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

Several compounds of the Na₂(Mn_1-xFe_x ²⁺)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 Fe ²⁺ 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 M ²⁺-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 splitfings 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.