OBSERVATION OF AN INCOMMENSURATE PHASE IN THE STABLE PHASE SEQUENCE OF DEUTERATED THIOPHENE BY POWDER NEUTRON-DIFFRACTION

Deuterated thiophene has been studied by powder neutron diffraction sp ectroscopy from approximately 60 K to melting temperature. Preliminary DSC measurements have shown that the thermodynamic behaviours of hydr ogenated and deuterated thiophene are almost identical, so that the co nclusions drawn from the present structural studies of C4D4S are valid for both compounds. The structures of stable phases IV and V have bee n observed for the first time: phase V lattice is obtained by multiply ing by 2 parameter c of orthorhombic phase III and has been assigned s pace group P2(1) ma; phase IV also corresponds to a superstructure of phase III, but, as expected because of the quasi-fivefold molecular sy mmetry and of its reorientational dynamics, it can be thought to be an incommensurate phase.