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Crystal chemistry of the hydrothermally synthesized Na₂(Mn_1–xFe_x²⁺)₂Fe³⁺(PO₄)₃ alluaudite-type solid solution

¹ Laboratoire de Minéralogie, B18, Université de Liège, B-4000 Sart-Tilman, Belgium
² Institut für Mineralogie, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44780 Bochum, Germany
³ Département de Physique, B5, Université de Liège, B-4000 Sart-Tilman, Belgium
⁴ Department of Chemistry, University of Missouri-Rolla, Missouri 65409-0010, U.S.A.

Correspondence: ^* E-mail: fhatert{at}ulg.ac.be

Several compounds of the Na₂(Mn_1–xFe²⁺_x)₂Fe³⁺(PO₄)₃ solid solution have been hydrothermally synthesized at 400 °C and 1 kbar; pure alluaudite-like compounds have been obtained for x = 0.00, 0.25, 0.50, 0.75, and 1.00. Rietveld refinements of the powder X-ray diffraction patterns indicate the presence of Na⁺ at the A1 and A2' sites, Mn²⁺ and Fe²⁺ at the M1 site, and Mn²⁺, Fe²⁺, and Fe³⁺ at the M2 site. The presence of small amounts of Na⁺ at the M1 site and Mn²⁺ at the A1 site indicates a partially disordered distribution of these cations. An excellent linear correlation has been established between the M1-M2 distance and the energy of the infrared band attributed to the M²⁺-O vibrations. The Mössbauer spectra, measured between 85 and 295 K, were analyzed in terms of a model which includes the next-nearest neighbor interactions at the M2 and M1 crystallographic sites. Fe²⁺ and Fe³⁺ isomer shifts are typical of the alluaudite structure and exhibit the expected second-order Doppler shift. The derived iron vibrating masses and Mössbauer lattice temperatures are within the range of values expected for iron cations in an octahedral environment. The Fe²⁺ and Fe³⁺ quadrupole splittings are typical of the alluaudite structure, and the temperature dependence of the Fe²⁺ quadrupole splitting was fit with the Ingalls model, which yielded a ground state orbital splitting of ca. 460 to 735 cm^–1 for the Fe²⁺ sites. The isomer shifts and quadrupole splittings of Fe²⁺ at the M1 site are larger than those of Fe²⁺ at M2, indicating that the M1 site is both larger and more distorted than the M2 site.

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