Crystal chemistry of the divalent cation in alluaudite-type phosphates: A structural and infrared spectral study of the Na1.5(image)1.5Fe1.5(PO4)3 solid solutions (x=0 to 1, M2+=Cd2+, Zn2+)

Compounds of the Na1.5(Mn1−xMx2+)1.5Fe1.5(PO4)3 (M2+=Cd2+, Zn2+) alluaudite-type solid solutions were synthesized by solid-state reactions in air, between 800 and 900 °C, and were investigated by X-ray powder diffraction and infrared spectroscopy. The site occupancy factors of the Na1.5(Mn1−xCdx2+)1.5Fe1.5(PO4)3 compounds, obtained from the Rietveld refinements of the X-ray powder patterns, indicate that the replacement of Mn2+ by Cd2+ takes place on the M(1) site for x=0.25 and 0.50, and then on the M(1) and M(2) sites for x=0.75 and 1.00. Small amounts of Cd2+ were also detected on the A(1) site, compensated by small amounts of Na+ occurring on M(1). This partially disordered distribution is probably due to the similar effective ionic radii of Cd2+ and Na+. The cationic distributions in the Na1.5(Mn1−xZnx2+)1.5Fe1.5(PO4)3 solid solution indicate that Zn2+ was first introduced in the M(2) site, before to replace Mn2+ in the M(1) site. The unit-cell parameters show a significant increase when Mn2+ is replaced by Cd2+, and a significant decrease when it is replaced by Zn2+. The variations of the β angle were also correlated with the M(1)–M(2) distances. The infrared spectra show the displacement of an absorption band at ca. 400–425 cm−1, which is assigned to the vibrations of the M2+ cations localized on the M(1) site. This assignment is confirmed by the excellent correlations between the M(1)–O bond distances and the energy of the absorption band.