Browsing by Author "Chhoeun, J"

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    Aliovalent M site substitutions of [PO4]3− and [HfO4]4− for [AlO4]5− and [GaO4]5− in the Sr3MO4F-type anti-perovskite oxyfluoride
    (IOP Publishing, 2019-01-17) Keil, ST; Chhoeun, J; Avdeev, M; Sullivan, E
    Rare earth-free phosphor materials have been synthesized via aliovalent doping of [PO4]3− and [HfO4]4− for [AlO4]5− and [GaO4]5− in the Sr3MO4F host lattice (space group I4/mcm). High temperature synthesis was used to synthesize the novel products Sr2.5-xBa0.5Al1-xPxO4F (0.05 ≤ x ≤ 0.15), Sr2.5-xBa0.5Ga1-xPxO4F (0.05 ≤ x ≤ 0.15) and (0.025 ≤ x ≤ 0.075). The structures of these oxyfluorides were analyzed via powder X-ray diffraction (PXRD) and neutron powder diffraction (NPD), and their photoluminescent properties were analyzed through spectrofluorimetry before and after being treated under reducing conditions. Optimal doping was found at a concentration of x = 0.10 for Sr2.5-xBa0.5Al1-xPxO4F (a = 6.8762(4) Å; c = 11.1273(6) Å) resulting in a maximum emission at 454 nm when excited at 214 nm and x = 0.05 for the Ga analog (a = 6.87591(7) Å; c = 11.3722(2) Å) corresponding to an emission centered around 529 nm from excitation with 227 nm. When doping with Hf4+ the optimal concentration for was x = 0.025 (a = 6.8607(1) Å; c = 11.1682(3) Å), resulting in a broad emission between 410–600 nm when excited at 251 nm. © 2019 The Electrochemical Society
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    Designing novel tunable Mn-based inorganic oxyfluoride pigments
    (Elsevier, 2024-06) Lehr, B; Zurowski, G; Chhoeun, J; Kumar, K; Nolis, G; Shanahan, J; Kilpatrick, K; Rojas, K; Cabana, J; Kissel, D; Avdeev, M; Sullivan, E
    A high tunability of green-blue colors in the manganese-doped oxyfluoride Sr2.5A0.5Mn0.1MO4F (A = Ca, Sr, Ba; M = Al, Ga) and anion-deficient Sr2.5A0.5Mn0.1MO4-αF1-δ (A = Ca, Sr, Ba; M = Al, Ga) is reported, and the chromophores responsible for this intense pigmentation are investigated. The hues exhibited by these materials are quantified via diffuse reflectance UV/Vis spectroscopy and measurement of their direct band gaps via Tauc plot. It is shown that choice of A cation (A = Ca, Sr, Ba) and M cation (M = Al, Ga) for as-synthesized phases Sr2.5A0.5Mn0.1MO4F yield a wide range of green colors (band gap range 2.70–2.96 eV). Treatment of these phases under reducing conditions according to Sr2.5A0.5Mn0.1MO4-αF1-δ (A = Ca, Sr, Ba; M = Al, Ga) induces anion non-stoichiometry, shifting the observed colors to a wide range of blue/blue-purple hues (band gaps from 3.31 to 3.66 eV), showing potential as tunable inorganic blue pigments. Density field theory (DFT) calculations support the preferential occupation of the smaller 8-coordinate Sr(2) site by the substituted Mn2+ cation. X-ray absorption near-edge structure (XANES) data reveal more subtle nuances in the interplay between formal manganese oxidation state, crystallographic site and observed hue. In general, for as synthesized (green) Sr2.5A0.5Mn0.1MO4F (A = Ca, Sr, Ba; M = Al, Ga), the edge position in Mn K-edge XANES is consistent with mixed Mn3+-Mn4+ oxidation state whilst a clear pre-edge structure suggesting that Mn is present on a tetrahedral site. This would suggest that during the reduction step, Mn3+/Mn4+ is reduced to entirely Mn2+ and migrates from the tetrahedral to the Sr(2) lattice site. © 2024 Elsevier Ltd. All rights reserved.