Charge-density waves in the mixed-valence two-dimensional metal ${\mathrm{K}}_3$${\mathrm{Cu}}_8$${\mathrm{S}}_6$

In order to study the physical properties of ${\mathrm{K}}_3$ ${\mathrm{Cu}}_8$ ${\mathrm{S}}_6$, we have carefully defined the synthetic conditions for ${\mathrm{KCu}}_4$ ${\mathrm{S}}_3$, ${\mathrm{K}}_3$ ${\mathrm{Cu}}_8$ ${\mathrm{S}}_6$, and ${\mathrm{KCu}}_3$ ${\mathrm{S}}_2$. ${\mathrm{K}}_3$ ${\mathrm{Cu}}_8$ ${\mathrm{S}}_6$ is apparently a kinetic phase and must be trapped with a minimum amount of either ${\mathrm{KCu}}_4$ ${\mathrm{S}}_3$ or ${\mathrm{KCu}}_3$ ${\mathrm{S}}_2$. Magnetic susceptibility and electrical resistivity data display behavior typical of charge-density-wave systems. In view of the low-dimensional metallic nature of ${\mathrm{K}}_3$ ${\mathrm{Cu}}_8$ ${\mathrm{S}}_6$ and preliminary x-ray data, a second-order reversible transition at $T_1$=152±1 K has been identified as the onset of an incommensurate lattice. In the region of 50–60 K, a first-order transition with considerable hysteresis seems to be a structural transition to a commensurate superlattice state.