Abstract

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.

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