1Supplementary data for this paper are available from the IUCr electronic archives (Reference: GP5058 ). Services for accessing these data are described at the back of the journal.

2The choice of crystallographic axes in this paper corresponds to the one in Drebushchak et al. (2009BB15), since the structure of the [alpha] polymorph at 100 K was taken as a starting model for the refinement of the high-pressure polymorph. The advantages of such a choice of the crystallographic axes were discussed in Drebushchak et al. (2009BB15).