Elsevier

Thermochimica Acta

Volume 513, Issues 1–2, 20 January 2011, Pages 112-118
Thermochimica Acta

Isomorphism and phase diagram of Pb5(PO4)3F–Pb5(PO4)3Cl system

https://doi.org/10.1016/j.tca.2010.11.020Get rights and content

Abstract

Compounds of composition Pb5(PO4)3FxCl1−x (0  x  1), which are synthetic analogues of minerals pyromorphite, flourapatite, and endlichite, were synthesized for the first time by high-temperature solid phase reactions. X-ray diffraction and IR spectroscopy were used to determine the structure of the compounds and revealed complete miscibility in the solid phase of the Pb5(PO4)3F–Pb5(PO4)3Cl binary system. Adiabatic reaction calorimetry was used to measure standard enthalpies of mixing and formation and show that the regular solutions model is applicable to the Pb5(PO4)3F–Pb5(PO4)3Cl system. Differential thermal analysis in tandem with high-temperature X-ray diffraction were used to study the phase diagram and characterize phase transitions.

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:4.5Pb(NO3)2+0.5xPbF2+0.5(1x)PbCl2+3(NH4)2HPO4Pb5(PO4)3FxCl1x+9NO2+2.25O2+6NH3+4.5H2O

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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