David G. Free

University of Oxford, Oxford, England, United Kingdom

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Publications (14)68.82 Total impact

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    ABSTRACT: Oxidative deintercalation of copper ions from the sulfide layers of the layered mixed-valent manganite oxide sulfide Sr2MnO2 Cu 1.5S2 results in control of the copper-vacancy modulated superstructure and the ordered arrangement of magnetic moments carried by the manganese ions. This soft chemistry enables control of the structures and properties of these complex materials which complement mixed-valent perovskite and perovskite-related transition metal oxides.
    APL Materials 04/2015; 3(4):041520. DOI:10.1063/1.4918973 · 2.79 Impact Factor
  • Alex J Corkett · David G Free · Simon J Clarke ·
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    ABSTRACT: High-resolution X-ray and neutron powder diffraction are used to reveal details of the spin-reorientation transition in the layered oxide pnictide CeMnAsO. Above 38 K, the localized moments on Mn(2+) are antiferromagnetically ordered in a checkerboard fashion within the antifluorite-type MnAs planes and are oriented perpendicular to the planes. Below 38 K, reorientation of these moments into the planes commences. This is complete by 34 K and is coincident with long-range ordering of the Ce(3+) moments. The Ce(3+) and Mn(2+) moments have an arrangement that is different in detail from that in the isostructural NdMnAsO and PrMnSbO. There is no evidence for structural distortion, as found for PrMnSbO and related Pr(3+)-containing compounds, although there is evidence for a very slight (0.025%) misfit between the magnetic and structural cells below the spin-reorientation transition. It is clarified that neutron powder diffraction methods are unable to distinguish between collinear and noncollinear arrangements of manganese and lanthanide moments when the moments have a component parallel to the MnAs planes. A proposal from computational analysis that NdMnAsO and CeMnAsO should adopt different magnetic structures on the basis of the different balances between biquadratic and antisymmetric exchange interactions should be tested using alternative methods.
    Inorganic Chemistry 12/2014; 54(3). DOI:10.1021/ic5026608 · 4.76 Impact Factor
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    ABSTRACT: The effect of the isovalent substitution of Sr2+ by Ca2+ on the structure and superconducting properties of Sr2-xCaxVO3FeAs is described in the compositional range 0 <= x <= 0.5. SQUID magnetometry measurements reveal that after an initial increase in T-c, which is maximised at 29.5 K in Sr1.95Ca0.05VO3FeAs. a rapid suppression of superconductivity is observed with increasing x. XANES spectra of Sr2-xCaxVO3FeAs collected on the Fe and V absorption K-edges show that the position of both edges are invariant with composition within the experimental uncertainty. A combination of synchrotron X-ray powder diffraction and neutron powder diffraction techniques is used to rationalise the observed changes in T-c with x, in terms of changes to the structure of the FeAs layer upon partial Ca substitution. These findings demonstrate that superconductivity in the Fe-based superconductors is extremely sensitive to the crystal structure with T-c maximised in samples with regular FeAs4-tetrahedra.
    ChemInform 08/2014; 216(33):91–98. DOI:10.1002/chin.201433009
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    ABSTRACT: The title compound is prepared from a mixture of La2O3, Se, and Zn using Al powder as an oxygen getter (alumina crucibles in an evacuated silica tube, 1100 °C, 12 h).
    ChemInform 05/2013; 44(21). DOI:10.1002/chin.201321012
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    ABSTRACT: The title compound is synthesized from mixtures of SrO, Mn, MnO2, Se, and KCl as a flux (alumina crucible, 800 °C, 7 d).
    ChemInform 03/2013; 44(11). DOI:10.1002/chin.201311017
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    ABSTRACT: The quaternary transition metal oxyselenide La(2)O(2)ZnSe(2) has been shown to adopt a ZrCuSiAs-related structure with Zn(2+) cations in a new ordered arrangement within the [ZnSe(2)](2-) layers. This cation-ordered structure can be derived and described using the symmetry-adapted distortion mode approach. La(2)O(2)ZnSe(2) is an direct gap semiconductor with an experimental optical band gap of 3.4(2) eV, consistent with electronic structure calculations.
    Inorganic Chemistry 01/2013; 52(4). DOI:10.1021/ic302484x · 4.76 Impact Factor
