ORCID Profile
0000-0003-0984-3058
Current Organisation
Panalytical
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Publisher: Mineralogical Society
Date: 04-2014
DOI: 10.1180/MINMAG.2014.078.2.07
Abstract: Lead-containing (Pb- B-X )-2 H apatites encompass a number of [ A F ] 4 [ A T ] 6 [( BO 4 ) 6 ] X 2 compounds used for waste stabilization, environmental catalysis and ion conduction, but the influence of the stereochemically active lone-pair electrons of Pb 2+ on crystal chemistry and functionality is poorly understood. This article presents a compilation of existing structural data for Pb apatites that demonstrate paired electrons of Pb 2+ at both the A F and A T results in substantial adjustments to the Pb F O 6 metaprism twist angle, φ. New structure refinements are presented for several natural varieties as a function of temperature by single-crystal X-ray diffraction (XRD) of vanadinite-2 H (ideally Pb 10 (VO 4 ) 6 C l2 ), pyromorphite-2 H (Pb 10 (PO 4 ) 6 Cl 2 ), mimetite- 2H/M (Pb 10 (As 5+ O 4 ) 6 Cl 2 ) and finnemanite-2 H (Pb 10 (As 3+ O 3 ) 6 Cl 2 ). A supercell for mimetite is confirmed using synchrotron single-crystal XRD. It is suggested the superstructure is necessary to accommodate displacement of the stereochemically active 6s 2 lone-pair electrons on the Pb 2+ that occupy a volume similar to an O 2− anion. We propose that depending on the temperature and concentration of minor substitutional ions, the mimetite superstructure is a structural adaptation common to all Pb-containing apatites and by extension apatite electrolytes, where oxide ion interstitials are found at similar positions to the lonepair electrons. It is also shown that plumbous apatite framework flexes substantially through adjustments of the Pb F O 6 metaprism twist-angles (φ) as the temperature changes. Finally, crystalchemical [100] zoning observed at submicron scales will probably impact on the treatment of diffraction data and may account for certain inconsistencies in reported structures.
Publisher: Royal Society of Chemistry (RSC)
Date: 2013
DOI: 10.1039/C3TA10518K
Publisher: International Union of Crystallography (IUCr)
Date: 08-09-2009
DOI: 10.1107/S0021889809031021
Abstract: The performance of photocatalytic titania powders is regulated, in part, by nonstoichiometry and the proportions of the crystalline and amorphous components. These variables can be quantitatively established by Rietveld analysis of diffraction data when internal standards are used to fix absolutely the crystallochemical parameters during quantitative phase analysis and to correct for mass absorption. Here, fixed-wavelength neutron and multiple-wavelength X-ray powder diffraction are used to assess phase development in alkoxide-derived titania gel as a function of temperature. In this manner, it is shown that the amorphous gel is progressively replaced by anatase for temperatures ≤ 773 K, and that during the reconstructive transition to rutile (773–873 K) aperiodicity increases as anatase is broken down to clusters of TiO 6 octahedra, with a fraction (∼10 wt%) of this short-range order persisting to 1273 K. Microabsorption correlates with X-ray energy, leading to systematic aberrations in the Rietveld scale factors connected to microstructural evolution which accompanies phase development during heat treatment. These changes are consistent with encapsulation of anatase and rutile by ubiquitous non-diffracting materials. The appearance of significant quantities of an intervening disordered phase during the dimorph transformation is supportive of recent kinetic models however, its impact on catalytic activity remains to be determined.
Publisher: Elsevier BV
Date: 10-2007
Publisher: Elsevier BV
Date: 26-08-2010
Publisher: Royal Society of Chemistry (RSC)
Date: 2009
DOI: 10.1039/B907704A
Abstract: Apatite-like materials are of considerable interest as potential solid oxide fuel cell electrolytes, although their structural vagaries continue to attract significant discussion. Understanding these features is crucial both to explain the oxide ion conduction process and to optimise it. As the composition of putative P6(3)/m apatites with ideal formula [A(I)(4)][A(II)(6)][(BO(4))(6)][X](2) is varied the [A(I)(4)(BO(4))(6)] framework will flex to better accommodate the [A(II)(6)X(2)] tunnel component through adjustment of the A(I)O(6) metaprism twist angle (varphi). The space group theory prescribes that framework adaptation during phase changes must lead to one of the maximal non-isomorphic subgroups of P6(3)/m (P2(1), P2(1)/m, P1[combining macron]). These adaptations correlate with oxygen ion conduction, and become crucial especially when the tunnels are filled by relatively small ions and/or partially occupied, and if interstitial oxygens are located in the framework. Detecting and completely describing these lower symmetry structures can be challenging, as it is difficult to precisely control apatite stoichiometry and small departures from the hexagonal metric may be near the limits of detection. Using a combination of diffraction and spectroscopic techniques it is shown that lanthanum strontium germanate oxide electrolytes crystallise as triclinic (A), monoclinic (M) and hexagonal (H) bi-layer pseudomorphs with the composition ranges: [La(10-x)Sr(x)][(GeO(4))(5+x/2)(GeO(5))(1-x/2)][O(2)] (0 <or=x<or= 1) apatite-2A[La(10-x)Sr(x)][(GeO(4))(5+x/2)(GeO(5))(1-x/2)][O(2)] (1 <or=x<or= 2) apatite-2M[La(10-x)Sr(x)][(GeO(4))(6)][O(2)][H(delta)] (2 <or=x<or= 2.96) apatite-2M[La(10-x)Sr(x)][(GeO(4))(6)][O(2)][H(delta)] (2.96 <or=x<or= 5.32) apatite-2HFurthermore, at typical fuel cell operating temperatures apatite-2A and apatite-2M will transform to apatite-2H, with the latter showing the highest conduction. The results show that small twist angles and high symmetry enhance oxygen mobility with these properties tailored by adjusting the relative size of the framework to tunnel. This information can hence aid in the design of new materials with improved oxide ion conductivity.
Publisher: American Chemical Society (ACS)
Date: 02-09-2011
DOI: 10.1021/JA206441X
Abstract: Melilite-type gallium oxides are potential intermediate temperature electrolytes for solid oxide fuel cells. Single crystals of [CaNd](2)[Ga](2)[Ga(2)O(7)](2) grown using an optical floating zone furnace have been investigated using transmission electron microscopy and powder and single-crystal X-ray diffraction. The anion array topologically conforms to a [(3.5.4.5)(2), 3.5.3.5] network that contains distorted pentagonal tunnels. The distortion is necessary to achieve space filling and accommodate structural misfit between the layers. Satisfactory bond lengths and angles are obtained through two-dimensional modulation in the tetragonal based plane, leading to five-dimensional symmetry in the superspace group P(4⁻)2(1)m(α,α,0)00s((a⁻)a,0)000, α = 0.2319(2), with modulation vectors q(1) = α(a* + b*) and q(2) = α(-a* + b*). Both displacive and occupational modulations are found. Through this mechanism, melilites are primed to accommodate mobile oxygen interstitials, suggesting a rational approach to crystallochemical tailoring that will enhance ionic diffusion and optimize electrolyte performance.
No related grants have been discovered for Martin Schreyer.