Crystal structure and spectroscopic studies of Na2Pb8(PO4)6

Abstract

The structure of sodium lead apatite Na2Pb8(PO4)6 has been determined using single-crystal X-ray diffraction. Crystal data are: hexagonal, space group P63/m, Z = 1, a = 972.1(4) pm, c = 718.6(5) pm, V = 588.1(5) 10 6 pm 3 , and R = 0.038. The result of charge distribution analysis confirmed the structural hypotheses. In the Raman spectrum, the two bands observed at 935 cm -1 and at 966 cm -1 , have been attributed to the Ag-E2g splitting, in the hexagonal C6h symmetry, of the nondegenerate symmetric stretching ν1(PO4).

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

“…As an example, the IR spectra of samples Pb 8– x Gd x Na 2 (PO 4 ) 6 O x /2 are shown in Figure . The spectra display absorption bands which, according to refs , , and , can be assigned to internal vibrations of (PO 4 ) 3– ions: symmetric bending ν 2 mode at 445 cm –1 , asymmetric bending ν 4 at 539 and 580 cm –1 , and asymmetric stretching ν 3 mode at 987 and 1051 cm –1 .…”

“…Such lacunary apatites may be considered as derivatives of lead hydroxyapatite Pb 10 (ZO 4 ) 6 (OH) 2 produced by substitution under the scheme 2Pb 2+ + 2OH – → 2A + + 2□. Then they were studied by X-ray diffraction (XRD) − and IR, Raman, and NMR spectroscopies . The availability of hexagonal structural channels free from hydroxide anions OH – creates necessary preconditions for the sodium cationic conductivity in Pb 8 Na 2 (ZO 4 ) 6 .…”

“…Mean interatomic distances M1–O are larger than M2–O by approximately 0.15 Å. − One might expect that smaller Na + ions would occupy smaller M2 sites. In spite of such geometrical considerations, Na + locates predominantly at M1 sites.…”

“…With the same approach, Pb–O bonds in the M2 6-fold coordination polyhedra are more covalent in character because the mean distance ⟨M2–O⟩ (2.533 Å) is smaller than the sum of the ionic radii of O 2– (1.40 Å) and Pb 2+ (1.19 Å) . So, Pb cations in the apatite Pb 8 Na 2 (PO 4 ) 6 behave in two different ways: (a) Pb1 cations with stereochemically inactive lone pairs 6s 2 are engaged in an almost totally ionic bond Pb1/Na–O in the mixed M1 site; (b) Pb2 cations are engaged in a Pb2–O bond with more covalent character, where its lone pair 6s 2 becomes stereochemically active and constitutes the seventh ligand of Pb cations. , …”

Crystal Structure Refinements of the Lead(II) Oxoarsenates(V) Pb2As2O7, Pb(H2AsO4)2, Pb5(AsO4)3OH and NaPb4(AsO4)3 from Single-Crystal X-ray Data

“…A check for a possibly missed higher symmetry using the ADDSYM feature in PLATON [54] gave no indication for a higher spacer group symmetry on basis of the current X-ray data. However, the corresponding phosphate phase NaPb 4 (PO 4 ) 3 (single-crystal X-ray data) has also been reported to crystallize in space group P6 3 /m in the apatite-type of structure [67,68]. The question of whether the lower space group symmetry of NaPb 4 (AsO 4 ) 3 is a systematic feature or is related with different preparation conditions (hydrothermal versus ceramic route) and associated polymorphism must remain unanswered for the time being.…”