Orientational ordering in the low-temperature stable phases of deuterated thiophene

The stable structures of deuterated thiophene C(4)D(4)S were investigated at 155 (phase III), 115 (phase IV), 100 and 1.5 K (phase V) by neutron powder diffraction. At 155 K, thiophene is orthorhombic with space group Pbnm. Although there is some degree of in-plane orientational disorder, molecules begin to order along two symmetrically equivalent main orientations. At 115 K the structure is incommensurate, with a wavevector q approximately 0.55a*. At 100 K and below, there is a doubling of the a cell parameter and the structure space group is P2(1). For the first time, it is shown that, unlike C(4)H(4)S, phase V of C(4)D(4)S is not an orientational glass: thiophene molecules are perfectly ordered and are oriented within the molecular plane along two alternating directions, corresponding to the two main orientations observed at 155 K. This ordering probably originates in the slowing down of the in-plane reorientational dynamics upon deuteration.