Abstract
A rare transition from positive to negative thermal expansion in an organic herringbone stacked structure is reported and related to an isosymmetric phase transition. Abstract 4,4'-(benzo[b]benzo[4,5]thieno[2,3-d]thiophene-2,7-diyl)bis(butan-1-ol), BTBT-C4OH, displays a continuous type 0 first-order isosymmetric phase transition at 200 K which is accompanied by a continuous change of the thermal expansion along the b-axis from positive to negative. The equivalent isotropic atomic displacement parameters for all non-hydrogen atoms as well as all the eigenvalues of the anisotropic atomic displacement tensor show discontinuous behavior at the phase transition. The eigenvalues of the translational tensor in a rigid-body description of the molecule are all discontinuous at the phase transition, but the librational eigenvalues are discontinuous only in their temperature derivative. BTBT-C4OH displays a similar type of quasi-supercritical phase transition as BTBT-C6OH, despite the difference in molecular packing and the very large difference in thermal expansion magnitudes.