Fourier transform infrared spectroscopic study of hydroxylpyromorphite Pb10(PO4)6OH2–hydroxylmimetite Pb10(AsO4)6(OH)2 solid solution series

Abstract

A total of seven compounds of the hydroxylmimetite Pb10(AsO4)6(OH)2–hydroxylpyromorphite Pb10(PO4)6(OH)2 (HPY–HMI) solid solution series were synthesized at 80°C from aqueous solutions and characterized using XRD, SEM–EDS and FTIR. The substitution effect of [PO4]3− anions by [AsO4]3− on systematic changes in lattice parameters and spectral properties has been explained based on correlation between chemical composition and the peak positions. The substitution results in systematic linear increase in unit cell parameters and unit cell volume. Isomorphic substitutions are apparent in IR as a change in the position and intensity of bands derived from phosphates, arsenates and hydroxyl ions. Main bands attributed to stretching and bending vibrations appear at 1109–948cm−1 and 577–511cm−1 for phosphates in HPY and bending vibrations at 853–747cm−1 for arsenates in HMI. The most sensitive for substitution are v1-PO4 and v3-PO4 bands which shift toward lower wavenumbers from 931cm−1 to 911cm−1 for v1-PO4 and from 1109cm−1 to 1086cm−1 for v3-PO4. Arsenate bands are less sensitive for substitution: v3-AsO4 shift from 853cm-1 to 831cm−1. The strong correlations between the positions of the vibrational modes and the As/As+P ratios can be used to determine the composition of the phases from the hydroxylmimetite–hydroxylpyromorphite series.