Crystal chemistry of the hydrothermally synthesized Na-2(Mn1-xFex2+)(2)Fe3+(PO4)(3) alluaudite-type solid solution

Several compounds of the Na-2(Mn1-xFex2x)(2)Fe3+(PO4)(3) solid solition have been hydrothermally synthesized at 400 degrees 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 Al 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 Al 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 Mossbauer 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 Mossbauer lattice temperatures are within the range of values expected for iron cations in all 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.