First-Principles Insights into the Stability Difference between ABX3 Halide Perovskites and Their A2BX6 Variants

Since ABX(3) halide perovskites exhibit intrinsic instability, their A(2)BX(6) variants have been examined as potential alternatives for optoelectronic applications. The better stability of A(2)BX(6) variants compared to that of ABX(3) halide perovskites was previously attributed to simply the better valence stability of B4+ compared to that of B2+, as exemplified by Sn4+ and Sn2+ However, the atomic origin of the stability difference between ABX(3) and A(2)BX(6) remains unclear. In this work, taking CsSnCl3 and Cs2SnCl6 as examples, we provide a general first-principles understanding of the stability difference between ABX(3) and A(2)BX(6). We find that Cs2SnCl6 exhibits stronger Sn-Cl bonds, a larger chemical window, and a lower density of defects than CsSnCl3. Owing to its unique crystal structure, Cs2SnCl6 possesses a more stable (111) surface without dangling Sn-Cl bonds; thus, the adsorption and diffusion of water are more difficult. We conclude that the stronger bonds and the more stable surfaces are the primary enablers of the better stability of A(2)BX(6) compared to that of ABX(3).