Neutron Diffraction Evidence of Structural Phase Transitions at 150 K and 90 K in Trimethylammonium-lodide-7,7,8,8, Tetracyanop-Quinodimethane (TMA+, TCNQ2/3-, (I−3)1/3)

Neutron photographs have been obtained down to 7 K and additional neutron data have been recorded on a 4-circle diffractometer for single crystals of the title compound. They show that below 150-160 K weak satellites appear at 1/3b of the main TMA-TCNQ lattice reflections. This is consistent with a charge transfer of 0.66 e and suggest that the transition is of a “metal-insulator” type rather than a “semiconductor-semiconductor” type. Below 80-90 K a second transition occurs which is associated with a splitting of reflections along ∼c. The main lattice is then triclinic and the satellites are still present.