Crystal Growth and the Structure of a New Quaternary Adamantine Cu□GaGeS4

Abstract

Single crystals of quaternary adamantine-type Cu□GaGeS4 were grown using the chemical vapor transport technique, with iodine as the transport agent. Dark red transparent crystals were grown in a temperature gradient of DT = 900–750 °C. Chemical characterization by X-ray fluorescence showed the off-stoichiometric composition of Cu□GaGeS4 crystals—in particular, a slight Ge deficiency was observed. By X-ray diffraction, Cu□GaGeS4 was found to adopt the chalcopyrite-type structure with the space group . Cation distribution in this structure was analyzed by multiple energy anomalous synchrotron X-ray diffraction, and it was found that Cu and vacancies occupied the 4a site, whereas Ga and Ge occupied the 4b site. The band gap energies of several off-stoichiometric Cu□GaGeS4 crystals were determined by UV-Vis spectroscopy and ranged from 2.1 to 2.4 eV. A non-linear correlation of the band gap energy with the Ge content of the compound was shown to follow the usual bowing behavior of semiconductor alloys, with a bowing parameter of = −1.45 (0.08).