Single-crystal Raman spectroscopy of natural paulmooreite Pb2As2O5 in comparison with the synthesized analog

Abstract

The single-crystal Raman spectra of the natural mineral paulmooreite Pb2As2O5 from the Langban, Filipstad district, Varmland province, Sweden, are presented for the first time. It is a monoclinic mineral containing an isolated [As2O5]4− dimer. Unpolarized single-crystal spectra of the natural and synthetic samples compare favorably with each other and are characterized by strong bands around 186 and 140 cm−1 and three medium bands at 800–700 cm−1. Band assignments were made based on band symmetry and spectral comparison between experimental band positions and those resulting from Hartree-Fock calculation of an isolated [As2O5]4− anion complex. Spectral comparison was also made with lead arsenites such as synthetic PbAs2O4 and Pb2(AsO2)3Cl and natural finnemanite to determine the contribution of the terminal and bridging O in paulmooreite. Bands at 760–733 cm−1 were assigned to terminal As-O vibrations, whereas stretches of the bridging O occur at 562 and 503 cm−1. The single-crystal spectra showed good mode separation, allowing bands to be assigned a symmetry species of A g or B g.