Isomorphous Substitution of Rare-Earth Elements in Lacunary Apatite Pb8Na2(PO4)6
| Autor: | A. V. Ignatov, S. N. Loboda, E. I. Get’man, Lyudmyla I. Ardanova, Mohammed A. B. Abdul Jabar, Vadim V. Prisedsky |
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| Rok vydání: | 2016 |
| Předmět: |
Lanthanide
Ionic radius Chemistry Rietveld refinement 02 engineering and technology 010402 general chemistry 021001 nanoscience & nanotechnology 01 natural sciences Apatite 0104 chemical sciences Inorganic Chemistry Crystallography Isomorphous substitution visual_art Vacancy defect visual_art.visual_art_medium Atomic number Physical and Theoretical Chemistry 0210 nano-technology Lacunary function |
| Zdroj: | Inorganic Chemistry. 55:2165-2173 |
| ISSN: | 1520-510X 0020-1669 |
| DOI: | 10.1021/acs.inorgchem.5b02571 |
| Popis: | 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 (xmax in Pb8-xLnxNa2(PO4)6Ox/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 2Pb(2+) + □ → 2Ln(3+) + 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 Pb(2+), 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 6s(2) electron pair on Pb(2+). 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. |
| Databáze: | OpenAIRE |
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