Structure of a new form of silicon phosphate (SiP ₂O ₇) synthesized at high pressures and temperatures

A new high-pressure phase of SiP ₂O ₇ 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 P2 ₁/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 SiO ₆ octahedra in a corner-sharing arrangement with P ₂O ₇ dimers, the same structural elements and vertex-sharing present in all the low-pressure forms of SiP ₂O ₇. However, the network is more condensed: the topology of the packing of SiO ₆ octahedra and P ₂O ₇ 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 SiP ₂O ₇. The structural data indicates that the P ₂O ₇ dimers are linear (P-O-P angle=180°), an unusual feature for phosphates.