The phases of [(CH3)4N](ClO4) at low temperature

The low-temperature crystal structures of tetramethylammonium perchlorate, [(CH(3))(4)N](ClO(4)), are analysed. At 210 K, a collection of 376 unique reflections on a single crystal gave R = 0.0567 for space group P4/nmm, with a(1) = 8.2376 (14), c(1) = 5.8256 (12) A and Z = 2, where the ClO(4) groups are disordered over four orientations. Below T(c) = 170 K, these groups order in four sublattices, each ion gradually choosing one unique orientation. At 150 K, the crystal is microtwinned. 1389 unique reflections were refined in the orthorhombic space group P2(1)2(1)2, with a(3) = 11.714 (3), b(3) = 11.784 (3), c(3) = 5.8265 (9) A, Z = 4 and R = 0.087. At 30 K, Rietveld refinement gave the same structure as is found at 150 K, with a clear difference between a(3) and b(3) [a(3) = 11.566 (2), b(3) = 11.806 (2) and c(3) = 5.729 (1) A]. The structural models and the phase transition are explained on the basis of electrostatic octopole-octopole interactions among the ClO(4) groups within layers perpendicular to the c axis. The order parameters of the low-temperature phase are discussed in relation to the structural results.