On the Electrochemically Grown Quasi-One-Dimensional KCu7- x S4 Series (0 ⩽ x ⩽ 0.34):  Nonstoichiometry, Superlattice, and Unusual Phase Transitions

• Published in: Chemistry of materials 1998-10, Vol.10 (10), p.3172-3183
• Main Authors: Li, He, Mackay, Richard, Hwu, Shiou-Jyh, Kuo, Yung-Kang, Skove, Malcolm J, Yokota, Yasuhiro, Ohtani, Tsukio
• Format: Article
• Language: English
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm980233d

Whiskers of the quasi-one-dimensional copper(I) sulfide series KCu7 - x S4 (0 ≤ x ≤ 0.34) 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 K2S 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 KCu7S4 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 KCu6.86S4 phase. At room temperature, KCu6.66(4)S4 crystallizes in the tetragonal space group I4/m (No. 87) with a = 10.179(2) Å, c = 3.790(2) Å, V = 392.7(2) Å3, and Z = 2; KCu6.82(3)S4 in I4/m (No. 87) with a = 10.177(2) Å, c = 3.822(2) Å, V = 395.8(2) Å3, and Z = 2; KCu7.00(2)S4 in P4/n (No. 85) with a = 10.177(2) Å, c = 7.722(2) Å, V = 799.8(3) Å3, and Z = 4. Refined cell volumes increase linearly with increasing copper content. The extended framework resembles the (NH4)Cu7S4 structure. It consists of quasi-one-dimensional Cu4S4 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+ cation along the tetrahedral chain. The nonstoichiometry and cation ordering are likely responsible for the anomalies observed in transport properties (Phys. Rev. B. 1998, 57, 3315−3325). 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, along with preliminary electron diffraction studies and temperature-dependent magnetic susceptibility measurements, of the title series are presented.