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

Abstract

Solid state ²³Na and ³¹P 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 ²³Na and ³¹P solid state NMR data confirm the similarities between the two structures, while the ²³Na 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)_nⁿ⁻ with Na⁺ and Mg²⁺/Zn²⁺ ions in distorted octahedral sites located between the polyphosphate chains.