Effects of pressure on the two polymorphs of [Co(Nh(3))(5)No-2]I-2: The anisotropy of lattice distortion and a phase transition

The effect of pressure on the two polymorphs of [Co(NH3)(5)NO2]I-2 (phase I -orthorhombic, S.G. Pnma; phase II-monoclinic, S.G. C2/m) was studied by X- ray powder diffraction in a diamond anvil cell (DAC). In the presence of th e ethanol-methanol-water mixture used as a pressure-transmitting liquid pol ymorph I was shown to undergo a phase transition at pressures between 0.45 GPa and 0.65 GPa. The diffraction pattern of the high-pressure phase (phase III) could be indexed as tetragonal with lattice parameters similar to tho se, which were previously reported for polymorph II in a 'pseudotetragonal setting'. The lattice distortions of phases II and III were studied at pres sures up to 3.2 GPa and 3.7 GPa, correspondingly, and were shown to be very similar. Phases II and III were supposed to be very closely related. If po ly(chlortri-fluorethylen)-oil was used as a pressure-transmitting medium, n o phase transitions were observed in phase I of [Co(NH3)(5)NO2]I-2 at least up to 1.8 GPa (the point when poly(chlortrifluorethylen)-oil becomes solid ), and the anisotropy of lattice distortion could be measured.