Mössbauer spectral evidence for next-nearest neighbor interactions within the alluaudite structure of Na_1-xLi_xMnFe₂(PO₄)₃

The Mössbauer spectra of the Na_1-xLi_xMnFe₂(PO₄)₃ compounds, with x = 0.00, 0.25, 0.50, and 0.75, have been measured between 90 and 295 K and analyzed in terms of a model which takes into account the next-nearest neighbor interactions within the alluaudite structure. Surprisingly, the spectra reveal an unexpected presence of iron(II) in these compounds; the amount of iron(II) is observed to decrease from 19 to 15 atomic percent of the total iron content with increasing x. The temperature dependence of the Fe²⁺ and Fe³⁺ isomer shifts agrees with that expected from a second-order Doppler shift and the resulting iron vibrating masses and Mössbauer lattice temperatures are within the expected range of values for iron cations in an octahedral environment. The temperature dependence of the Fe²⁺ quadrupole splitting has been fit with the Ingalls' model and the results yield a ground state orbital splitting of ca. 500 cm^-1 for the iron(II) sites. The compositional dependence of the isomer shifts and Fe²⁺ content can be understood in terms of a decrease in the unit-cell volume with increasing substitution of sodium by lithium, a substitution which does not influence the observed quadrupole splittings.