  The discovery of double perovskites A2B(I)B(III)X6 (A=monovalent cation;
B(I)/B(III) = metal cation; X = halogen) as Pb(II)-free alternatives has
attracted widespread attention, making it possible to introduce d-block metal
cations (e.g., d0 and d10) into halide perovskites. However, at present, there
are quite limited insights into the underlying bonding orbitals for d-block
metal cation-based halide perovskites. Here, we present an orbital engineering
strategy to construct two-dimensional (2D) electronic structures in
three-dimensional (3D) halide perovskites by rationally controlling the d
orbitals of the metal cations to hybridize with the halide p orbitals. Taking
Cs2Au(I)Au(III)I6 as an example, we demonstrate that the flat conduction band
and valence band at the band edges can be achieved simultaneously by combining
two metal cations with different d orbital configurations using the
first-principles calculations. The predicted carrier mobilities show huge
anisotropy along the in-plane and out-of-plane directions in Cs2Au(I)Au(III)I6,
further confirming the 2D electronic properties. In addition, the anisotropic
static dielectronic constants and Young's modulus are also observed. More
importantly, it is found that Cs2Au(I)Au(III)I6 has excellent mechanical
flexible and an ultra-small shear modulus among halide perovskites. Our work
provides valuable guidance for achieving low-dimensional electronic
characteristic in three-dimensional halide perovskites for novel electronic
applications.
