Browsing by Author "Green, RL"

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    Structural changes and self-activated photoluminescence in reductively annealed Sr3AlO4F
    (2015-08-01) Green, RL; Avdeev, M; Vogt, T
    White light emission of self-activated photoluminescence (PL) in Sr3AlO4F under 254 nm light is only observed after annealing in a reducing atmosphere of 5%H2/95%Ar. High-resolution neutron powder diffraction reveals that the FSr6 octahedrons and AlO4 tetrahedrons in this anti-perovskite structure are closer packed in reduced than in air-annealed samples which show no PL. Careful analysis of temperature-dependent neutron powder diffraction data establishes smaller isotropic displacement parameters for Sr(1) and O in Sr3AlO4F annealed in a reducing atmosphere indicating that the denser packing of the polyhedral sub-units leads to a slightly deeper potential for the Sr(1) and O atoms. Both the air- and reductively-annealed samples have identical thermal expansion within the temperature range between 3 and 350 K. The Debye temperatures were calculated using the atomic displacement parameters and show no significant differences between the air and reductively annealed samples making the Debye temperature a bad proxy for self-activated PL. © 2015 Elsevier Inc.
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    Structural distortions and self-activated photoluminescence in reductively annealed
    (Elsevier, 2019-08-01) Green, RL; Avdeev, M; Pierre, V
    A tetragonal oxyfluoride Ba2SrGaO4F, a novel material in the Sr3AlO4F family of materials, has been synthesized via solid-state method and exhibits self-activated photoluminescence (PL) under 254 nm light after annealing in a reducing atmosphere of 4%H2/96% Ar. The structure of this material is determined by powder x-ray diffraction and confirmed using high-resolution neutron powder diffraction, which reveals similarities to Sr3AlO4F-type oxyfluorides and Ba2GaLaO5. Analysis of the refined Ba2SrGaO4F structure at room temperature reveals subtle changes in the polyhedral sub-units between the reductively annealed and air-annealed samples, which are correlated with the PL emission. © 2019 Elsevier Inc.
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    Structure, stability, and photoluminescence in the anti-perovskites Na3W1−xMoxO4F (0≤x≤1)
    (Elsevier, 2015-10-01) Sullivan, E; Avdeev, M; Blom, DA; Gahrs, CJ; Green, RL; Hamaker, CG; Vogt, T
    Single-phase ordered oxyfluorides Na3WO4F, Na3MoO4F and their mixed members Na3W1−xMoxO4F can be prepared via facile solid state reaction of Na2MO4·2H2O (M=W, Mo) and NaF. Phases produced from incongruent melts are metastable, but lower temperatures allow for a facile one-step synthesis. In polycrystalline samples of Na3W1−xMoxO4F, the presence of Mo stabilizes the structure against decomposition to spinel phases. Photoluminescence studies show that upon excitation with λ=254 nm and λ=365 nm, Na3WO4F and Na3MoO4F exhibit broad emission maxima centered around 485 nm. These materials constitute new members of the family of self-activating ordered oxyfluoride phosphors with anti-perovskite structures which are amenable to doping with emitters such as Eu3+. © 2015 Elsevier Inc.
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    Synthesis and structural characterization of the hexagonal anti-perovskite Na2CaVO4F
    (Elsevier, 2017-06-01) Green, RL; Avdeev, M; Vogt, T
    The structural details of the ordered hexagonal oxyfluoride Na2CaVO4F prepared by solid-state synthesis using stoichiometric amounts of V2O5, CaCO3, Na2CO3 and NaF were characterized using high-resolution neutron powder diffraction. The structural changes between 25 °C and 750 °C revealed that the two structural subunits in this material behave different when heated: there is an expansion of the face-shared FNa4Ca2 octahedra while the VO4 tetrahedra due to increased thermal disorder reveal marginal bond contractions. Bond valences and the global instability index point to significant structural disorder at 750 °C. © 2017 Elsevier Inc.