A new oxygen coordination for phosphorus at high pressure

Contrary to gases, condensed phases (liquids and solids) are often considered as incompressible. This is obviously not the case in the range of pressures encountered in the deep Earth. When high pressures are applied to crystalline phases, their density is found to increase. This density increase is accommodated by the shortening of atomic bonds to a certain point where, under favorable kinetics condition, a phase transition can eventually be triggered ; In the case of a reconstructive phase transition, the atomic arrangement in the structure is then modified and a density jump can occur. Minerals that compose the Earth are mostly ceramics. They can be described as formed by a periodic association of more or less regular oxygen polyhedra which host cationic species. Quartz, for example, is made of polymerized SiO₄ (tetrahedral) groups in the three dimensions. When pressure is applied onto quartz, silicon gets squeezed in the oxygen tetrahedron and the structure may re-arrange to make room around silicon. This is achieved by changing the silicon environment from four to six oxygen neighbours (6-fold coordination). In the very high-pressure forms of silica ; e.g. stishovite, silicon is therefore located in an oxygen octahedron. Phosphates are also composed of oxygen tetrahedra in the form of PO₄ groups. However, phosphorus (P⁵⁺) is smaller than silicon (Si⁴⁺), the pressure required to make phosphorus feel unconfortable in the tetrahedron is therefore higher. As consequence all phosphate minerals occurring in nature are PO₄ phosphates. In order to check whether phosphorus has the ability to form PO₆ groups in the Earth, we have used the SiO₂ stishovite structure as a sort of crystallographic template. We’ve tried to replace some SiO₆ groups by PO₆ groups in the stishovite structure. There is a small complication by trying to do so which is that SiO₆ has an 8- charge whereas PO₆ has a 7- charge. The direct substitution of PO₆ by SiO₆ would therefore break the crystal electroneutrality. Charge compensation is required, and for each SiO₆ replaced by a PO₆, another SiO₆ has been replaced by AlO₆^9- in order to maintain the overall crystal neutrality.
On the chemical point of view, we basically tried to solubilize AlPO₄ in SiO₂ stishovite. Electron microprobe data showed that we were able to incorporate around 1-2 wt.% AlPO₄ in stishovite at 18 GPa and 1600°C [1].
Well, we could have readily concluded that since stishovite is made of oxygen octahedra, the solubilization of some AlPO₄ would imply some P atoms to be in octahedral position as well. This reasoning would also apply to Al (octahedrally coordinated Al is a common feature in minerals). Minerals are tricky ; nanodomains of AlPO₄ with tetrahedral phosphorus might have formed in the stishovite structure. We therefore needed an appropriate probe which provides specific information about the phosphorus atomic environment. X-ray absorption spectroscopy at phosphorus K-edge and ³¹P-MAS Nuclear Magnetic Resonance are the right tools to unravel the atomic environment around phosphorus. In a series of two papers ([1], [2]), we have demonstrated that phosphorus is bonded to six oxygen atoms in the AlPO₄-doped stishovite that we synthesized under extreme pressure and temperature conditions.
So yes, phosphorus can adopt six-fold oxygen coordination in the deep Earth. By the way, it was the first time that PO₆ groups were ever synthesized and characterized ! Consequently, we evidenced with this work a new oxygen coordination for phosphorus.

[1] Brunet, F., A. M. Flank, J. P. Itie, T. Irifune, and P. Lagarde (2007) Experimental evidence of sixfold oxygen coordination for phosphorus. Am. Mineral., 92, 989-993.
[2] Stebbins, J., Kim, N., Brunet, F. and Irifune, T. (2009) Confirmation of octahedrally coordinated phosphorus in AlPO4-containing stishovite by 31P NMR, Eur. J. Mineral., 21, 667-671.http://www.schweizerbart.de/resources/downloads/paper_previews/73654.pdf