Sodium percarbonate between 293 and 100 K.

Sodium carbonate perhydrate, Na(2)CO(3).1.5H(2)O(2), commonly known as sodium percarbonate, has been structurally characterized at temperatures ranging from 293 down to 100 K. As the temperature drops, there is a reversible phase change between 250 and 240 K, in which the crystallographic mirror plane of the room-temperature space group, Cmca, is lost, transforming it to Pbca. This transformation is accompanied by a reversal in the expansion coefficient of cell side b, which, like the other two sides, initially contracts on cooling. The two crystallographically independent hydrogen peroxide sites undergo the largest change, where the disorder seen at room temperature gradually diminishes and is almost completely absent at 100 K, thus indicating dynamic rather than static disorder. It is noteworthy that, despite Cmca being the preferred room-temperature space group in the current work, two previous single-crystal studies rejected this space group in favour of non-centrosymmetric Aba2 [Carrondo et al. (1977). J. Chem. Soc. Dalton Trans. pp. 2323-2331; Adams & Pritchard (1977). Acta Cryst. B33, 3650-3653]. A re-evaluation of these determinations indicates that they are identical to the current structure and should be re-assigned to Cmca.