L-threo-D-galacto-Octitol: a curious non-classical order-disorder polytype with an 88 Å axis

GalC8 [L-threo-D-galacto-octitol, systematic name (2S,3R,4R,5R,6R,7S)-octane-1,2,3,4,5,6,7,8-octol] crystallizes with a pseudo-tetragonal lattice featuring a long (∼88 Å) c axis. The bulk of the structure possesses P4₃2₁2 (Z' = 1) pseudo-symmetry, whereby molecules are arranged linearly with eight molecules in a translation period, thus explaining the long c axis. According to this pseudo-symmetry, adjacent molecules are alternately related by 2₁ screw rotations (in 〈100〉 directions) and twofold rotations (in 〈110〉 directions). Due to an asymmetrical hydrogen-bonding network, symmetry is broken in the vicinity of the twofold rotation axes, leading to an overall P2₁ (Z' = 4) space-group symmetry. The lost symmetry is retained as a twin operation. The structure can be classified as a `non-classical' OD polytype, in the sense that there are nonequivalent layer contacts, yet all members of the polytype family are locally equivalent. The main polytype is not of a maximum degree of order (MDO) and can be described as an alternation of fragments of the two MDO polytypes with P4₃ and P2₁2₁2₁ symmetry, respectively.