The discovery of the iron-based oxypnictide superconductor LaFeAsO1 xFx has reinvigorated research on superconductors, and has been quickly followed by a number of studies of a series of lanthanide iron-based oxypnictide superconductors. Of these oxypnictide superconductors, SmFeAsO1 xFx exhibits the highest critical temperature (Tc) of 55K. Charge doping from reservoir layers consisting of LnO1 xFx (Ln: lanthanide) to conduction layers consisting of FeAs, is considered to be controlled by the substitution of oxygen atoms with fluorine atoms. Although there have been discussions comparing the superconducting properties of iron-based oxypnictides and copper-based oxide superconductors, experimental data for new oxypnictide superconductors remain relatively scarce. Recently, Kito et al. have succeeded in preparing a series of fluorine-free lanthanide iron-based oxypnictide superconductors by a high-pressure technique. Carrier doping is controlled in this case by oxygen vacancies formation instead of fluorine replacement. The crystallographic structures of such fluorine-free iron-based oxypnictide superconductors, and particularly the order or disorder of oxygen vacancies, should thus be investigated. In this short note, the crystallographic structure of NdFeAsO1 y (1 y 1⁄4 0:5 nominal) with Tc 1⁄4 54K, synthesized by a high-pressure technique, is evaluated by electron diffraction analyis and electron microscopy. The space group thus elucidated has assisted subsequent quantitative structural analysis by Rietveld neutron diffractometry. NdFeAsO0:5 specimens for microscopy observation were prepared by argon-ion milling. Transmission electron microscopy (TEM) was performed using JEM 4000FX and JEM2010 instruments at 200 kV. The present samples were polycrystalline with a grain size of several hundreds of nanometers. No other phases could be clearly detected, although some lattice defects were observed. Figure 1 shows the electron diffraction pattern of the material taken on the [001] zone axis. No intensity of diffuse scattering due to the short-range ordering of oxygen vacancies is observed. However, very weak spot intensities are visible in the diffraction pattern taken over a longer exposure period, as indicated by arrows in the figure. These spot intensities correspond to positions hk0; hþ k 1⁄4 2nþ 1, and might indicate the long-range ordering of oxygen vacancies. Figure 2 shows an electron diffraction pattern taken on the [100] zone axis. Strong diffraction spots are apparent at 0k0; k 1⁄4 2nþ 1 in this case. Figure 3 shows the convergent beam electron diffraction (CBED) pattern observed on the [100] zone axis. Gjonnes–Moodie lines appear in the CBED discs at 0k0; k 1⁄4 2nþ 1. The diffraction intensity at 0k0; k 1⁄4 2nþ 1 and the weak spot intensities in Fig. 1 are thus considered to be due to multiple scattering. No other extra reflections caused by the long-range ordering or diffuse scattering due to the short-range ordering of vacancies are observed. Oxygen vacancies are concluded to be in a disordered state. No diffracted intensity associated with vacancy ordering is also observed in powder neutron diffraction measurement. The observation of electron diffraction spots corresponding to the 0th-order Laue zone and the firstorder Laue zone on the [001] zone axis indicates that the extinction rule is hk0 with hþ k 1⁄4 2n, suggesting the space group P4=n (No. 85) or P4=nmm (No. 129) for this crystal. P4=n has a fourfold rotational axis about the [001] zone axis, while P4=nmm has a 4mm symmetry around the Fig. 1. Electron diffraction pattern on the [001] zone axis. Weak spot diffractions are indicated by white arrows.
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Mar 3, 2016
Dai-Ning Cho + 4
