Crystal Growth of Tl4CdI6: A Wide Band Gap Semiconductor for Hard Radiation Detection

We report the synthesis, physical characterization, and crystal growth of Tl(4)Cdl(6). We show that this material has good photoconductivity and is a promising semiconductor for room temperature X-ray and gamma-ray detection. Large single crystals were grown by the vertical Bridgman method and cut to dimensions appropriate for detector testing. Single crystal X-ray diffraction refinements confirm that Tl(4)Cdl(6) crystallizes in the tetragonal crystal system with a centrosymmetric space group of P4/mnc, with a calculated density of 6.87 g/cm(3). Thermal analysis and high-temperature synchrotron powder diffraction studies were used to determine phase relationships and crystallization behavior during crystal growth. We have elucidated the reason for different colors encountered when synthesizing or growing single crystals of Tl(4)Cdl(6) (yellow, red, and black), and it is the presence of a small amount of TlI impurity. We report proper crystal growth conditions to obtain essentially pure yellow Tl(4)Cdl(6) crystals. The material having the yellow color has a band gap of 2.8 eV. First-principles density functional theory calculations indicate a direct band gap at the Gamma point of the Brillouin zone. The Tl(4)Cdl(6) crystals have a resistivity of 1019 Omega.cm. Photoconductivity measurements on the as-grown crystals show mobility-lifetime product on the order of 10(-4) cm(2)/V for both electrons and holes. The promising detector properties of this material are confirmed by preliminary measurements showing a clear spectral response to an Ag X-ray source, which classifies Tl(4)Cdl(6) as an emerging material for radiation detection.