Phase properties of a zigzag chain lattice gas with Coulomb interactions.

It has been reported that the phase transitions found in the quasi-one-dimensional sulfide KCu(7-x)S4 are most likely due to vacancy ordering involving Cu+-ion diffusion along the Cu(2)-Cu(2) zigzag chains. Our previous studies with both a self-consistent method and Monte Carlo simulations confirmed that phase transitions indeed exist in a one-dimensional (1D) lattice gas system in which vacancy ordering is involved. In this paper, we calculate the more nearly real case of KCu6.88S4 and further investigate the angular dependence of the phase properties in a partially occupied 1D zigzag chain with various particle occupancies. The calculated results suggest that the phase transitions that occur in the quasi-one-dimensional material KCu(7-x)S4 are presumably due to both intrachain and interchain interactions between the partially occupied Cu+ zigzag chains. Most interestingly, we found that the average particle distribution of the lowest free energy state is a linear superposition of two other solutions with different particle distributions for occupancy n(av)=1/2.