Understanding the evolution of double perovskite band structure upon dimensional reduction.

Recent investigations into the effects of dimensional reduction on halide double perovskites have revealed an intriguing change in band structure when the three-dimensional (3D) perovskite is reduced to a two-dimensional (2D) perovskite with inorganic sheets of monolayer thickness ( = 1). The indirect bandgap of 3D CsAgBiBr becomes direct in the = 1 perovskite whereas the direct bandgap of 3D CsAgTlBr becomes indirect at the = 1 limit. Here, we apply a linear combination of atomic orbitals approach to uncover the orbital basis for this bandgap symmetry transition with dimensional reduction.