On the Electrochemically Grown Quasi-One-Dimensional KCu7-xS4 Series (0 ⩽ x ⩽ 0.34): Nonstoichiometry, Superlattice, and Unusual Phase Transitions

Abstract

Whiskers of the quasi-one-dimensional copper(I) sulfide series were grown by employing electrochemical (E-Chem) methods via anodic dissolution of copper electrodes. The compound series can be prepared at 110 C in ethylenediamine solution of polysulfide K{sub 2}S{sub n} (n = 5,6) electrolytes and, in some cases, CuCl. Single-crystal structure analysis has reconfirmed the once ambiguous space group issue, and the study has also revealed that the KCu{sub 7}S{sub 4} phase reported by Ohtani et al. is nonstoichiometric. The present investigations on as-grown whiskers show that the previously observed unusual insulator-to-metal transition can be reproduced by the KCu{sub 6.86}S{sub 4} phase. Refined cell volumes increase linearly with increasing copper content. The extended framework resembles the (NH{sub 4})Cu{sub 7}S{sub 4} structure. It consists of quasi-one-dimensional Cu{sub 4}S{sub 4} columns interlinked by tetrahedral copper chains. New evidence shows that the title series exhibits complicated structure patterns with regard to the ordering of the Cu{sup +} cation along the tetrahedral chain. The nonstoichiometry and cation ordering are likely responsible for the anomalies observed in transport properties. A self-consistent model with respect to local ordering is discussed, and a recently proposed diffusive one-dimensional ordering model at the transitions is reiterated. The detailed synthesis and structure, alongmore » with preliminary electron diffraction studies and temperature-dependent magnetic susceptibility measurements, of the title series are presented.« less