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    ABSTRACT: The synthesis, crystal structure and magnetic properties of the brown oxide selenide Sr2Mn2O4Se are described. The compound crystallises in space group Pnma with room temperature lattice parameters a = 9.4825(1), b = 10.3356(1) and c = 6.23895(8) Å. The structure contains Mn3+ ions in highly distended MnO4Se2 octahedra with selenide ions in the axial positions. These octahedra share two of their faces with neighbouring octahedra with the result that all the manganese and oxide ions are involved in the formation of undulating [MnO4/2]– chains directed along the a axis, which form corrugated chains along the c axis by sharing the selenide vertices of the octahedra. These chains are isolated from one another along the b axis. Magnetic susceptibility measurements show that the compound does not obey the Curie-Weiss law below room temperature and there is a magnetic transition at 160(5) K associated with antiferromagnetic ordering of the Mn3+ moments. The magnetic structure measured at 5 K is described by a k = (0, 0, 0) propagation vector and nearest neighbour moments are coupled antiferromagnetically.
    Zeitschrift für anorganische und allgemeine Chemie 12/2012; 638(15):2532-2537. DOI:10.1002/zaac.201200254 · 1.16 Impact Factor
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    ABSTRACT: The discovery of high-temperature superconductivity in a layered iron arsenide has led to an intensive search to optimize the superconducting properties of iron-based superconductors by changing the chemical composition of the spacer layer between adjacent anionic iron arsenide layers. Superconductivity has been found in iron arsenides with cationic spacer layers consisting of metal ions (for example, Li(+), Na(+), K(+), Ba(2+)) or PbO- or perovskite-type oxide layers, and also in Fe(1.01)Se (ref. ) with neutral layers similar in structure to those found in the iron arsenides and no spacer layer. Here we demonstrate the synthesis of Li(x)(NH(2))(y)(NH(3))(1-y)Fe(2)Se(2) (x~0.6; y~0.2), with lithium ions, lithium amide and ammonia acting as the spacer layer between FeSe layers, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far. We have determined the crystal structure using neutron powder diffraction and used magnetometry and muon-spin rotation data to determine the superconducting properties. This new synthetic route opens up the possibility of further exploitation of related molecular intercalations in this and other systems to greatly optimize the superconducting properties in this family.
    Nature Material 10/2012; 12(1). DOI:10.1038/nmat3464 · 36.50 Impact Factor
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    ABSTRACT: DOI: 10.1021/cm301486v The series Sr2MnO2Cu1.5(S1–xSex)2 (0 ≤ x ≤ 1) contains mixed-valent Mn ions (Mn2+/Mn3+) in MnO2 sheets which are separated by copper-deficient antifluorite-type Cu2−δCh2 layers with δ 0.5. The compounds crystallize in the structure type first described for Sr2Mn3Sb2O2 and are described in the I4/mmm space group at ambient temperatures. Below about 250 K, ordering between Cu+ ions and tetrahedral vacancies occurs which is long-range and close to complete in the sulfide-containing end member of the series Sr2MnO2Cu1.5S2 but which occurs over shorter length scales as the selenide content increases. The superstructure is an orthorhombic 2√2a × √2a × c expansion in Ibam of the room temperature cell. For x > 0.3 there are no superstructure reflections evident in the X-ray or neutron diffraction patterns, and the I4/mmm description is valid for the average structure at all temperatures. However, in the pure selenide end member, Sr2MnO2Cu1.5Se2, diffuse scattering in electron diffractograms and modulation in high resolution lattice image profiles may arise from short-range Cu/vacancy order. All members of the series exhibit long-range magnetic order. In the sulfide-rich end member and in compounds with x < 0.1 in the formula Sr2MnO2Cu1.5(S1–xSex)2, which show well developed superstructures due to long-range Cu/vacancy order, the magnetic structure has a (1/41/4 0) propagation vector in which ferromagnetic zigzag chains of Mn moments in the MnO2 sheets are coupled antiferromagnetically in an arrangement described as the CE-type magnetic structure and found in many mixed-valent perovskite and Ruddlesden–Popper type oxide manganites. In these cases the magnetic cell is an a × 2b × c expansion of the low temperature Ibam structural cell. For x ≥ 0.2 in the formula Sr2MnO2Cu1.5(S1–xSex)2 the magnetic structure has a (0 0 0) propagation vector and is similar to the A-type structure, also commonly adopted by some perovskite-related manganites, in which the Mn moments in the MnO2 sheets are coupled ferromagnetically and long-range antiferromagnetic order results from antiferromagnetic coupling between planes. In the region of the transition between the two different structural and magnetic long-range ordering schemes (0.1 < x < 0.2) the two magnetic structures coexist in the same sample. The evolution of the competition between magnetic ordering schemes and the length scale of the structural order with composition in Sr2MnO2Cu1.5(S1–xSex)2 suggest that the changes in magnetic and structural order are related consequences of the introduction of chemical disorder.
