Browsing by Author "Pramana, SS"

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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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    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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    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.