High-pressure synthesis and crystal structure of silicon phosphate hydroxide, SiPO4(OH)

A new high-pressure phase, silicon phosphate hydroxide, was prepared at 8.3+/-0.5GPa and 1000 deg. C in >98% purity. From X-ray diffraction on a pseudo-merohedrally twinned crystal, it was found that SiPO4(OH) crystallizes in a monoclinic cell with space group P21/n (No. 14), a=6.8446(11)A,b=6.8683(13)A,c=6.8446(11)A,β=119.77(1)o, and Z=4. The refinement gave a conventional Robs of 0.0320 and wRobs of 0.0864 for the overlapped data from both twin components. In the structure, SiO6 octahedra form chains along [101], with PO4 tetrahedra alternating along the chain in the b-direction. The parallel chains link up with tetrahedral corners from other chains to form a 3-dimensional network. SiPO4(OH) belongs to a structural family that includes HgSeO4.H2O. It is also related to the SbOPO4 structure by a small distortion that lowers the symmetry from C2/c in SbOPO4 to P21/c(P21/n) in SiPO4(OH)