Orthorhombic Structures as Inorganic Halide Perovskite Models for High-Throughput DFT Investigations

Although high-symmetry Pm-3m space group
cubic
models are computationally efficient for high-throughput density functional
theory (DFT) calculations of inorganic ternary (ABX3) halide
perovskites (HPs), they frequently predict band gaps (Eg) that disagree with experiment. Conversely, while low-symmetry
cubic polymorphous networks (PN) comprised of 160 to 320 atoms incur significantly greater computational cost, they predict Eg’s that are more closely aligned with
experiment. In this study, we compare the DFT total energies and Eg’s predicted by four high-symmetry structure
models (Pnma orthorhombic, R3m trigonal, P4/mbm tetragonal, and Pm-3m cubic) to
cubic PNs for 5 experimentally characterized ternary HPs and find
that the orthorhombic model computes Eg’s with the smallest MAD of 0.23 eV relative to the PNs. Pair
distribution functions and DFT-computed total energies show that octahedral
tilting, which is present in the 20-atom orthorhombic and 160-atom
cubic PN models but not in the 5-atom cubic models, stabilizes all
5 compositions in our benchmarking set. We also find that imposing
PN constraints when generating and optimizing these orthorhombic structures
by fixing the unit cell lattice vectors and displacing the atoms prior
to ionic relaxation with DFT, which we call the orthorhombic surrogate
model (OSM), lowers the MAD of Eg predictions
to 0.09 eV. Our OSM predicts the PN band gaps of an additional 95
theoretical inorganic ternary HPs with MAD of 0.08 eV, supporting
its usage in high-throughput DFT investigations to closely estimate
PN band gaps with much less computational expense.