First-principles study on structural stability and reaction with H2O and O2 of vacancy-ordered double perovskite halides: Cs2(Ti, Zr, Hf)X6

Vacancy-ordered double perovskite halides (A(2)BX(6)) have been recently considered as the alternative lightabsorber in solar cell. However, structural degradations were observed after air exposure. In this work, we started with the structural optimization of Cs-2(Ti, Zr, Hf)X-6, where X = I, Br, and Cl, by using density functional theory (DFT) with the Perdew-Burke-Ernzerhof (PBE) approximation and found that the lattice mismatches of these materials with the same halides are significantly small (similar to 1%), revealing the potential of mixed alloys among B-site atoms. Next, the formation enthalpy and chemical potential diagrams of these materials were calculated indicating the thermodynamic stability and the allowed ranges of the constituent elements' chemical potentials. In addition, all decomposition energy of double perovskite were extracted and show that Cs-2(Ti, Zr, Hf)X-6 are stable against competing binary phases unlike the pristine perovskite Cs(Ti, Zr, Hf)X-3, which favor to decompose into binary compounds. Finally, the reaction of Cs-2(Ti, Zr, Hf)X-6 with H2O and O-2 were considered for evaluation of their stability in ambient condition. We found that Cs-2(Ti, Zr, Hf)X-6 are lasting after reaction with H2O but for O-2 only Cs2ZrCl6 and Cs2HfCl6 are durable. As a consequence, Cs2ZrCl6 and Cs2HfCl6 would fulfill candidates of all-inorganic perovskite for photovoltaic and optoelectronic applications.