Unusual physical properties of KCu{sub 7{minus}x}S{sub 4} at diffusive one-dimensional ordering transitions

Single crystals of quasi-one-dimensional sulfides KCu{sub 7{minus}x}S{sub 4} with x=0.00,0.12, and 0.34 were prepared by electrochemical methods. Temperature-dependent properties of resistance, thermal conductivity, thermoelectric power, and heat capacity C{sub P} were studied on single-crystal samples. These measurements show several anomalies which we associate with phase transitions, with the transition temperatures and physical properties depending strongly on x. Most interestingly, measurements of C{sub P} with an ac technique show very unusual hysteresis and frequency dependence. The present work indicates that these phase transitions are due to vacancy ordering involving Cu{sup +}-ion diffusion along the Cu(2)-Cu(2) zigzag chains. {copyright} {ital 1998} {ital The American Physical Society}