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Title: The influence of stereochemically active lone-pair electrons on crystal symmetry and twist angles in lead apatite-2H type structures
Authors: Baikie, Tom
Schreyer, Martin
Wei, Fengxia
Herrin, Jason Scott
Ferraris, Cristiano
Brink, Frank
Topolska, Justyna
Piltz, R. O.
Price, Jason
White, Timothy John
Keywords: Crystal symmetry; Lone-pair Electrons; PB Apatite Structures; Twist Angles
Issue Date: 2014
Source: Baikie, T., Schreyer, M., Wei, F., Herrin, J. S., Ferraris, C., Brink, F., et al. (2014). The influence of stereochemically active lone-pair electrons on crystal symmetry and twist angles in lead apatite-2H type structures. Mineralogical Magazine, 78(2), 325-345.
Series/Report no.: Mineralogical Magazine
Abstract: Lead-containing (Pb-B-X)-2H apatites encompass a number of [AF ]4[AT ]6[(BO4)6]X 2 compounds used for waste stabilization, environmental catalysis and ion conduction, but the influence of the stereochemically active lone-pair electrons of Pb2+ 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 Pb2+ at both the AF and AT results in substantial adjustments to the PbFO6 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-2H (ideally Pb10(VO4)6Cl2), pyromorphite-2H (Pb10(PO4)6Cl2), mimetite-2H/M (Pb10(As5+O4)6Cl2) and finnemanite-2H (Pb10(As3+O3)6Cl2). 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 6s2 lone-pair electrons on the Pb2+ that occupy a volume similar to an O2– 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 lone-pair electrons. It is also shown that plumbous apatite framework flexes substantially through adjustments of the Pb F O6 metaprism twist-angles (ϕ) as the temperature changes. Finally, crystal-chemical [100] zoning observed at submicron scales will probably impact on the treatment of diffraction data and may account for certain inconsistencies in reported structures.
ISSN: 0026-461X
DOI: 10.1180/minmag.2014.078.2.07
Rights: © 2014 The Mineralogical Society. This is the author created version of a work that has been peer reviewed and accepted for publication by Mineralogical Magazine, The Mineralogical Society. It incorporates referee’s comments but changes resulting from the publishing process, such as copyediting, structural formatting, may not be reflected in this document. The published version is available at: [].
Fulltext Permission: open
Fulltext Availability: With Fulltext
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