Searching for better X-ray and γ-ray photodetectors: structure-composition properties of the TlPb 2 Br 5- x I x quaternary system.

Developing X-ray and γ-ray detectors with stable operation at ambient temperature and high energy resolution is an open challenge. Here, we present an approach to search for new detector materials, combining binary photodetector compounds. More specifically, we explore quaternary TlPb 2 Br 5- x I x compositions, relying on materials synergy between TlBr, TlI, and PbI 2 photodetectors. We discover a broad solid solution in the TlPb 2 Br 5 -'TlPb 2 I 5 ' section, which can be derived from a new quaternary compound, TlPb 2 BrI 4 , by partial substitution of Br by I atoms on the 4c site or by replacement of I by Br atoms on the 16l site. We carry out a thorough crystallographic analysis of the new TlPb 2 BrI 4 compound and prepare a high-quality standardized structure file. We also complete the phase diagram of the TlPb 2 Br 5 -'TlPb 2 I 5 ' section, based on 21 alloys. Furthermore, we synthesize a series of high quality centimeter-sized TlPb 2 Br 5- x I x single crystals ( x = 2, 2.5, 3, 3.5, 4, 4.5) by the Bridgman-Stockbarger method and study their structure and properties using a combination of experimental techniques (X-ray diffraction, X-ray photoelectron spectroscopy, and absorption spectroscopy) and theoretical calculations.