Electronic structure of Fe-bearing lazulites

Abstract

The Fe end-members scorzalite [Fe2+Al23+(PO4)(2)(OH)(2)] and barbosalite [Fe2+Fe23+(PO4)(2)(OH)(2)] of the lazulite series have been investigated by Mossbauer and diffuse reflectance spectroscopy, and by electronic structure calculations in the local spin density approximation. The measured quadrupole splitting (DeltaE(Q) = -3.99 mm/s) in scorzalite is in quantitative agreement with the calculated value (DeltaE(Q) = -3.90 mm/s), as well as its temperature dependence. The optical spectrum of barbosalite can be resolved into three peaks at 8985 cm(-1), 10980 cm(-1), and 14110 cm-1. These positions correlate well with the two calculated spin-allowed d-d transitions at 8824 cm-1 and 11477 cm-1, and with an intervalence charge transfer transition at about 14200 cm-1. The calculated low-temperature magnetic structure of barbosalite is characterized by a strong antiferromagnetic coupling (J = -84.6 cm-1) within the octahedral Fe3+-chains, whereas a weak antiferromagnetic coupling within the trioctabedral subunit cannot be considered as conclusive. The analysis of the charge and spin densities reveals that more than 90% of the covalent part of the iron-ligand bonds arises from the Fe(4s,4p)-electrons. Clusters of at least 95 atoms are required to reproduce the available experimental data with quantitative accuracy.