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High Pressure Research
An International Journal
Volume 17, 2000 - Issue 2
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Original Articles

Effects of pressure on the two polymorphs of [Co(NH3)5NO2]I2: The anisotropy of lattice distortion and a phase transition

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Pages 79-99 | Received 13 Oct 1999, Accepted 17 Nov 1999, Published online: 19 Aug 2006
 

Abstract

The effect of pressure on the two polymorphs of [CO(NH3)5NO2]I2 (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 the ethanol-methanol-water mixture used as a pressure-transmitting liquid polymorph 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 those, which were previously reported for polymorph II in a 'pseudotetragonal setting'. The lattice distortions of phases II and III were studied at pressures 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 poly(chlortrifluorethylen)-oil was used as a pressure-transmitting medium, no phase transitions were observed in phase I of [CO(NH3)5NO2I2 at least up to 1.8 GPa (the point when poly(chlortrifluorethylen)-oil becomes solid), and the anisotropy of lattice distortion could be measured.

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