Towards a more reliable symmetry determination from powder diffraction: a redetermination of the low-temperature structure of 4-methylpyridine-N-oxide.

The low-temperature structure of 4-methylpyridine-N-oxide was previously determined in symmetry P4(1) [Damay et al. (2006), Acta Cryst. B62, 627-633]. Using a recently published symmetry-determination method it was found that the true symmetry of the structure is P4(1)2(1)2. The structure was refined in the new space group using X-ray and neutron data. The previously published structure is close to the newly refined structure, but the new structure is in agreement with the results of rotational tunneling spectroscopy, and, in contrast to the structure in symmetry P4(1), does not require a twofold degeneracy of the tunneling bands.