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Accurate density data yield the primary information required for a thermodynamic model of molecular disorder. A detailed analysis of the structure from neutron Bragg data shows that the protons of the ammonia groups perform nearly unhindered rotations at room temperature as well as at 30 K. The nuclear density distribution is derived from a combination of conventional crystallographic split-atom density interpolation and maximum-entropy density reconstruction. The crystallographic density interpolation allows unique phases to be determined, i.e. phases that are independent of details of the model, for the measured Fourier components. MaxEnt reconstruction eliminates series-termination effects and provides for higher quality in the spatial resolution compared with standard Fourier maps. The proton density obtained in [Co(NH3)6](PF6)2 at both temperatures is concentrated on a plane perpendicular to the fourfold crystal axis and is found to be nearly circular, with a weak tetragonal contribution superimposed. This is a strong indication of nearly free uniaxial rotation in this compound. The proton density is analysed in terms of an anharmonic orientational potential, which couples rotational and translational motion. The nearly unhindered rotation in this compound is a consequence of the quasi-eightfold symmetry built up by the next-neighbour F atoms surrounding the NH3 groups in their plane of rotation.

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Crystallographic Information File (CIF)
Contains datablocks text, sh0067a, sh0067b

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