Abstract
Five lead-arsenate apatites (mimetites)-Pb5(AsO4)3X—where X denotes fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and hydroxide (OH)—were synthesized via precipitation from aqueous solutions. The crystal structures were determined through Rietveld refinement of powder synchrotron X-ray data. All the compounds crystallized in the hexagonal class symmetry (space group P63/m). The Rietveld refinement indicated that mimetite-Cl, -Br, -I, and -OH had an anion deficiency at position X. Substitution of halogens in a mimetite structure brought about systematic changes in unit-cell parameters, interatomic distances, and metaprism twist angles φ, proportional to the substituted halogen’s ionic radius. Mimetite-OH did not follow the linear correlations determined within the series. Twist angle φ, a useful device for monitoring changes in apatite topology, ranged from 20.34° for mimetite-F to 11.42° for mimetite-I. The geometric method has been proposed for determining the diameter of hexagonal channels hosting halogens in apatites. A comparison of the results with halogenated pyromorphites showed similar systematic trends: the substitutions in mimetites have comparable effect on the interatomic distances as in their phosphorous analogues.
Highlights
The crystal structure and crystal chemistry of apatites have been analyzed in detail in recent reviews ([1,2,3,4,5], and the literature cited therein)
The basic apatite structure is predominantly hexagonal with space group
This paper presents the synthesis of several lead-arsenic apatites containing F, Cl, Br, I, and OH ions on the anionic sites
Summary
The crystal structure and crystal chemistry of apatites have been analyzed in detail in recent reviews ([1,2,3,4,5], and the literature cited therein). The generalized formula of apatite is written as [M(1)]2 [M(2)]3 (TO4 ) X. The basic apatite structure is predominantly hexagonal with space group. Oxide ions occupy two special positions O(1) and O(2), and one general position O(3), around the T site. Site cations are divalent (e.g., Ca2+ , Pb2+ , Sr2+ , Ba2+ ), the T cation is pentavalent (e.g., P5+ , As5+ , V5+ ), and the X anion monovalent (e.g., OH− , F− , Cl− , Br− , I− ). Charge neutrality can be maintained by other ions (CO3 2− , REE3+ , S6+ , Si4+ , Na+ , etc.) and by vacancies [6]
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