Optical properties of an unusual form of thin chalcosite (Cu2S) crystals

In thin single crystals of orthorhombic chalcosite (Cu2S) prepared by chemical conversion from CdS crystals, the direction of the crystallographic c-axis is usually retained during the conversion (in the plane of the crystals). However, in rare cases it is the b-axis that takes the direction of the original c-axis. The directions of the crystal axes both in the usual and in the unusual form of chalcosite have been deduced from the preferential directions of growth of djurleite (Cu1.96S) in the matrix of chalcosite. The absorption index and the refractive index have been measured for wavelengths between 0.5 and 1.1 μm for a crystal of the unusual form. The combined data measured on the two forms represent the optical properties of chalcosite for polarizations along the three crystal axes and for a polarization in the ab plane. The lowest value of the band gap (1.09 eV) is measured when the light is polarized along the b-axis. A few consequences of the data for the role of chalcosite in CdSCu2S solar cells are stated.