Novel ionic plastic phase of [(CH3)4N]SCN obtainable above 455 K studied by proton magnetic resonance, electrical conductivity and thermal measurements

Abstract

A new high-temperature solid phase of tetramethylammonium thiocyanate has been found above the phase transition at 455 K, where a large entropy change (15 J K^–1 mol^–1) was observed. X-ray powder patterns recorded at ca. 470 K showed that this phase forms a CsCl-type cubic lattice with a_O= 600 pm, indicating that the SCN^– anions take highly disordered orientations. The presence of rapid three-dimensional self-diffusion, as well as the overall rotation of the cations in this phase, was revealed by the measurements of ¹H NMR spin–lattice relaxation times, T₁ and T_1ρ, the second moment of NMR absorptions and the electrical conductivity. The large ionic-diffusion constant (of the order of 10^–13 m² s^–1) obtained around 490 K is roughly the same as those reported for ‘ionic plastic’ crystals. An activation energy of ca. 110 kJ mol^–1 was determined for cationic self-diffusion. These results imply that this new phase is the first example of an ionic plastic crystal containing tetramethylammonium cations.