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 Långban, Filipstad district, Värmland 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.