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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 …