Structures of two new high-pressure forms of AlPO₄ by X-ray powder diffraction and NMR spectroscopy

The crystal structures of two new high-pressure AlPO₄ phases are reported. One phase synthesized at 6 GPa and 1523 K is triclinic (P1̄) whilst the other phase synthesized at 7 GPa and 1773 K is monoclinic (P2 ₁/C). ³¹P MAS (magic-angle spinning) NMR suggests three tetrahedral P sites with equal abundance in both phases. ²⁷Al 3Q MAS NMR spectra provided evidence for two octahedral sites and one five-coordinated Al site in each phase. The crystal structures were solved using an ab initio structure determination technique from synchrotron powder X-ray diffraction data utilizing the local structural information from NMR, and were further refined by the Rietveld method. Both phases contain doubly bent chains made of six edge-shared Al polyhedra (including five-coordinated Al), which are joined by PO₄ tetrahedra. The P1̄ phase is isostructural with FeVO ₄ and AlVO₄. The two phases differ in the packing manner of the chains. This study has demonstrated that the combined application of ab initio structure determination via X-ray powder diffraction and solid-state NMR spectroscopy is a powerful approach to the rapid solution of complex inorganic crystal structures.