cell of Ln2O2Fe2OSe2 with O ,Se ,Fe ,andLn ions in red, yellow, blue, and green, respectively, 2+ showing [Ln2O2] fluorite-like layers built from edge-shared OLn4 tetrahedra in red; (b) shows the in-plane arrangement of magnetic moments for the 2-k model, with O2− (red), Se2− (yellow), and Fe2+ (blue) ions shown, and Fe2+ moments shown by blue arrows; the three intraplanar exchange interactions J1, J2,andJ2 are also shown; (c) and (d) show plots of unit cell parameters a and c for Ln2O2Fe2OSe2 determined from Rietveld analysis using room temperature XRPD data (data in blue are described in this work including quench-cooled Ce2O2Fe2OSe2); data in red are determined for the main () and secondary (×) phases in two phase samples of “Ce2O2Fe2OSe2”; green points are those reported for Ce2O2Fe2OSe2 by Ni et al. [10]; (e) illustrates the magnetic ordering of Ce (green) and Fe (blue) moments [O(1) sites are omitted for clarity]. (10 μA h) scans were collected every 4 K on cooling from analysis (EDX) measurements were made over areas of the 120 K to 8 K and a final 25 min (15 μA h) scan was collected sample and at 65–100 points on each sample. at 1.5 K. For Nd2O2Fe2OSe2, several 16 min (10 μA h) scans were collected between 1.5 K and 12 K followed by a further 5 on warming to 300 K. Rietveld refinements [30]were III. RESULTS performed using Topas Academic software [31,32]. Sequential The synthesis method described gave a single-phase sam- refinements were controlled using local subroutines. The high ple of Nd2O2Fe2OSe2 by x-ray and neutron analysis (see ◦ d-spacing 59 bank of data (10.5–85.0 ms, 1–8 A)˚ was used to Supplemental Material [33]). Quenching the sealed reaction study the magnetic behavior of the two materials. The nuclear tube of Ce2O2Fe2OSe2 from the reaction temperature into iced and magnetic refinements were carried out with separate water gave an almost single-phase sample of Ce2O2Fe2OSe2 nuclear and magnetic phases (lattice parameters and scale (NPD data revealed the presence of ∼3% by weight of factor of the magnetic phase were constrained to be multiples a second phase similar to the main phase which was of the nuclear phase). Magnetic susceptibility was measured included in refinements). See Supplemental Material [33] using a Magnetic Properties Measurement System (Quantum for further analyses. Rietveld refinements using NPD data Design, MPMS). Field-cooled and zero-field-cooled data for both Ln = Ce, Nd phases are in good agreement with were collected on warming from 2 K to 292 K at 5 K the nuclear structure previously reported [10]. Refining site intervals in an applied magnetic field of 1000 Oe. Field occupancies (with Ln site fully occupied) gave compositions ± × 4 sweep measurements to 5 10 Oe were made at 292 K close to stoichiometric (Ce2O1.99(1)Fe2.00(1)O0.99(1)Se2.0(1) and and at 12 K. Scanning electron microscopy for compositional Nd2O1.93(1)Fe1.96(1)O0.913(9)Se2.05(1)). Site occupancies were analysis of Ce2O2Fe2OSe2 samples used a Hitachi SU-70 fixed at unity for subsequent refinements.