Cation distribution in cubic NaM(PO3)3 (M = Mg or Zn) using X-ray powder diffraction and solid state NMR

Solid state Na-23 and P-31 magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy and X-ray powder diffraction have been used in combination to study the structure of the model phosphate phases NaMg(PO3)(3) and NaZn(PO3)(3). The similar X-ray scattering of Na and Mg make determination of the cation distribution in NaMg(PO3)(3) difficult using X-ray data alone. However, the structure of NaZn(PO3)(3) was successfully refined and an initial model for the cation distribution in the magnesium analogue proposed. The Na-23 and P-31 solid state NMR data confirm the similarities between the two structures, while the Na-23 also suggested three sodium sites in a 2:1:1 ratio, consistent with the proposed crystallographic model. This allowed for a successful refinement of the structure of NaMg(PO3)(3), confirming that the structures are indeed isostructural. Both structures contain chains of (PO3)(n)(n)- with Na+ and Mg2+/Zn2+ ions in distorted octahedral sites located between the polyphosphate chains.