Non-stoichiometry in Cs2HfCl6 material and study of its influence on the CsCl–HfCl4 phase diagram using DSC analysis

The influence of non-stoichiometry on the thermal behavior of cesium hafnium chloride (Cs 2 HfCl 6 ) was investigated using differential scanning calorimetry (DSC) under conditions simulating Bridgman crystal growth. CsCl–HfCl 4 mixtures with compositions close to the stoichiometric 2:1 molar ratio and deviations of ± 5 and ± 10 mol% HfCl 4 were sealed in evacuated silica ampoules and analyzed over temperature range of 100–900 °C. The measured endothermic and exothermic effects were correlated with the CsCl–HfCl 4 pseudobinary phase diagram. The stoichiometric composition exhibited a single melting endothermic effect at 820 °C, consistent with the congruent melting of Cs 2 HfCl 6 . In contrast, samples with excess or deficiency of HfCl 4 showed eutectic and liquidus transitions shifted relative to theoretical values, reflecting the volatility of HfCl 4 and pressure-dependent equilibrium in the closed ampoule. The presence of α–β phase transition in samples with excess of HfCl 4 underlined the crucial role of HfCl 4 depletion due to sublimation. These results demonstrate that both the stoichiometry and the ratio between the CsCl melt volume, and the ampoule free volume strongly affect the equilibrium and decomposition behavior of Cs 2 HfCl 6 . The findings provide critical thermodynamic insights for optimizing crystal growth of Cs 2 HfCl 6 scintillators in sealed systems, ensuring reproducibility, phase purity, and process safety.