Single crystal of the mixed-halogen compound Cs2HfCl3Br3 was grown by the vertical Bridgman method, and we investigated its crystal structure and optical properties. Single-crystal X-ray diffraction and energy dispersive X-ray spectroscopy analyses indicated the chemical composition to be Cs2Hf(Cl0.578(6)Br0.422(6))6 (= Cs2HfCl3.47(4)Br2.53(4), approximately Cs2Hf(Cl0 58Br0.42)6) where the number in the parentheses denotes the estimated standard deviation in the last digit. Cs2Hf(Cl0.58Br0.42)6 was found to have a cubic Fm3‾m potassium hexachloroplatinate structure consisting of Cs+ cations and Hf(Cl0 . 58Br0.42)6 2− anions. The emission peak of Cs2HfCl3Br3 was red-shifted by Br substitution, and the photoluminescence decays of Cs2HfCl3Br3 were accelerated when compared to those of Cs2HfCl6. Cs2HfCl3Br3 had an emission peak at 450 nm under X-ray irradiation, while under UV-excitation, two photoluminescence emission peaks at 420 and 525 nm were observed. The decay profile of the 420-nm emission band had a single-exponential component with a decay time of 1.95 ± 0.01 μs under the 280-nm-excitation. On the other hand, 525-nm-emisison had two decay components of 0.28 ± 0.03 μs (11%) and 6.54 ± 0.03 μs (89%) under 305-nm-photons excitation.
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