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The structure of lawsonite, CaAl 2 (Si 2 O 7 )(OH) 2 · H 2 O, was refined from X-ray single­crystal data at six temperatures between 110 and 500 K. Two reversible phase transitions were found to occur: (1) Below 273(5) K the space group changes from Cmcm to Pmcn, accompanied by the loss of the --m mirror plane because of rotation of both H 2 O and OH groups. This transition was monitored by the appearance of C-centering forbidden reflec­tions (h + k = 2n + 1) and by nonlinear changes of cell constants and optical birefringence values vs. temperature. (2) Below 155(5) K the space group symmetry is further reduced to P2 1 cn, accompanied by the loss of the m-- mirror plane because of unidirectional shifts of the x coordinates of the H atoms of the H 2 O and OH groups, thus resulting in a polar structure.

Low-temperature phase transitions and the role of hydrogen bonds in lawsonite