Donor–anion interactions at the charge localization and charge ordering transitions of (TMTTF)₂AsF₆ probed by NEXAFS

Abstract

High-resolution near-edge X-ray absorption fine structure (NEXAFS) measurements at the As M-edge, F K-edge and S L-edge of the Fabre salt (TMTTF)2AsF6 were performed from room temperature (RT) to 90 K, allowing to reach the charge localization regime below Tρ ≈ 230 K and to cross the charge ordering (CO) transition at TCO ≈ 102 K. The F K-edge and S L-edge spectra exhibit several transitions which have been indexed on the basis of first-principles DFT calculations. Upon cooling from RT significant energy shifts up to +0.8 eV and -0.4 eV were observed in transitions exhibited by the F 1s and S 2p spectra respectively, while the As 3p doublet does not show a significant shift. Opposite energy shifts found in the F 1s and S 2p spectra reflect substantial thermal changes in the electronic environment of F atoms of the anion and S atoms of TMTTF. The changes found around the charge localization crossover suggest an increase of the participation of the S d orbitals in the empty states of TMTTF as well as an increase of the strength of donoranion interactions. A new F 1s pre-edge signal detected upon entry into the CO phase is a clear fingerprint of the symmetry breaking occurring at TCO. We propose that this new transition is caused by a substantial mixing between the HOMO of the AsF6(-) anion and the unoccupied part of the TMTTF HOMO conduction band. Analysis of the whole spectra also suggests that the loss of the inversion symmetry associated with the CO is due to an anion displacement increasing the strength of SF interactions. Our data show unambiguously that anions are not, as previously assumed, innocent spectators during the electronic modifications experienced by the Fabre salts upon cooling. In particular the interpretation of the spectra pointing out a thermally dependent mixing of anion wave functions with those of the TMTTF chains demonstrates for the first time the importance of anion-donor interactions.

The metal-insulator transition in the organic conductor β″-(BEDT-TTF)2Hg(SCN)2Cl

“…This implies an enhancement of the effective electron-electron correlation in β ′′ -(D 8 -BEDT-TTF) 2 Hg(SCN) 2 Cl as the transfer integrals are smaller. In case of one-dimensional organic conductor it was noted previously that the donor-anion interaction is important for establishing charge order [30].…”

Near-Edge X-ray Absorption Fine Structure Investigation of the Quasi-One-Dimensional Organic Conductor (TMTSF)2PF6

“…Here we report NEXAFS measurements and a density functional theory (DFT) investigation of (TMTSF) 2 PF 6 and we will compare the results with those of our previous study of (TMTTF) 2 AsF 6 . 12 The paper is organized as follows: experimental and computational methods are described in section II; NEXAFS spectra are presented in section III; the energy levels of PF 6 , TMTSF and (TMTSF) 2 PF 6 are discussed in section IV; then in section V the NEXAFS spectra will be discussed on the basis of the electronic structure 6 calculations and finally, in section VI, we summarize the results commenting the differences with the case of the Fabre salt (TMTTF) 2 AsF 6 .…”

Electron‐ and X‐Ray Spectroscopies of Organic Charge‐Transfer Complexes

“…As a model system showing several phase transitions we studied the Fabre salt (TMTTF) 2 X . These experiments focused on changes in the electronic structure when crossing the metal‐insulator to charge‐order (CO) transition at 154 K. In order to have access to all elements in the large unit cell of (TMTTF) 2 SbF 6 (in particular Sb and F), HAXPES was the method of choice because of its large information depth.…”

“…The F K‐edge NEXAFS spectrum presents clear features at three photon energies of 688.2, 693.5, and 697 eV, marked with 1, 2, and 3 in Figure (a). For (TMTSF) 2 PF 6 signals 1 and 2 are being shifted to higher photon energies in comparison to signals of (TMTTF) 2 AsF 6 . NEXAFS spectra exhibit a small variation of the total intensity upon increasing of the angle θ , because of the varying photon penetration depth with θ .…”