Crystal Chemistry of the Hydrothermally Synthesized Na₂(Mn1-xFex2+)₂Fe3+(PO₄)₃ Alluaudite-type Solid Solution


Several compounds of the Na₂(Mn1-xFex2+)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 reÞ nements 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 Þ t 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.




National Science Foundation of Belgium
Ministere de la Region Wallonne

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