Crystal chemistry of the divalent cation in alluaudite-type phosphates: A structural and infrared spectral study of the Na-1.5(Mn1-xMx2+)(1.5)Fe-1.5(PO4)(3) solid solutions (x = 0 to 1, M2+ = Cd2+, Zn2+)

摘要

Compounds of the Na-1.5(Mn1-xMx2+)(1.5)Fe-1.5(PO4)(3) (M2+ = Cd2+, Zn2+) alluaudite-type solid solutions were synthesized by solid-state reactions in air, between 800 and 900 degrees C, and were investigated by X-ray powder diffraction and infrared spectroscopy. The site occupancy factors of the Na-1.5(Mn1-xCdx2+),,, x Fe-1.5(PO4)(3) compounds, obtained from the Rietveld refinements of the X-ray powder patterns, indicate that the replacement of Mn2+ by CA(2+) 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 Na-1.5(Mn1-x,Zn-x(2+))(1.5)Fe-1.5(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 beta 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.