Thermal Behavior of Pyromorphite (Pb10(PO4)6Cl2): In Situ High Temperature Powder X-ray Diffraction Study

Abstract

Pyromorphite is one of the important end member lead apatites that has potential applications in environment remediation. The thermal behavior of natural pyromorphite, Pb10(PO4)6Cl2, has been investigated up to 1373 K at room-pressure using a powder X-ray diffraction device equipped with a heating system. Pyromorphite experiences melting and decomposing at 1373 K into lead phosphate (Pb3(PO4)2), and lead dioxide (PbO2) with reaction with air. The fit of the temperature–volume data yields a linear volume expansion coefficient αV = 4.5 (±0.02) × 10−5 K−1. The linear expansion coefficients for lattice parameters present the anisotropic thermal expansibility, i.e., αa = (±0.06) × 10−5 K−1 and αc = 2.2 (±0.06) × 10−5 K−1. We looked into the crystal chemistry and proposed an expression to quantitatively evaluate the structural evolution of pyromorphite upon high temperature by calculating the twist angle of the Pb(1)O6 metaprism, which decreases at elevated temperatures. A distinct drop of the twist angle was observed at ~1100–1200 K, which might be a sign for the phase transition to a low symmetric subgroup. The variation of the twist angle is more sensitive than that of the unit cell; therefore, it can be applied to monitor the structural and phase changes of apatite group materials in general.