ELECTRONIC STRUCTURE OF FE-BEARING LAZULITES

Abstract

The Fe end-members scorzalite [Fe2+ Al2 3+ (PO4)2(OH)2] and barbosalite [Fe 2+ Fe2 3+ (PO4)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 (DEQ = -3.99 mm/s) in scorzalite is in quantitative agreement with the calculated value (DEQ = -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 mag-netic structure of barbosalite is characterized by a strong antiferromagnetic coupling (J = -84.6 cm -1 ) within the octahedral Fe 3+ -chains, whereas a weak antiferromagnetic coupling within the trioctahedral subunit cannot be considered as conclusive. The analysis of the charge and spin densi-ties 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.