The crystal structure and temperature dependence of the elpasolite Tl 2 LiYCl 6

Tl 2 LiYCl 6 (TLYC) is an analog to Cs 2 LiYCl 6 , which is currently an industry-standard inorganic scintillator for radiation detection with good gamma–neutron discrimination. The presence of thallium ( Z = 81) instead of cesium ( Z = 55) in the elpasolite structure increases the density of the compound and its stopping power for gamma rays. This work investigates the impact of the Tl atom on the elpasolite structure. Single-crystal X-ray diffraction at room temperature and powder neutron diffraction with temperature control were used to characterize the crystal structure of TLYC between 296 and 725 K. The presence of Tl leads to a distortion of the cubic elpasolite structure at room temperature: a tetragonal P 4 2 crystal structure (space group 77, a = 10.223, c = 10.338 Å) is identified for TLYC at 296 K. A structural transition to the cubic elpasolite Fm 3 m phase (space group 225) is observed at 464 K. The thermal expansion of the material for each crystal direction is well described by a linear relationship, except for the region between 400 and 464 K where the lattice parameters converge.