Systematic Variations in the Charge-Glass-Forming Ability of Geometrically Frustrated $\hat{I}¸$-(BEDT-TTF)${}_{2}X$ Organic Conductors

Abstract

The critical cooling rate $R_{\mathrm{c}}$ above which charge ordering is kinetically avoided upon cooling, which results in charge-glass formation, was investigated for the geometrically frustrated system $\hat{I}¸$-(BEDT-TTF)${}_{2}X$. X-ray diffraction experiments revealed that $\hat{I}¸$-(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 $\hat{I}¸$-type compounds, thus demonstrating the lower charge-glass-forming ability of $X$ $=$ TlCo(SCN)${}_4$. In accounting for the systematic variations of $R_{\mathrm{c}}$ among the three $\hat{I}¸$-(BEDT-TTF)${}_{2}X$, 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.

4 pages, 3 figures. Selected for Editors' Choice