    Chemistry of Materials 07/2012; 24(14):2802. DOI:10.1021/cm301486v · 8.35 Impact Factor
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    ABSTRACT: The interplay and coexistence of superconducting, magnetic and structural order parameters in NaFe{1-x}Co{x}As has been studied using SQUID magnetometry, muon-spin rotation and synchrotron x-ray powder diffraction. Substituting Fe by Co weakens the ordered magnetic state through both a suppression of T_N and a reduction in the size of the ordered moment. Upon further substitution of Fe by Co the high sensitivity of the muon as a local magnetic probe reveals a magnetically disordered phase, in which the size of the moment continues to decrease and falls to zero around the same point at which the magnetically-driven structural distortion is no longer resolvable. Both magnetism and the structural distortion are weakened as the robust superconducting state is established.
    Physical review. B, Condensed matter 12/2011; 85(5). DOI:10.1103/PhysRevB.85.054503 · 3.66 Impact Factor
  • Emma E. McCabe · David G. Free · John S. O. Evans ·
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    ABSTRACT: The title compound is prepared from stoichiometric mixtures of CeO2O, Se, and Fe powders using Al powder as an oxygen-getter in a second crucible (two alumina crucibles in an evacuated quartz tube, 1050 °C, 12 h).
    ChemInform 04/2011; 42(16). DOI:10.1002/chin.201116017
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    ABSTRACT: We report the synthesis of a series of manganese-containing oxychalcogenides with general formula A2O2Mn2OSe2 (A = La, Ce, Pr) as well several new A2O2Fe2OSe2 materials (A = La−Sm). We report the structural, magnetic, and conduction properties of the manganese-containing materials: La2O2Mn2OSe2, Ce2O2Mn2OSe2, and Pr2O2Mn2OSe2. These materials are isostructural with La2O2Fe2OSe2, and are shown to undergo a phase transition on cooling, apparently related to displacement of oxide ions from the [Mn2O]2+ plane. Peak splitting has been observed in the Pr3+ containing material at low temperatures, consistent with a reduction to orthorhombic symmetry. All of the manganese containing materials order antiferromagnetically on cooling with k = (0, 0, 0), TN = 164−184 K, and have a 12 K Mn2+ moment of around 4 μB. The magnetic structure of La2O2Co2OSe2 is also reported. Conductivity studies have shown that La2O2Mn2OSe2 and La2O2Co2OSe2 are both semiconducting, with activation energies of 0.24 and 0.35 eV.Keywords: characterization of materials; inorganic solids and ceramics; magnetic materials
    ChemInform 02/2011; 23(6). DOI:10.1021/cm1035453
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    Emma E McCabe · David G Free · John S O Evans ·
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    ABSTRACT: In this communication we report the synthesis, structural and preliminary physical characterisation of a new layered oxyselenide Ce(2)O(2)FeSe(2). This material, containing a 1D portion of the structure of the layered FeSe-related superconductors, is a semiconductor with a band gap of around 0.64 eV and orders antiferromagnetically at low temperatures.
    Chemical Communications 01/2011; 47(4):1261-3. DOI:10.1039/c0cc03477k · 6.83 Impact Factor
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    ABSTRACT: Two new oxyselenide materials have been synthesized with composition La2O2MSe2 (M = Mn, Fe). They adopt a new structure type, the β structure, which has been solved and refined from powder X-ray and neutron diffraction data. The structure is described by Ama2 symmetry with unit cell 17.5 Å × 16.6 Å × 4.0 Å and consists of sheets of MSen polyhedra separated by La2O2Se blocks. A structural phase transition occurs on cooling involving ordering of M cations and a symmetry reduction to a primitive structure of Pna21 symmetry. Both manganese and iron analogues are antiferromagnetic at low temperature. β-La2O2FeSe2 is a semiconductor in the temperature of 150−300 K with a band gap of approximately 0.7 eV while β-La2O2MnSe2 is insulating at room temperature with band gap of 1.6 eV.
    ChemInform 10/2010; 22(22). DOI:10.1021/cm1023103

Publication Stats

163 Citations
68.82 Total Impact Points


  • 2011-2015
    • University of Oxford
      • • Department of Chemistry
      • • Inorganic Chemistry Laboratory
      Oxford, England, United Kingdom
  • 2010-2013
    • Durham University
      • Department of Chemistry
      Durham, England, United Kingdom