Chemical Origin of the Stability Difference between Copper(I)- and Silver(I)-Based Halide Double Perovskites

Recently, Cu^I - and Ag^I -based halide double perovskites have been proposed as promising candidates for overcoming the toxicity and instability issues inherent within the emerging Pb-based halide perovskite absorbers. However, up to date, only Ag^I -based halide double perovskites have been experimentally synthesized; there are no reports on successful synthesis of Cu^I -based analogues. Here we show that, owing to the much higher energy level for the Cu 3d¹⁰ orbitals than for the Ag 4d¹⁰ orbitals, Cu^I atoms energetically favor 4-fold coordination, forming [CuX₄ ] tetrahedra (X=halogen), but not 6-fold coordination as required for [CuX₆ ] octahedra. In contrast, Ag^I atoms can have both 6- and 4-fold coordinations. Our density functional theory calculations reveal that the synthesis of Cu^I halide double perovskites may instead lead to non-perovskites containing [CuX₄ ] tetrahedra, as confirmed by our material synthesis efforts.