Theory of phase transitions in charge-transfer complexes: Anthracene–tetracyanobenzene

The phase transition in anthracene tetracyanobenzene (A-TCNB) is identified as of mixed order–disorder and displacive character. This arises from coupling between the anthracene orientation, treated as a pseudospin, and librational phonons of Bg symmetry at the M point in the high-temperature Brillouin zone. Suitable coupling is provided by the orientational dependence of charge-transfer interactions, enhanced by dielectric effects due to the highly polarizable TCNB; an expression is deduced for the electrostatic energy in the disordered structure. This coupling raises the critical temperature in the Landau mean field treatment, but further coupling with totally symmetric acoustic phonons makes the transition of first order at a still higher temperature. The transition temperature, pseudospin ordering, and orientational displacements are derived. Strong coupling favors a displacive transition, weak coupling an order–disorder transition. Similar considerations should apply to phase transitions in other charge-transfer crystals.