Crystal growth of Tl₄CdI₆: A wide band gap semiconductor for hard radiation detection

We report the synthesis, physical characterization, and crystal growth of Tl₄CdI₆. We show that this material has good photoconductivity and is a promising semiconductor for room temperature X-ray and γ-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 Tl₄CdI₆ crystallizes in the tetragonal crystal system with a centrosymmetric space group of P4/mnc, with a calculated density of 6.87 g/cm³. 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 Tl₄CdI₆ (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 Tl₄CdI₆ 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 point of the Brillouin zone. The Tl₄CdI₆ crystals have a resistivity of 10¹⁰ ω·cm. Photoconductivity measurements on the as-grown crystals show mobility-lifetime product on the order of 10^-4 cm²/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 Tl₄CdI₆ as an emerging material for radiation detection.