Systematic Variations in the Charge-Glass-Forming Ability of   Geometrically Frustrated $\theta$-(BEDT-TTF)$_2X$ Organic Conductors
  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. 
T. Sato, F. Kagawa, K. Kobayashi, A. Ueda, H. Mori, K. Miyagawa, K.   Kanoda, R. Kumai, Y. Murakami, and Y. Tokura
cond-mat.str-el