Understanding the relationship between the structural and dielectric changes at the order-disorder transition in TEA(TCNQ)₂

Triethylammonium bis-7,7,8,8-tetracyanoquinodimethane (TEA(TCNQ)₂) shows an interesting dielectric behaviour, at approximately 220 K, where both the capacitance and the loss exhibit clear anomalies. This corresponds to the order-disorder transition where it is believed that the TEA cations freeze within the structure promoting both a structural change which manifests itself also as a change in dielectric properties. To understand this further, we have coupled quasi-elastic neutron scattering and other techniques to provide information on how these TEA cations are actually moving and what motions freeze out. We find that two things happen; firstly at 100 K we believe the methyl groups start rotating but at 220 K, the TEA cations rotate with a spehrical radius of ~1.7 Angstroms.