Spinodal decomposition of the charge ordering in the $\theta $-based organic salt

We have measured with X-ray diffraction, resistivity and thermopower the spinodal decomposition of the long range charge ordering in the organic compounds $\theta $-(BEDT-TTF)$_{2}$RbZn(SCN)$_{4}$ and $\theta $-(BEDT-TTF)$_{2}$RbCo(SCN)$_{4}$ . Details of this spinodal decomposition reveals that the charge ordering growth in these compounds follows at first a two dimensional diffusion controlled mode followed abruptly by a change to a three dimensional diffusion controlled growth. Thermopower reveals that the long ranged charge ordered state in these compounds is characteristic of a system with strong on-site repulsion with narrow bandwidths. In contrasts to this, the thermopower of the short ranged charge ordering is anomalous not only in its value but also in its temperature dependence.