An X-ray Rietveld, infrared, and Mössbauer spectral study of the NaMn(Fe_1-xIn_x)₂(PO₄) ₃ alluaudite-type solid solution

Several compounds of the NaMn(Fe_1-xIn_x)₂ (PO₄)₃ solid solution were synthesized by solid state reaction in air: pure alluaudite-like compounds were obtained for x = 0.00 to 1.00. X-ray Rietveld refinements indicate the presence of Na⁺ at the A1 and A2′ sites, Mn²⁺ at the M1 site, and Fe²⁺, Fe³⁺, and In³⁺ at the M2 site. The presence of small amounts of In³⁺ at the M1 site, and Mn²⁺ at the M2 site, indicates a partially disordered distribution between these cations. A good correlation was also established between the M1-M2 bond distance and the β angle of the alluaudite-like compounds. The disordered distribution of Fe²⁺, Fe³⁺, and In³⁺ at the M2 site is confirmed by the broadness of the infrared absorption bands. The Mössbauer spectra, measured between 90 and 295 K, were analyzed in terms of a model that takes into account the next-nearest neighbor interactions around the M2 crystallographic site. In all cases these spectra reveal the unexpected presence of small amounts of Fe²⁺ at the M2 site, an amount that decreases as the In³⁺ content increases. The Fe²⁺ and Fe³⁺ isomer shifts are typical of the alluaudite structure and vary with temperature, as expected from a second-order Doppler shift. The derived iron vibrating masses and Mössbauer lattice temperatures are within the expected range of values for iron cations in an octahedral environment. The Fe²⁺ and Fe³⁺ quadrupole splittings are also typical of the alluaudite structure and the temperature dependence of the Fe²⁺ quadrupole splitting was fit with the model of Ingalls (1964), which yielded a ground state orbital splitting of ca. 380 to 570 cm^-1 for the Fe²⁺ sites.