Crystal chemistry of the hydrothermally synthesized Na2(Mn1-xFex2+)2Fe3+(PO4)3 alluaudite-type solid solution

Abstract

Several compounds of the Na 2 (Mn 1 - x Fe 2 + x ) 2 Fe 3 + (PO 4 ) 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, Mn 2 + and Fe 2 + at the M1 site, and Mn 2 + , Fe 2 + , and Fe 3 + at the M2 site. The presence of small amounts of Na + at the M1 site and Mn 2 + 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 M 2 + -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 Ml crystallographic sites. Fe 2 + and Fe 3 + 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 an octahedral environment. The Fe 2 + and Fe 3 + quadrupole splittings are typical of the alluaudite structure, and the temperature dependence of the Fe 2 + quadrupole splitting was fit with the Ingalls model, which yielded a ground state orbital splitting of ca. 460 to 735 cm - 1 for the Fe 2 + sites. The isomer shifts and quadrupole splittings of Fe 2 + at the Ml site are larger than those of Fe 2 + at M2, indicating that the Ml site is both larger and more distorted than the M2 site.