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Silicon in five-fold coordination by oxygen is believed to play an important role in determining the transport properties (e.g. diffusivity, viscosity) of silicate melts at the pressures and temperatures typical of the Upper Mantle of the Earth. Understanding such processes on an atomic scale has been hindered previously by the absence of such five-fold coordinated silicon in crystalline compounds in which its crystal chemical properties could be more easily quantified. The first inorganic crystal structure to contain silicon coordinated by five oxygens was described in the Annual Report of 1996, while its transformation to six-coordinated silicon under pressure was described in last year's Annual Report.

A calcium aluminosilicate phase of composition CaAl₄Si₂O₁₁ (CAS phase) had previously been reported in the products of several different experiments at high pressures and temperatures around 14 GPa and 1500°C. We synthesised single-crystals up to ~200 µm in size at these conditions from a stoichiometric mixture of oxides in the 1000 ton multianvil press. Initial single-crystal X-ray diffraction measurements confirmed earlier suggestions that the new CAS phase was similar in structure to a class of synthetic materials known as "hexagonal Ba-ferrites"; a typical member of this family has the chemical formula BaFe₄Ti₂O₁₁. These ferrite structures contain Fe atoms coordinated by five oxygen atoms in a trigonal-bipyramidal geometry, suggesting that the Si in the CAS phase might occupy this trigonal-bipyramidal site, and it would then be the second crystal structure to contain Si five-coordinated by oxygens. However, careful structure refinement and analysis of the X-ray diffraction intensities indicated that although the Si atoms may occupy the trigonal-bipyramidal cavity, they are significantly displaced from its centre and so obtain the tetrahedral coordination typical of low-pressure silicates (Fig. 3.3-3). Thus, at least at room conditions, the CAS phase does not contain five-coordinated silicon, although the possibility exists that the silicon atoms might be moved back to the central five-coordinated position under the application of high pressure. Further single-crystal studies are under way to investigate this possibility. Such a transition would be the four-to-five analogue of the five to six-fold coordination change previously observed for Si in CaSi₂O₅ at high pressures.
 

Fig. 3.3-3: The crystal structure of CaAl4Si2O11 as determined by single-crystal X-ray diffraction. The M1 and M2 octahedra contain Al and/or Si, while the four-coordinated cations occupying one-half of the trigonal-bipyramidal cavities are labelled T.