X-ray diffraction and Raman spectroscopy study of F, Cl, Br, I and OH substitutions in lead arsenate apatites (mimetites) Pb5(AsO4)3X

Five Pb-As bearing apatites Pb₅(AsO₄)₃X, where X stands for F, Cl, Br, I and OH, were synthesized by precipitation from an aqueous solution and analyzed with powder X-ray diffraction and Raman spectroscopy. High-resolution high-quality powder diffraction data were obtained at the 11-BM beamline of the Advanced Photon Source at Argonne National Laboratory, Argonne, IL, USA and the structure Rietveld refinements of mimetites with different halogenic substitutions were provided.

Mimetites precipitated from aqueous solutions crystallize in hexagonal crystal system (space group P6₃/m). The lattice parameters a and c, as well as the volume of the unit cell, increase with the increasing ionic radius of halogen substitution: a = 10.081Å, 10.247Å, 10.310Å, 10.351Å; c = 7.426Å, 7.442Å, 7.473Å, 7.528Å; V = 653.515ų; 676.716ų, 688.019ų, 698.402ų for Pb₅(AsO₄)₃F, Pb₅(AsO₄)₃Cl, Pb₅(AsO₄)₃Br, Pb₅(AsO₄)₃I, respectively. The OH^- ion has similar effect on the lattice parameter a = 10.187Å but much stronger effect on parameter c = 7.525Å and overall volume V = 676.274ų than halogens.

Systematic linear relations between the unit cell parameters and the Pb(2) – Pb(2) distance as well as the size of AsO₄ tetrahedra are observed. The distance between the Pb(2) – Pb(2) increases (from 4.093Å for Pb₅(AsO₄)₃F to 4.674Å for Pb₅(AsO₄)₃I) indicating the systematic increase in the radius of hexagonal channels occupied by halogens and OH. In contrast, the size (volume) of AsO₄ tetrahedra decreases (from 2.474ų for Pb₅(AsO₄)₃F to 2.025ų for Pb₅(AsO₄)₃I) with the substitution and with the increasing size of the unit cell.

These structural effects affect the Raman spectra of substituted mimetites resulting in systematic shift of the position of the bands. The most sensitive to isomorphic substitutions are symmetric stretching vibrations ν₁ of the As-O bond in [AsO₄] tetrahedra, position of which range from 813 cm^-1 for Pb₅(AsO₄)₃F to 810 cm^-1 for Pb₅(AsO₄)₃I. This, however, is not due to the increase in the mass of substituted anion. The position of the bands is directly affected by the increasing length of As-O bond: the increase in As-O bond length shifts the position of v₁ vibrations towards lower wavenumbers.

Financial support for the research was provided by the Polish National Science Centre (NCN) grant No. 2017/27/N/ST10/00776.