Two-step neutral-ionic phase transition in organic charge-transfer compounds: Possible staging effect

A two-step neutral-ionic transition has been discovered in a 1:1 mixed-stack charge-transfer crystal, (3,3’,5,5’) tetramethylbenzidine-(7,7,8,8) tetracyanoquinodimethane (TMB-TCNQ) under pressure. The fraction of ionized molecules changes discontinuously at two critical pressures, ${\mathit{P}}_1$∼6 and ${\mathit{P}}_2$∼20 kbar, demonstrating a characteristic feature of a staging phenomena. This result is interpreted in terms of a frustration effect of Coulomb interactions in the crystal, as predicted theoretically by Hubbard and Torrance.