Confirmation of octahedrally coordinated phosphorus in AlPO4-containing stishovite by 31P NMR

Using ³¹P and ²⁷Al Nuclear Magnetic Resonance (NMR) spectroscopy, we confirm the discovery of six-coordinated phosphorus (^VIP) in stishovite that contains about 1 wt% P₂O₅ and Al₂O₃ (synthesized at 18 GPa), as described in a recent study based on micro-X-ray Absorption Near-Edge Structure (XANES) data at the phosphorus and aluminum K-edges in a similar sample. We also confirm the presence of six-coordinated Al (^VIAl) in the stishovite. The ^VIP is marked by a ³¹P NMR peak far below the known range for phosphate groups in solid oxide materials, at −158 ppm. A sample of pure AlPO₄ made under the same conditions contained only ^IVP, as well as ^IVAl, ^VIAl, and a minor amount of ^VAl. Observed chemical shifts and peak shapes in this material, along with X-ray powder diffraction data (XRPD), indicate a mixture of the high-pressure form of AlPO₄ (Cmcm, CrVO₄-structure), its low-cristobalite form, an amorphous phase, and one or more unknown crystalline phases.