Systematic Variations in the Charge-Glass-Forming Ability of Geometrically Frustrated $\theta$-(BEDT-TTF)$_2X$ Organic Conductors

TL;DR

This study investigates how geometric frustration influences the charge-glass-forming ability in $ heta$-(BEDT-TTF)$_2X$ organic conductors, revealing that increased frustration enhances glass formation by slowing charge ordering kinetics.

Contribution

It provides systematic analysis linking charge frustration levels to charge-glass-forming ability in $ heta$-(BEDT-TTF)$_2X$ compounds, highlighting the role of frustration in kinetic slowing.

Findings

Higher charge frustration correlates with better charge-glass-forming ability.

Faster cooling rates are required to avoid charge order in less frustrated compounds.

Charge frustration slows down the kinetics of charge ordering.

Abstract

The critical cooling rate $R_{\rm c}$ above which charge ordering is kinetically avoided upon cooling, which results in charge-glass formation, was investigated for the geometrically frustrated system $\theta$-(BEDT-TTF)$_2X$. X-ray diffraction experiments revealed that $\theta$-(BEDT-TTF)$_2$TlCo(SCN)$_4$ exhibits a horizontally charge-ordered state, and kinetic avoidance of this state requires rapid cooling of faster than 150 K/min. This value is markedly higher than that reported for two other isostructural $\theta$-type compounds, thus demonstrating the lower charge-glass-forming ability of $X$ $=$ TlCo(SCN)$_4$. In accounting for the systematic variations of $R_{\rm c}$ among the three $\theta$-(BEDT-TTF)$_2X$, we found that stronger charge frustration leads to superior charge-glass former. Our results suggest that charge frustration tends to slow the kinetics of charge ordering.