DMTTF-CA revisited: temperature-induced valence and structural instability

TL;DR

This study investigates the temperature-driven valence and structural changes in the DMTTF-CA organic crystal, revealing continuous ionicity variation, a phase transition at 65 K, and precursor modes for dimerization.

Contribution

It provides detailed spectroscopic evidence of the valence and structural instability in DMTTF-CA, highlighting the role of stack dimerization and identifying Peierls mode precursors.

Findings

DMTTF-CA remains neutral across the temperature range.

A phase transition occurs at 65 K involving dimerization.

Coexistence of ionic species observed near the transition.

Abstract

We report a detailed spectroscopic investigation of temperature-induced valence and structural instability of the mixed-stack organic charge-transfer (CT) crystal 4,4'-dimethyltetrathiafulvalene-chloranil (DMTTF-CA). DMTTF-CA is a derivative of tetrathiafulvalene-chloranil (TTF-CA), the first CT crystal exhibiting the neutral-ionic transition by lowering temperature. We confirm that DMTTF-CA undergoes a continuous variation of the ionicity on going from room temperature down to $\sim$ 20 K, but remains on the neutral side throughout. The stack dimerization and cell doubling, occurring at 65 K, appear to be the driving forces of the transition and of the valence instability. In a small temperature interval just below the phase transition we detect the coexistence of molecular species with slightly different ionicities. The Peierls mode(s) precursors of the stack dimerization are identified.