Suppression of a Structural Phase Transition by an Orientational Disorder of Counteranions in an Organic Conductor,...

In 2022, we reported that the β″-(BEDT-TTF)₂ClC₂H₄SO₃ salt (1) has a phase transition at 210 K on cooling and 260 K on heating, which was suggested as a nondoped-to-doped transition. During the synthesis of 1, a second salt was coproduced, the structure and properties of which are reported here. The new salt β″-β″-(BEDT-TTF)₂ClC₂H₄SO₃ (2) has the same composition and is isomorphous to 1L, the low-temperature phase of 1. However, the salt shows no phase transition. 1H, the high-temperature phase of 1, displays rotational disorders of the -SO₃^- groups. In contrast, the anions in 2 have no rotational disorders but have orientational disorders. Approximately 13% of the anions in 2 have their orientation as opposed to the remaining anions. The geometrically calculated shortest O···O distance is 2.24 Å if neighbor -SO₃^- groups are able to rotate freely. This is 1.1 Å shorter than the sum of the van der Waals radii, and therefore, this is too close to allow free rotation. Therefore, it is proposed that 2 has no rotational disorder, which leads to no phase transition as is observed in 1.