Relationship between solid state NMR parameters and X-ray structural data in tricadmium phosphates

P-31 and Cd-113 MAS NMR spectra of solid beta'-tricadmium phosphate (beta'-TCdP) show a number of highly resolved resonances that agree well with the number of independent crystallographic sites indicated by the results of X-ray diffraction studies. A correlation of the P-31 chemical shifts with the crystallographic sites for the six different PO43- groups in the unit cell of beta'-TCdP has been obtained by a method based on the computation of bond strength at oxygen atoms in phosphate moieties. The assignment of the Cd-113 resonances has been carried out on the basis of the relationship between the asymmetry of the chemical shift tensor (evaluated by analysis of the spinning side bands intensities in the MAS spectrum) and a geometric parameter related to the distortion from the bipyramidal trigonal coordination at each cadmium center. Samples of tricadmium phosphate with different degrees of magnesium substitution for cadmium were investigated by P-31 MAS NMR, Cd-113 MAS MMR, and X-ray diffraction. The results of these investigations showed that the magnesiums distribute randomly in the cadmium sites, inducing a marked decrease in the order of the structure.