Electronic structure of Fe-bearing lazulites

M. Grodzicki 1 , G.J. Redhammer 1 , G. Amthauer 1 , V. Schünemann 2 , A.X. Trautwein 2 , B. Velickov 3  and P. Schmid-Beurmann 4
  • 1 Institute of Mineralogy, University of Salzburg, Hellbrunner Str. 34, A-5020 Salzburg, Austria
  • 2 Institute of Physics, Medical University, Ratzeburger Allee 160, D-23536 Lübeck, Germany
  • 3 Institute of Applied Geosciences, TU-Berlin, Ernst-Reuter-Platz 1, D-10587 Berlin, Germany
  • 4 Institute of Mineralogy and Petrography, University of Kiel, Ludewig-Meyn-Str. 10, D-24098 Kiel, Germany

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 Mössbauer and diffuse reflectance spectroscopy, and by electronic structure calculations in the local spin density approximation. The measured quadrupole splitting (ΔEQ = -3.99 mm/s) in scorzalite is in quantitative agreement with the calculated value (ΔEQ = -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.

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