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

¹ Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA
² Laboratoire de Géologie, Ecole normale supérieure, CNRS, 24 rue Lhomond, 75005 Paris, France
³ Geodynamics Research Center, Ehime University, Matsuyama, 790-8577 Japan

^* Corresponding author, e-mail: stebbins{at}stanford.edu

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.

Key-words: phosphorus, stishovite, aluminum phosphate, high pressure phases, NMR spectroscopy.