Role of hydrogen bonding in charge-ordered organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ probed by $^{127}$I nuclear quadrupole resonance

TL;DR

This study uses $^{127}$I NQR to investigate how hydrogen bonding influences charge ordering in the organic conductor $ ext{α}$(BEDT-TTF)$_2$I$_3$, revealing the importance of anion-donor interactions and local anion librations.

Contribution

It demonstrates the role of hydrogen bonding and anion-donor interactions in charge ordering, supported by NQR spectra and relaxation data.

Findings

Only one I$_3$ site shows electric field gradient differentiation.

Charge modulation in BEDT-TTF layers alone is insufficient.

Local libration of I$_3$ anions dominates nuclear relaxation.

Abstract

We present $^{127}$I nuclear quadrupole resonance spectra and nuclear relaxation of $\alpha$-(BEDT-TTF)$_2$I$_3$ that undergoes a charge-ordering transition. Only one of the two I$_3$ anion sites shows a significant differentiation in the electric field gradients across the first-order transition. The charge modulation only in the BEDT-TTF layers can not reproduce; instead, an anion-donor interaction accompanied by hydrogen bonding is necessary. The dominating source for the nuclear relaxation is the local libration of the I$_3$ anions, but an anomalous peak is detected just below the transition, as observed by $^{13}$C NMR.