Luminescence and Charge Trapping in Cs₂HfCl₆ Single Crystals: Optical and Magnetic Resonance Spectroscopy Study

Single crystals of Cs₂HfCl₆ have recently been identified as promising scintillating material. Furthermore, extended first-principles calculations of Cs₂HfCl₆ electronic and optical properties have been reported, which, among others, indicate the self-trapping of holes and electrons in the form of VK and polaron centers, respectively. Renewed interest in halide scintillators appeared in the late 1990s when rare-earth-based halides were studied and later on the Ce-doped LaCl₃, LaBr₃, and CeBr₃ were discovered as scintillators with both the high light yield and excellent energy resolution. Further optimization of the LaBr₃:Ce by divalent ion codoping provided the material with the best energy resolution within single-crystal scintillators. Moreover, a growing interest in materials from the elpasolite group, such as Cs₂LiYCl₆:Ce, Cs₂LiLaCl₆:Ce, Cs₂NaLaBr₆:Ce, Cs₂NaLaI₆:Ce, and Cs₂LiLaBr₆:Ce, and those with mixed anions has arisen.(10, 11) Among them, the Li-containing compositions can also be considered for thermal neutron detection. Most recently, SrI₂:Eu single crystal was rediscovered, having ultrahigh light yield and excellent energy resolution as well. Similarly, the LuI₃:Ce single crystals have been repeatedly addressed and found to be very competitive in the view of achievable light yield. All of the above-mentioned materials are highly hygroscopic, which makes their practical application difficult and expensive.

Fragment of the Cs₂HfCl₆ lattice with models of Vk centers. The Vk(a) center is created by two Cl ions belonging to the same octahedron. The Vk(b) center is created by one Cl lattice ion and interstitial Cl ion placed along [100] crystal directions.

¹Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00 Prague 6, Czech Republic