An X-ray scattering study of charge-density waves in K sub 3 Cu sub 8 S sub 6.

K sub 3 Cu sub 8 S sub 6 is a mixed valent, metastable phase that can be trapped as KCU sub 4 S sub 3 is heated to form KCU sub 3 S sub 2. K sub 3 Cu sub 8 S sub 6 has a monoclinic structure with [Cu sub 4 S sub 4] chains parallel to the monoclinic axis. The chains are linked by an intervening tetrahedral network to form planes. Two phase transitions had previously been identified in K sub 3 Cu sub 8 S sub 6, a second-order transition at 153K and a first-order transition at 55K. In this x-ray scattering study we show that the second-order transition, which is associated with a decrease in the conductivity, corresponds to the onset of an incommensurate charge-density wave (CDW) with a wave vector of q = (0 (1-delta)/2 0). The first-order transition, which is accompanied by an increase in the conductivity, results in a significant modification of the host structure and the creation of a superlattice with a commensurate periodicity given by q = (1/2 1/2 0). The temperature dependence of the CDW order parameter is highly unusual in this compounds.