Browsing by Author "Slater, PR"

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    A combined single crystal neutron/x-ray diffraction and solid-state nuclear magnetic resonance study of the hybrid perovskites CH3NH3PbX3 (X = I, Br and Cl)
    (Royal Society of Chemistry, 2015-03-31) Baikie, T; Barrow, NS; Fang, Y; Keenan, PJ; Slater, PR; Piltz, RO; Gutmann, MJ; Mhaisalkara, SG; White, TJ
    The 1H and 13C NMR spectra in methylammonium lead halide perovskites, CH3NH3PbX3 (X = I, Br and Cl) show that the CH3NH3+ units undergo dynamic reorientation, as the organic component tumbles in the perovskite cage. In addition, the differences in the anomalously long relaxation times of the protons associated with the CH3 and not the NH3 groups indicate that only the amine end of the CH3NH3+ group is interacting with the inorganic network. Using this information, we have refined some single crystal X-ray and neutron diffraction data to probe their unusual structures in more detail. Furthermore, impedance spectroscopy has been used to monitor the high-temperature phase transition of CH3NH3PbI3, which confirms a significant increase in conductivity, when it is in its high temperature and higher symmetry structural regime. The optical band-gaps of each halide perovskite were determined using UV-visible spectroscopy and are consistent with previous reports. © Royal Society of Chemistry
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    Crystallographic correlations with anisotropic oxide ion conduction in aluminum-doped neodymium silicate apatite electrolytes
    (American Chemical Society, 2013-04-09) An, T; Baikie, T; Wei, FX; Pramana, SS; Schreyer, MK; Piltz, RO; Shin, JF; Wei, J; Slater, PR; White, TJ
    To better understand the oxide ion conduction mechanism of rare earth silicate apatites as intermediate temperature electrolytes for solid oxide fuel cells (SOFC), the effect of lower valent metal doping on the performance of Nd(28+x)/3AlxSi6-xO26 (0 <= x <= 2) single crystals has been examined. The measurement of ionic conductivity via AC impedance spectroscopy showed that the conductivities were anisotropic and superior along the c direction. An interesting aspect from the impedance studies was the identification of a second semicircle with capacitance similar to that of a grain boundary component, despite the fact that polarized optical microscopy and electron backscattered diffraction showed that the single crystals consisted of a single grain. This semicircle disappeared after long-term (up to 3 months) annealing of the single crystals at 950 degrees C, also leading to a reduction in the bulk conductivity. In order to explain these observations, single-crystal X-ray diffraction studies were performed both before and after annealing. These studies found the undoped crystal conformed to P6(3)/m, but with the 0(3) oxygen positions, that participate in conduction, split nonstatistically across two sites with a shortened Si-O(3) bond. Consequently, the bond valence sum (BVS) of the Si (4.20) is larger than the formal valence. Fourier difference maps of the Al-doped crystals contain regions of excess scattering, suggesting the possible lowering of symmetry or creation of superstructures. After long-term annealing, the single crystal structure determinations were of higher quality and the experimental and nominal compositions were in better agreement. From these observations, we propose that in the as-prepared single crystals there are regions of high and low interstitial content (e.g., Nd9.67Si6O26.5 and Nd9.33Si6O26), and the second semicircle relates to the interface between such regions. On annealing, Nd redistribution and homogenization removes these interfaces and also reduces the number of interstitial oxide ions, hence eliminating this second semicircle while reducing the bulk conductivity. The results therefore show for the first time that the conductivity of apatite materials containing cation vacancies is affected by the thermal history.© 2013, American Physical Society.
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    Hydrothermal Synthesis, Structure Investigation, and Oxide Ion Conductivity of Mixed Si/Ge-Based Apatite-Type Phases
    (ACS Publications, 2014-05-02) Li, HN; Baikie, T; Pramana, SS; Shin, J F; Keenan, PJ; Slater, PR
    Apatite-type oxides ([AI4][AII6][(BO4)6]O2), particularly those of the rare-earth silicate and germanate systems, are among the more promising materials being considered as alternative solid oxide fuel cell electrolytes. Nonstoichiometric lanthanum silicate and germanate apatites display pure ionic conductivities exceeding those of yttria-stabilized zirconia at moderate temperatures (500–700 °C). In this study, mixed Si/Ge-based apatites were prepared by hydrothermal synthesis under mild conditions rather than the conventional solid-state method at high temperatures. Single-phase and highly crystalline nanosized apatite powders were obtained with the morphology changing across the series from spheres for the Si-based end member to hexagonal rods for the Ge-based end member. Powder X-ray and neutron analysis found all of these apatites to be hexagonal (P63/m). Quantitative X-ray microanalysis established the partial (<15 at%) substitution of La3+ by Na+ (introduced from the NaOH hydrothermal reagent), which showed a slight preference to enter the AI 4f framework position over the AII 6h tunnel site. Moreover, retention of hydroxide (OH–) was confirmed by IR spectroscopy and thermogravimetric analysis, and these apatites are best described as oxyhydroxyapatites. To prepare dense pellets for conductivity measurements, both conventional heat treatment and spark plasma sintering methods were compared, with the peculiar features of hydrothermally synthesized apatites and the influence of sodium on the ionic conductivity considered.© 2014, American Chemical Society.
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    Structural study of the apatite Nd8Sr2Si6O26 by laue neutron diffraction and single-crystal raman spectroscopy
    (American Chemical Society, 2014-08-20) An, T; Orera, A; Baikie, T; Herrin, JS; Piltz, RO; Slater, PR; Sanjuán, ML
    A single-crystal structure determination of Nd8Sr2Si6O26 apatite, a prototype intermediate-temperature electrolyte for solid oxide fuel cells grown by the floating-zone method, was completed using the combination of Laue neutron diffraction and Raman spectroscopy. While neutron diffraction was in good agreement with P6₃/m symmetry, the possibility of P6₃ could not be convincingly excluded. This ambiguity was removed by the collection of orientation-dependent Raman spectra that could only be consistent with P6₃/m. The composition of Nd8Sr2Si6O26 was independently verified by powder X-ray diffraction in combination with electron probe microanalysis, with the latter confirming a homogeneous distribution of Sr and the absence of chemical zonation commonly observed in apatites. This comprehensive crystallochemical description of Nd8Sr2Si6O26 provides a baseline to quantify the efficacy of cation vacancies, oxygen superstoichiometry, and symmetry modification for promoting oxygen-ion mobility. © 2014 American Chemical Society