Browsing by Author "White, TJ"

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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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    Crystal chemistry of the alkaline-earth apatites A(10)(PO4)(6)CuxOy(H)(z) (A = Ca, Sr and Ba).
    (Royal Society of Chemistry, 2009-06-17) Baikie, T; Ng, GMH; Madhavi, S; Pramana, SS; Blake, K; Elcombe, MM; White, TJ
    The crystal chemistry of the cuprate apatites A(4)(I)A(6)(II)(PO4)(6)CuxOy(H)(z) (A = Ca, Sr and Ba) was investigated by powder X-ray (PXRD) and neutron diffraction (PND) and X-ray photoelectron spectroscopy (XPS). The refined crystal structures confirmed earlier X-ray diffraction studies that showed copper resides in the apatite channels and additionally, located hydrogen. For all materials copper is primarily divalent (Cu2+) but in the calcium and strontium analogues co-exists with minor Cu3+. This is in contrast with a previous work where Cu1+ and Cu2+ were reported. © 2009, Royal Society of Chemistry
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    Crystal chemistry of vanadium-bearing ellestadite waste forms
    (American Chemical Society, 2018-07-16) Fang, Y; Page, SJ; Rees, GJ; Avdeev, M; Hanna, JV; White, TJ
    Vanadate ellestadites Ca10(SiO4)x(VO4)6–2x(SO4)xCl2, serving as prototype crystalline matrices for the fixation of pentavalent toxic metals (V, Cr, As), were synthesized and characterized by powder X-ray and neutron diffraction (PXRD and PND), electron probe microanalysis (EPMA), Fourier transform infrared spectroscopy (FTIR), and solid-state nuclear magnetic resonance (SS-NMR). The ellestadites 0.19 < x < 3 adopt the P63/m structure, while the vanadate endmember Ca10(VO4)6Cl2 is triclinic with space group P1̅. A miscibility gap exists for 0.77 < x < 2.44. The deficiency of Cl in the structure leads to short-range disorder in the tunnel. Toxicity characteristic leaching testing (TCLP) showed the incorporation of vanadium increases ellestadite solubility, and defined a waste loading limit that should not exceed 25 atom % V to ensure small release levels. © 2018 American Chemical Society
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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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    A multi-domain gem-grade Brazilian apatite
    (De Gruyter, 2012-10-01) Baikie, T; Schreyer, MK; Wong, CL; Pramana, SS; Klooster, WT; Ferraris, C; McIntyre, GJ; White, TJ
    A gem-grade apatite from Brazil of general composition (Ca,Na)10[(P,Si,S)O4]6(F,Cl,OH)2 has been studied using single-crystal X-ray and neutron diffraction together with synchrotron powder X-ray diffraction. Earlier electron microscopy studies had shown the nominally single-phase apatite contains an abundant fluorapatite (F-Ap) host, together with chloro-hydroxylapatites (Cl/OH-Ap) guest phases that encapsulate hydroxylellestadite (OH-El) nanocrystals. While the latter features appear as small (200–400 nm) chemically distinct regions by transmission electron microscopy, and can be identified as separate phases by synchrotron powder X-ray diffraction, these could not be detected by single-crystal X-ray and neutron analysis. The observations using neutron, X-ray and electron probes are however consistent and complementary. After refinement in the space group P63/m the tunnel anions F− are fixed at z = ¼ along <001>, while the anions Cl− and OH− are disordered, with the suggestion that O-H•••O-H••• hydrogen-bonded chains form in localized regions, such that no net poling results. The major cations are located in the 4f AFO6 metaprism (Ca+Na), 6h ATO6X tunnel site (Ca only), and 6h BO4 tetrahedron (P+Si+S). The structural intricacy of this gem stone provides further evidence that apatite microstructures display a nano-phase separation that is generally unrecognized, with the implication that such complexity may impact upon the functionality of technological analogues. © 2012, Mineralogical Society of America.
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    Triclinic apatites
    (International Union of Crystallography, 2007-04) Baikie, T; Mercier, PHJ; Elcombe, MM; Kim, JY; Le Page, Y; Mitchell, LD; White, TJ; Whitfield, PS
    Apatites commonly adopt P63/m hexagonal symmetry. More rarely, monoclinic chemical analogues have been recognized, including the biologically significant hydroxyapatite, Ca10(PO4)6(OH)2, but the driving force towards lower symmetry has not been systematically examined. A combination of diffraction observations and ab initio calculations for Ca10(AsO4)6F2 and Ca10(VO4)6F2 show these materials are triclinic apatites in which the AsO4 and VO4 tetrahedra tilt to relieve stress at the metal and metalloid sites to yield reasonable bond-valence sums. An analysis of the triclinic non-stoichiometric apatites La10 − x(GeO4)6O3 − 1.5x and Ca10(PO4)6(OH)2 − xOx/2 confirms this scheme of tetrahedral rotations, while Cd10(PO4)6F2 and Ca10(CrO4)6F2 are predicted to be isostructural. These distortions are in contrast to the better known P1121/b monoclinic dimorphs of chloroapatite and hydroxyapatite, where the impetus for symmetry reduction is ordered anion (OH− and Cl−) displacements which are necessary to obtain acceptable bond lengths. These results are important for designing apatites with specific structural and crystal-chemical characteristics. © 2007, International Union of Crystallography
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    Zinc hydroxyapatite catalyst for decomposition of 2-propanol
    (Wiley-V C H Verlag GMBH, 2012-06-20) Low, HR; Avdeev, M; Ramesh, K; White, TJ
    Hydroxyapatite (HAp) is attracting interest as a heterogeneous catalyst due to its “zeolitic” properties that allow tailoring of chemically active surface sites for specific applications. Here, the crystallographic modification of HAp through incorporation of zinc (Zn2+) was studied using diffraction and spectroscopic techniques. The preferential displacement of tunnel calcium (Ca2+) by covalently bonded Zn2+ inhibits crystal growth and promotes the retention of an amorphous (10-20 wt%) component. In combination, these factors create chemically active surfaces that allow Zn-HAp materials to effectively absorb carbon monoxide (*characteristic vibration at ~1711 cm-1) and catalyze the decomposition of 2-propanol. © 2012, Wiley-VCH Verlag GmbH & Co. KGaA