title: Isomorphous Substitution of Rare-Earth Elements in Lacunary Apatite Pb8Na2(PO4)6.
abstract: 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.
authors.affiliation: Department of Inorganic Chemistry, Donetsk National University , 24 Universitetskaya, Donetsk 83001, Ukraine.|Department of Analytical Chemistry, Donetsk National University , 21 600-richya, Vinnytsia, 21021, Ukraine.
authors.forename: Evgeni I
authors.initials: EI
authors.lastname: Get'man
authors.affiliation: Department of Inorganic Chemistry, Donetsk National University , 24 Universitetskaya, Donetsk 83001, Ukraine.
authors.forename: Stanislav N
authors.initials: SN
authors.lastname: Loboda
authors.affiliation: Department of Inorganic Chemistry, Donetsk National University , 24 Universitetskaya, Donetsk 83001, Ukraine.
authors.forename: Alexey V
authors.initials: AV
authors.lastname: Ignatov
authors.affiliation: Department of General Chemistry, Donetsk National Technical University , 58 Artema, Donetsk 83001, Ukraine.
authors.forename: Vadim V
authors.initials: VV
authors.lastname: Prisedsky
authors.affiliation: Department Medical Lab Technologies, Bilad Al-Rafidain University College , Baaqubah 32001 Iraq.
authors.forename: Mohammed A B
authors.initials: MA
authors.lastname: Abdul Jabar
authors.affiliation: Department of Chemistry and Geology, Minnesota State University at Mankato , 241 Ford Hall, Mankato, Minnesota 56001, United States.
authors.forename: Lyudmyla I
authors.initials: LI
authors.lastname: Ardanova