Crystal Structure of Low Chalcocite

Abstract

CHALCOCITE, Cu₂S, is hexagonal above 104° C, and has a small, hexagonal unit cell containing 2 Cu₂S, in which the sulphur atoms are arranged in nearly perfect hexagonal clOsest packing. Buerger and Wuensch¹ and Sadanaga, Ohmasa, and Morimoto 2 have shown that the copper atoms are distributed throughout the interstices between the sulphur atoms, almost in a fluid state, with only partial concentration at certain sites. Below the transition point a large superstructure is formed based on the hexagonal cell, as was demonstrated in 1944 by Buerger and Buerger³. This phase, commonly known as low chalcocite, was found by them to have an orthorhombic unit cell with a=11.92 Å, b=27.34 Å, c=13.44 Å, containing 96 Cu₂S. These authors showed that the only orthorhombic space group compatible with the hexagonal close-packed sulphur lattice and the observed extinctions was the noncentrosymmetric group Ab2m(C¹⁵_2v). All attempts since then to solve the structure on this basis have been fruitless. I have discovered that the true symmetry of low chalcocite is mono-clinic, and with this knowledge have been able to find a complete solution of the structure problem. Following a brief preliminary report of this work⁴, refinement of the structure has proceeded slowly, with interruptions, but is now complete. In this article the essential structural results are reported, and a full description of the structure problem and its crystal chemical implications will be given elsewhere.