Cu2I2Se6 is a new wide-bandgap semiconductor with high stability and great potential toward hard radiation and photon. detection. Cu2I2Se6 crystallizes in the rhombohedral R (3) over barm space group with a density of d = 5.287 g.cm(-3) and a wide bandgap E-g of 1.95 eV. First-principles electronic band structure calculations at the density functional theory level indicate an indirect bandgap and a low electron effective mass m*(e) of 0.32. The congruently melting compound was grown in centimeter size Cu2I2Se6 single crystals using a vertical Bridgman method. A high electric resistivity of similar to 10(12) Omega.cm is readily achieved, and detectors made of Cu2I2Se6 single crystals demonstrate high photosensitivity to Ag K alpha X-rays (22.4 keV) and show spectroscopic performance with energy resolutions under Am-241 alpha-particles (5.5 MeV) radiation. The electron mobility is measured by a time-of-flight technique to be similar to 46 cm(2).V-1.s(-1). This value is comparable to that of one of the leading gamma-ray detector materials, TIBr, and is a factor of 30 higher than mobility values obtained for amorphous Se for X-ray detection.