Neutral-ionic phase transition : a thorough ab-initio study of TTF-CA

TL;DR

This study uses first-principles density functional theory to analyze the neutral-ionic phase transition in TTF-CA, revealing how thermal effects influence electronic structure and charge transfer mechanisms.

Contribution

It provides a detailed ab-initio analysis of TTF-CA's phase transition, including charge density topology and the impact of thermal lattice contraction on electronic properties.

Findings

Thermal lattice contraction reduces the band gap in the neutral phase.

Charge transfer from TTF to CA is quantified using three different methods.

The study discusses a possible mechanism for the neutral-ionic phase transition.

Abstract

The prototype compound for the neutral-ionic phase transition, namely TTF-CA, is theoretically investigated by first-principles density functional theory calculations. The study is based on three neutron diffraction structures collected at 40, 90 and 300 K (Le Cointe et al., Phys. Rev. B 51, 3374 (1995)). By means of a topological analysis of the total charge densities, we provide a very precise picture of intra and inter-chain interactions. Moreover, our calculations reveal that the thermal lattice contraction reduces the indirect band gap of this organic semi-conductor in the neutral phase, and nearly closes it in the vicinity of the transition temperature. A possible mechanism of the neutral-ionic phase transition is discussed. The charge transfer from TTF to CA is also derived by using three different technics.