Isomorphous Substitution of Rare-Earth Elements in Lacunary Apatite Pb8Na2(PO4)6

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• 作者：Evgeni I. Get’man ; Stanislav N. Loboda ; Alexey V. Ignatov ; Vadim V. Prisedsky ; Mohammed A. B. Abdul Jabar ; Lyudmyla I. Ardanova
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：March 7, 2016
• 年：2016
• 卷：55
• 期：5
• 页码：2165-2173
• 全文大小：431K
• ISSN：1520-510X

文摘

The substitution of rare-earth elements (REEs) for Pb in the lacunary apatite Pb₈Na₂(PO₄)₆ with void structural channels was studied by means of powder X-ray diffraction (including the Rietveld refinement), scanning electron microscopy, energy-dispersive X-ray microanalysis, and IR spectroscopy and also measurements of the electrical conductivity. The substitution limits (x_max in Pb_8–xLn_xNa₂(PO₄)₆O_x/2) at 800 °C were found to decrease with the atomic number of the REE from 1.40 for La to 0.12 for Yb with a rapid drop from light to heavy lanthanides (between Gd and Tb). The REE atoms substitute for Pb predominantly at Pb2 sites of the apatite structure according to the scheme 2Pb²⁺ + □ → 2Ln³⁺ + O^2–, where □ is a vacancy in the structural channel. The substitution in lacunary apatite produces quite different changes in the structural parameters compared with broadly studied alkaline-earth hydroxyapatites. In spite of the much lower ionic radii of REE than that of Pb²⁺, the mean distances ⟨Pb1–O⟩ somewhat increase, whereas the distances ⟨Pb2–Pb2⟩ and ⟨Pb2–O4⟩ do not change considerably with the degree of substitution. This implies control of the substitution by not only spatial and charge accommodation of REE ions but also the availability of a stereochemically active 6s² electron pair on Pb²⁺. The high-temperature electrical conductivity shows dependence on the degree of substitution with a minimum at x = 0.2 indicative of a possible change of the type of conductivity.