Synthesis and characterisation of Cr3+-containing NASICON-related phases

Chromium-containing NASICON-related phosphates of the type Na(1+x)CrxM(2−x)P3O12) (M = Ti, Hf, Zr) have been synthesised by solid state reaction and structurally characterised by Rietveld refinement of the powder X-ray diffraction data. Materials of composition A(1+x)/2CrxZr(2−x)P3O12 (A = Cd, Ca, Sr), have also been prepared and characterised. The crystal structure of Na(1+x)CrxM(2−x)P3O12 corresponds to R-3c symmetry for x values ranging from 0.15 to 2.00, whereas compounds of composition A(1+x)/2CrxZr(2−x)P3O12 corresponding to R-3c are obtained when x ≤ 1.00 for Sr2+ and Ca2+, and x ≤ 1.50 for Cd2+. The polarizing effect of the two different metal ions A and M on the phosphorus atom and the P–O bond was studied by both 31P MAS NMR and infrared spectroscopy and shows that the electron density on the phosphorus, and thus the strength of the P–O bonds, are affected by both the interstitial (A) and the structural (M) metal ions.