Isomorphism and phase diagram of Pb5(PO4)3F–Pb5(PO4)3Cl system
Introduction
Compounds with the general formula MII5(AVO4)3L (MII = Ca, Sr, Ba, Cd, Pb; AV = P, As, V, Mn, Cr; L = OH, F, Cl, Br, I) are subjects of geochemistry and biochemistry. Most of the individual compounds of the aforementioned general formula and their base solid solutions are known as natural minerals, such as apatite Ca5(PO4)3F, hydroxyapatite Ca5(PO4)3OH, pyromorphite Pb5(PO4)3Cl, endlichite Pb5(PO4)3FxCl1−x, and some others [1], [2]. Therefore, both science and technology are interested in their complex exploration. This work offers the results of a physicochemical study of Pb5(PO4)3F–Pb5(PO4)3Cl system by X-ray diffraction methods, including high temperature experiments; IR spectroscopy; differential thermal analysis; and reaction calorimetry.
These compounds are structurally built of discrete phosphate tetrahedra linked to one another by lead polyhedra, which form joint layers (Fig. 1). Apatite-type structures typically offer two crystallographic positions for cations differing in coordination number and local symmetry. The lead atoms occupying the first positions 4f form polyhedra shaped as three-capped trigonal prisms PbO9 having symmetry C3 whose columns run along the threefold axis. Coordination number of lead atoms occupying the second position depends on type of ligand L: CR = 7 when L = F (2a) (distorted pentagonal bipyramids PbO6F), whereas CR = 8 when L = Cl (2b) (distorted two-capped trigonal prisms PbO6Cl2).
In consequence of differences of halogen positions chlorine atoms of pyromorphite structure are located between layers formed by PO4 tetrahedra, whereas fluorine atoms of fluorpyromorphite structure occupies positions in layers [3].
Section snippets
Sample
Samples of Pb5(PO4)3FxCl1−x solid solution was prepared by the solid-state reaction between lead(II) nitrate, lead(II) fluoride, lead(II) chloride, and ammonium hydrophosphate:
A reaction mixture of a set stoichiometry was placed in a porcelain crucible and calcined at 350 and then 700 °C for 10 h with dispersion in an agate mortar every 2 h.
Apparatus and measurement procedure
The phase individuality of synthesized compounds was monitored by X-ray
X-ray diffraction
We synthesized solid solutions of compositions Pb5(PO4)3FxCl1−x (x = 0, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875, 1) using solid-phase reactions (Fig. 2). As follows from literature X-ray crystallography data for individual compounds Pb5(PO4)3F and Pb5(PO4)3Cl and from our powder diffraction studies solid solutions based on them, the Pb5(PO4)3F–Pb5(PO4)3Cl system has complete solid miscibility, and unit cells of the compounds in question have hexagonal symmetry of space group P63/m (Table 1).
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Cited by (12)
A comprehensive guide to experimental and predicted thermodynamic properties of phosphate apatite minerals in view of applicative purposes
2015, Journal of Chemical ThermodynamicsThermal expansion of solid solutions in apatite binary systems
2015, Materials Research BulletinCitation Excerpt :We assume that this phenomenon is connected with the formation of superstructure in the solid solution due to the ordering of the P- and V-atoms in the same crystallographic position 6 h. (CaxPb1−x)5(PO4)3Cl system is characterized by more complicated character of the solid solution formation in comparison with the other two systems due to the number of substitutable atoms per unit cell: solid solutions can be synthesized only in the range of composition 0 < x < 0.7 [10–12]. Moreover, the substitution takes place in different crystallographic sites 4f and 6 h.
Low-temperature heat capacity and thermal expansion of synthetic caracolite Na<inf>3</inf>Pb<inf>2</inf>(SO<inf>4</inf>)<inf>3</inf>Cl
2014, Thermochimica ActaCitation Excerpt :It also should be noted, that in spite of the importance of apatites (including halide sulfates) there is a lack of investigations of these compounds especially of their thermodynamics and thermophysical properties. In our previous work we tried to solve that situation: thermodynamic modeling of phase diagrams was carried out for three binary systems of apatites [7–9], all thermodynamic functions were calculated for pentalead tris(vanadate) chloride Pb5(VO4)3Cl as one of the most industrially important apatite [10], thermal deformation were also described for a wide range of apatites [11–14]. In combination with some other investigations [15–20] on the topic it gives satisfactory, but not quite the full picture.
Apatite-structured compounds: Synthesis and high-temperature investigation
2012, Materials Chemistry and PhysicsCitation Excerpt :Monoclinic chlorapatite included to data bases without mar “high-temperature”. In recent publications [18] stoichiometric Ca5(PO4)3Cl was finally referred to space group P63/m of hexagonal symmetry, while nature mineral of composition Ca4.95(PO4)3.99Cl0.92 was determined in monoclinic (pseudohexagonal) symmetry with spac group P21/b [19]. There are practically no literature data about polymorphism of Pb-containing apatites except our previous publications [11,20,21].
Phase diagram of apatite system Ca<inf>10</inf>(PO<inf>4</inf>) <inf>6</inf>Cl<inf>2</inf>-Pb<inf>10</inf>(PO<inf>4</inf>)<inf>6</inf>Cl <inf>2</inf>
2011, Thermochimica ActaCitation Excerpt :There are a few endothermic effects on DTA curves, which are correspond to melting of individual components and solid solutions and polymorphic transitions of Ca10(PO4)6Cl2 and Pb10(PO4)6Cl2. Peculiarities of polymorphic transition accompanied of lowering of unit-cell symmetry were considered in our previous papers [5,6]. As far as temperatures of Ca10(PO4)6Cl2 phase transitions are concerned, there is only melting temperature in literature [10].