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Compounds with apatite structure having the composition M 10 (ZO 4 ) 6 X 2 (where M = Na + , K + , Ca 2+ , Sr 2+ , Ba 2+ , Pb 2+ , Cd 2+ , Y 3+ , La 3+ , REE etc.; Z = Si 4+ , Ge 4+ , P 5+ , V 5+ , As 5+ , S 6+ , Cr 6+ etc.; X = OH – , F – , Cl – , Br – , I – , O 2– , (vacancies) are characterized by various properties. As a result, they are intensively studied and may be used as bioactive, luminescent and laser materials, sensors, solid electrolytes, sorbents, catalysts. In crystal chemistry the apatite formula can be represented as [M(1)] 4 [M(2)] 6 (ZO 4 ) 6 X 2 . M(1) (4f position) is surrounded by nine oxygen atoms, which are the part of ZО 4 tetrahedra. M(2) (6h position) is surrounded by six oxygen atoms being the part of ZО 4 tetrahedra and X atom (position 2a) located in the structure channel. Cations in the position M(2) form triangles, their central axis coinciding with axis c. Repeating of this structure along axis c enables the formation of channels, in which X ions can be located and move. Such structure allows to make isomorphous substitution by different atoms in structural units M, Z, and X. The present study represents the results of isomorphous substitution of lead by REE of yttrium subgroup, which was mainly realized in the system Pb (8−x) Tb x Na 2 (PO 4 ) 6  (2−x/2) O (x/2) . Substitution of rare-earth elements (REEs, Ln: Tb, Dy, Ho, Tm, and Yb) for lead in the lacunary apatite Pb (8−x) Ln x Na 2 (PO 4 )6 (2−x/2) O (x/2) (0 ≤ x ≤ 2) in accordance to scheme 2 Pb 2+ +  → 2 Ln 3+ + O 2– has been studied by Xray powder diffraction (including the Rietveld refinement), scanning electron microscopy and FT-IR spectroscopy. Single phase solid solutions Pb (8−x) Ln x Na 2 (PO 4 ) 6  (2−x/2) O (x/2) are formed in the range from х = 0.00 to х = 0.55. By changing the parameters of elementary cells from the composition and the phase-vanishing method the solubility limits xmax of REE decreases with an REE atomic number increasing from 0.55 till 0.12 at 800 °C (xmax = 0.53–0.55 for Tb, xmax = 0.45 for Dy, xmax = 0.38 for Ho, xmax = 0.16–0.18 for Tm, and xmax = 0.12 for Yb) were established. Refinements of X-ray diffraction patterns by the Rietveld method show that substitution lead to Pb(2)—O(1,2,3) and Pb(2)—O(2) atomic distances decreases. This study shows that REE atoms substitute for Pb preferentially at the Pb(2) sites of the apatite structure.

vìsnik donecʹkogo nacìonalʹnogo unìversitetu ìmenì vasilâ stusa. serìâ hìmìčnì nauki/vìsnik donecʹkij nacìonalʹnij unìversitet ìmenì vasilâ stusa. serìâ hìmìčnì nauki

Ізоморфні заміщення у системі Pb(8−x)LnxNa2 (PO4)6(2−x/2)O(x/2), де Ln = Tb, Dy, Ho, Tm і Yb