Thermodynamics of Formation and Stability of Cs₄PbX₆ (X = Cl, Br, I) Halides

Thermodynamics of the formation of Cs₄PbX₆ (X = Cl, Br, I) halides, promising for various optoelectronic applications, was studied at 298.15 K via solution calorimetry and analysis of solution equilibria. The compounds in question were synthesized by a solid-state reaction technique from binary halides and characterized by powder X-ray diffraction. They were shown to be intrinsically stable with respect to decomposition to a mixture of either binary halides or CsPbX₃ and CsX (X = Cl, Br, I) under standard conditions at 298.15 K. The interaction with gaseous components of the ambient atmosphere, such as O₂, CO₂ and H₂O, was shown to be thermodynamically unfavorable, whereas the presence of liquid water is detrimental since the latter will degrade all the Cs₄PbX₆ (X = Cl, Br, I) halides leaching the CsX salts. The change in thermodynamic stability depending on the composition of the halides Cs₄PbX₆ is similar to that observed for the perovskites CsPbX₃, i.e., the chlorides are the most stable, and the iodides are the least stable to interaction with external gaseous agents. At the same time, complex halides Cs₄PbX₆ (X = Cl, Br, and I) are significantly more stable than the corresponding perovskite counterparts. Also, based on the results obtained, the thermodynamic guidelines for growing high-quality single crystals of various cesium-lead halides were discussed.