Vibrational spectroscopic study of synthetic analogs of schultenite PbHAsO4–“phosphoschultenite” PbHPO4 solid solution series

Abstract

Lead hydrogen arsenate PbHAsO4 and lead hydrogen phosphate PbHPO4, which can form a continuous solid solution series, have been extensively studied owing to their ferroelectric properties. PbHAsO4 has also been found to occur in weathered As- and Pb-rich outcrops and historically contaminated soils as a schultenite mineral. Although most ferroelectric studies were of substantial interest more than 30 years ago, the contamination of the Earth’s surface with toxic Pb and As remains a major concern for environmentalists. In this study, six compounds of PbHAsO4–PbHPO4 solid solution series were synthesized from aqueous solutions at ambient conditions. They were subsequently analyzed using Raman and Fourier transform infrared spectroscopy to describe their vibrational characteristics, which should help identify the series members in natural or synthetic samples in further environmental and mineralogical studies. Additionally, powder X-ray diffraction analyses were conducted to investigate the variations in the unit cell parameters along the series. The results showed linear variations in the band positions in both Raman and infrared spectra, which depend on the extent of the isomorphic substitution of (PO4)3− for (AsO4)3−. However, the bands become broader or split as a result of phosphate substitution, making it difficult to correlate the band positions with the molecular composition of an unknown phase. The X-ray diffraction data revealed linear variations of all lattice constants as a function of P. Moreover, a relevant finding of this study is the systematic deviation in the chemical compositions of the synthetic phases from the composition of aqueous solutions from which they precipitated, caused by the preferential incorporation of (AsO4)3− with respect to (PO4)3− into the structure.