Electronic structure of Fe-bearing lazulites

The Fe end-members scorzalite [Fe²⁺Al₂ ³⁺(PO_{4 2}(OH)₂] and barbosalite [Fe²⁺Fe₂ ³⁺(PO_{4 2}(OH)₂] of the lazulite series have been investigated by Mössbauer and diffuse reflectance spectroscopy, and by electronic structure calculations in the local spin density approximation. The measured quadrupole splitting (ΔE_Q = -3.99 mm/s) in scorzalite is in quantitative agreement with the calculated value (ΔE_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 trioctahedral 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.