Structure of a new form of silicon phosphate (SiP2O7) synthesized at high pressures and temperatures

A new high-pressure phase of SiP2O7 has been found and its crystal structure solved and refined from a single crystal grown at a condition of 16 GPa and 2000 °C and recovered to ambient conditions. The material is monoclinic, with the space group P21/c and lattice parameters a=4.3042(7) Å, b=7.1505(12) Å, c=6.2897(11) Å, β=103.805(2). The structure contains SiO6 octahedra in a corner-sharing arrangement with P2O7 dimers, the same structural elements and vertex-sharing present in all the low-pressure forms of SiP2O7. However, the network is more condensed: the topology of the packing of SiO6 octahedra and P2O7 dimers (represented by Si and the bridging oxygen that both lie on centers of symmetry) is that of the CsCl structure, with some distortion. The resulting phase is 11.2% to 22.3% denser than the various low-pressure forms of SiP2O7. The structural data indicates that the P2O7 dimers are linear (P–O–P angle=180°), an unusual feature for phosphates.