ON THE ELECTROCHEMICALLY GROWN QUASI-ONE-DIMENSIONAL KCU7-XS4 SERIES ESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.34) - NONSTOICHIOMETRY,SUPERLATTICE, AND UNUSUAL PHASE-TRANSITIONS

Whiskers of the quasi-one-dimensional copper(I) sulfide series KCu7-xS 4 (0 less than or equal to x less than or equal to 0.34) were grown by employing electrochemical (E-Chem) methods via anodic dissolution of copper electrodes. The compound series can be prepared at 110 degrees C in ethylenediamine solution of polysulfide K2Sn (n = 5, 6) electroly tes and, in some cases, CuCl. Single-crystal structure analysis has re confirmed the once ambiguous space group issue, and the study has also revealed that the KCu7S4 phase reported by Ohtani et al. is nonstoich iometric. The present investigations on as-grown whiskers show that th e previously observed unusual insulator-to-metal transition can be rep roduced by the KCu6.86S4 phase. At room temperature, KCu6.66(4)S-4 cry stallizes in the tetragonal space group I4/m (No. 87) with a = 10.179( 2) Angstrom, c = 3.790(2) Angstrom, V = 392.7(2) Angstrom(3), and Z = 2; KCu6.82(3)S-4 in I4/m (No. 87) with a = 10.177(2) Angstrom, c = 3.8 22(2) Angstrom, V = 395.8(2) Angstrom(3) and Z = 2; KCu7.00(2)S-4 in P 4/n (No. 85) with a = 10.177(2) Angstrom, c = 7.722(2) Angstrom,V= 799 .8(3) Angstrom(3), and Z = 4. Refined cell volumes increase linearly w ith increasing copper content. The extended framework resembles the (N H4)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 t he ordering of the Cu+ cation along the tetrahedral chain. The nonstoi chiometry 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 th e transitions is reiterated. The detailed synthesis and structure, alo ng with preliminary electron diffraction studies and temperature-depen dent magnetic susceptibility measurements, of the title series are pre sented.