Magnetic susceptibility of low-dimensional organic conductors with bis(ethylenedithio) tetrathiafulvalene: (BEDT-TTF${)}_3$(${\mathrm{ClO}}_4$${)}_2$ and (BEDT-TTF${)}_2$${\mathrm{ClO}}_4$(${\mathrm{C}}_2$${\mathrm{H}}_3$${\mathrm{Cl}}_3$${)}_{0.5}$

The magnetic susceptibilities, χ, have been measured for 2 two-dimensional metallic bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) perchlorate salts under slow or rapid cooling conditions. (BEDT-TTF${)}_3$(${\mathrm{ClO}}_4$ ${)}_2$ shows a semimetal-insulator transition due to a Peierls instability at 175 K which is independent of the cooling rate. (BEDT-TTF${)}_2$ ${\mathrm{ClO}}_4$(${\mathrm{C}}_2$ ${\mathrm{H}}_3$ ${\mathrm{Cl}}_3$ ${)}_{0.5}$ shows a semimetal-semimetal transition associated with nesting of a part of the Fermi surface at 25 K under slow cooling. Under rapid cooling this transition disappears. The dependence of the transition observed for χ on the cooling rate is confirmed by resistivity measurements. The properties and the origin of anomalies observed for χ with the two salts are discussed in connection with the motion of ethylene groups and ${\mathrm{C}}_2$ ${\mathrm{H}}_3$ ${\mathrm{Cl}}_3$ molecules.