Single crystals of quasi-one-dimensional sulfides ${\mathrm{KCu}}_{7-x}{\mathrm{S}}_4$ 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 $C_P$ 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 $C_P$ 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 ${\mathrm{Cu}}^{+}$-ion diffusion along the Cu(2)-Cu(2) zigzag chains.

• Received 27 August 1997

DOI:https://doi.org/10.1103/PhysRevB.57.3315