PRESSURE-INDUCED PHASE-TRANSITIONS IN MO8O23

The variation of lattice parameters with pressure was studied on singl e-crystals of Mo8O23 up to 6.03(7) GPa by X-ray diffraction methods. T he crystal, which at ambient conditions has an incommensurately modula ted phase with superspace group B P2/c/11 (0.188,1/2,0.133) is highly compressible, with a lock-in phase transition to a commensurate supers tructure below 0.94(7) GPa and a reversible first-order phase transiti on between 1.40(7) and 1.77(7) GPa. The compressibility of the high-pr essure phase results in an anisotropic crystal with lattice parameters alpha and beta only slightly changing with pressure contrary to the m uch more varying b and c. The high-pressure behaviour of both Mo8O23 p hases can qualitatively be described by different rotations of the MoO 6 octahedra leading to higher 0-0 coordination numbers and therewith t o denser packings of oxygen atoms.