X-ray diffraction and Raman spectroscopy study of F, Cl, Br, I and OH substitutions in lead arsenate apatites (mimetites) Pb5(AsO4)3X
Authors: Sordyl Julia , Puzio Bartosz , Borkiewicz Olaf , Manecki Maciej 
Published: March 2020

&lt;p&gt;Five Pb-As bearing apatites Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;X, 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.&lt;/p&gt;&lt;p&gt;Mimetites precipitated from aqueous solutions crystallize in hexagonal crystal system (space group P6&lt;sub&gt;3&lt;/sub&gt;/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 = 10.081&amp;#197;, 10.247&amp;#197;, 10.310&amp;#197;, 10.351&amp;#197;; c = 7.426&amp;#197;, 7.442&amp;#197;, 7.473&amp;#197;, 7.528&amp;#197;; V = 653.515&amp;#197;&lt;sup&gt;3&lt;/sup&gt;; 676.716&amp;#197;&lt;sup&gt;3&lt;/sup&gt;, 688.019&amp;#197;&lt;sup&gt;3&lt;/sup&gt;, 698.402&amp;#197;&lt;sup&gt;3&lt;/sup&gt; for Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;F, Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;Cl, Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;Br, Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;I, respectively. The OH&lt;sup&gt;-&lt;/sup&gt; ion has similar effect on the lattice parameter a = 10.187&amp;#197; but much stronger effect on parameter c = 7.525&amp;#197; and overall volume V = 676.274&amp;#197;&lt;sup&gt;3&lt;/sup&gt; than halogens.&lt;/p&gt;&lt;p&gt;Systematic linear relations between the unit cell parameters and the Pb(2) &amp;#8211; Pb(2) distance as well as the size of AsO&lt;sub&gt;4&lt;/sub&gt; tetrahedra are observed. The distance between the Pb(2) &amp;#8211; Pb(2) increases (from 4.093&amp;#197; for Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;F to 4.674&amp;#197; for Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;I) indicating the systematic increase in the radius of hexagonal channels occupied by halogens and OH. In contrast, the size (volume) of AsO&lt;sub&gt;4&lt;/sub&gt; tetrahedra decreases (from 2.474&amp;#197;&lt;sup&gt;3&lt;/sup&gt; for Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;F to 2.025&amp;#197;&lt;sup&gt;3&lt;/sup&gt; for Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;I) with the substitution and with the increasing size of the unit cell.&lt;/p&gt;&lt;p&gt;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 &amp;#957;&lt;sub&gt;1&lt;/sub&gt; of the As-O bond in [AsO&lt;sub&gt;4&lt;/sub&gt;] tetrahedra, position of which range from 813 cm&lt;sup&gt;-1&lt;/sup&gt; for Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;F to 810 cm&lt;sup&gt;-1&lt;/sup&gt; for Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;I. 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 v&lt;sub&gt;1&lt;/sub&gt; vibrations towards lower wavenumbers.&lt;/p&gt;&lt;p&gt;Financial support for the research was provided by the Polish National Science Centre (NCN) grant No. 2017/27/N/ST10/00776.&lt;/p&gt;