Browsing by Author "Evans, JSO"

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    3D transition metal ordering and Rietveld stacking fault quantification in the new oxychalcogenides La2O2Cu2–4xCd2xSe2
    (American Chemical Society, 2016-04-04) Ainsworth, CM; Lewis, JW; Wang, CH; Coelho, AA; Johnston, HE; Brand, HEA; Evans, JSO
    A number of LnOCuCh (Ln = La-Nd, Bi; Ch = S, Se, Te) compounds have been reported in the literature built from alternating layers of fluorite-like [Ln2O2]2+ sheets and antifluorite-like [M2Se2]2- sheets, where M is in the +1 oxidation state leading to full occupancy of available MSe4/2 tetrahedral sites. There is also a family of related LnOM0.5Se (Ln = La & Ce, M = Fe, Zn, Mn & Cd) compounds built from alternating layers of [Ln2O2]2+ sheets and [MSe2]2- sheets, where M is in the +2 oxidation state with half occupancy of available tetrahedral sites and complex ordering schemes in two dimensions. This paper reports a new family of compounds containing both +1 and +2 metal ions in the La2O2Cu2-4xCd2xSe2 family. We show how Cu1+ and Cd2+ ions segregate into distinct fully occupied and half occupied checkerboard-like layers respectively, leading to complex long-range superstructures in the third (stacking) dimension. To understand the structure and microstructure of these new materials we have developed and implemented a new methodology for studying low and high probability stacking faults using a Rietveld-compatible supercell approach capable of analyzing systems with thousands of layers. We believe this method will be widely applicable. © 2016 American Chemical Society.
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    Crystal structure and magnetic modulation in β−Ce2O2FeSe2
    (American Physical Society, 2017-08-11) Wang, CH; Ainsworth, CM; Champion, SD; Stewart, GA; Worsdale, MC; Lancaster, T; Blundell, SJ; Brand, HEA; Evans, JSO
    We report a combination of x-ray and neutron diffraction studies, Mössbauer spectroscopy, and muon spin relaxation (μ+SR) measurements to probe the structure and magnetic properties of the semiconducting β-Ce2O2FeSe2 oxychalcogenide. We report a structural description in space group Pna21 which is consistent with diffraction data and second harmonic generation measurements and reveal an order-disorder transition on one Fe site at TOD≈330K. Susceptibility measurements, Mössbauer, and μ+SR reveal antiferromagnetic ordering below TN=86K and more complex short range order above this temperature. 12 K neutron diffraction data reveal a modulated magnetic structure with q=0.444bN∗. © 2017 American Physical Society.
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    Infinitely adaptive transition-metal ordering in Ln2O2MSe2-type oxychalcogenides
    (American Chemical Society, 2015-04-30) Ainsworth, CM; Wang, CH; Johnston, HE; McCabe, EE; Tucker, MG; Brand, HEA; Evans, JSO
    A number of Ln2O2MSe2 (Ln = La and Ce; M = Fe, Zn, Mn, and Cd) compounds, built from alternating layers of fluorite-like [Ln2O2]2+ sheets and antifluorite-like [MSe2]2– sheets, have recently been reported in the literatures. The available MSe4/2 tetrahedral sites are half-occupied, and different compositions display different ordering patterns: [MSe2]2– layers contain MSe4/2 tetrahedra that are exclusively edge-sharing (stripe-like), exclusively corner-sharing (checkerboard-like), or mixtures of both. This paper reports 60 new compositions in this family. We reveal that the transition-metal arrangement can be systematically controlled by either Ln or M doping, leading to an “infinitely adaptive” structural family. We show how this is achieved in La2O2Fe1–xZnxSe2, La2O2Zn1–xMnxSe2, La2O2Mn1–xCdxSe2, Ce2O2Fe1–xZnxSe2, Ce2O2Zn1–xMnxSe2, Ce2O2Mn1–xCdxSe2, La2–yCeyO2FeSe2, La2–yCeyO2ZnSe2, La2–yCeyO2MnSe2, and La2–yCeyO2CdSe2 solid solutions. © 2015 American Chemical Society
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    Oxide ion and proton conductivity in highly oxygen-deficient cubic perovskite SrSc0.3Zn0.2Ga0.5O2.4
    (American Chemical Society, 2020-04-27) Fuller, CA; Berrod, Q; Frick, B; Johnson, MR; Avdeev, M; Evans, JSO; Evans, IR
    A series of Zn-substituted compounds, Sr2Sc1–xZnxGaO5–0.5x, based on the brownmillerite-type oxide ion conductor Sr2ScGaO5 have been synthesized, and a single-phase region has been identified at 0.4 ≤ x < 0.6. The structure and dynamics of Sr2Sc0.6Zn0.4GaO4.8 were investigated by X-ray and neutron diffraction, neutron total scattering and pair distribution function (PDF) analysis, impedance spectroscopy, and neutron spectroscopy. The material was found to be a highly disordered cubic perovskite with a remarkable level of oxygen deficiency across a large temperature range. These structural properties lead to an increase of oxide ion conductivity of about two orders of magnitude relative to the parent Sr2ScGaO5. The presence of proton conductivity and some water uptake was suggested by the impedance data and corroborated by thermogravimetric analysis (TGA), solid state nuclear magnetic resonance (NMR), variable temperature X-ray diffraction, and neutron spectroscopy. Both proton and oxide ion conductivity produced a measurable quasi-elastic neutron scattering (QENS) signal, and the onset of each dynamic process could be observed by monitoring the temperature dependence of the elastic and inelastic scattering intensities measured in fixed window scans. Neutron total scattering and PDF studies revealed a local structure that is markedly different from the perovskite average structure, and we propose that Sr2Sc0.6Zn0.4GaO4.8 contains a rare one-coordinate or terminal oxygen site. © 2020 American Chemical Society