Structural and 31P NMR investigation of Bi(MM′)2PO6 statistic solid solutions: Deconvolution of lattice constraints and cationic influences

Two solid solutions BiMxMg(2−x)PO6 (with M2+=Zn or Cd) have been studied through 31P MAS NMR. The analysis has been performed on the basis of refined crystal structures through X-ray diffraction and neutron diffraction. The BiZnxMg(2−x)PO6 does not provide direct evidence for sensitive changes in the phosphorus local symmetry. This result is in good agreement with structural data which show nearly unchanged lattices and atomic separations through the Zn2+ for Mg2+ substitution. On the other hand, the Cd2+ for Mg2+ substitution behaves differently. Indeed, up to five resonances are observed, each corresponding to one of the five first-cationic neighbour distributions, i.e. 4Mg/0Cd, 3Mg/1Cd, 2Mg/2Cd, 1Mg/3Cd and 0Mg/4Cd. Their intensities match rather well the expected weight for each configuration of the statistical Cd2+/Mg2+ mixed occupancy. The match is further improved when one takes into account the influence of the 2nd cationic sphere that is available from high-field NMR data (18.8 T). Finally, the fine examination of the chemical shift for each resonance versus x allows to de-convolute the mean Z/a2 effective field into two sub-effects: a lattice constraint-only term and a chemical-only term whose effects are directly quantifiable.