Comparative first-principles and machine-learned force field investigation of MAPbBrxI3-x and SbH4PbBrxI3-x perovskites: Electronic, structural, and dynamic properties

The structural, electronic, optical, and thermal properties of mixed-halide MAPbBrxI3-x and SbH4PbBrxI3-x (x = 0-3) perovskites are investigated using first-principles density functional theory and machine-learned force fields (MLFF). A crucial element of this work is the rigorous validation of the MLFF approach against the well-established MAPbBrxI3-x benchmark system, which confirmed the method's high fidelity in reproducing the experimentally established relative thermal-stability trends of the MA-based benchmark. Formation-energy calculations revealed that bromine incorporation significantly enhances thermodynamic favorability relative to the binary precursor phases in both series, with SbH4PbBr3 exhibiting the most favorable formation energy (-0.078 eV). MLFF molecular dynamics simulations demonstrated that Br-rich SbH4PbBrxI3-x alloys (x >= 2) maintain crystalline integrity within the 100 ps simulation window up to 500 K, while iodine-rich phases undergo structural collapse at elevated temperatures due to insufficient entropic stabilization. Electronic-structure analyses at the PBE+vdW and HSE+SOC+vdW levels showed that SbH+4 substitution systematically narrows the band gap by 0.13-0.35 eV relative to MA-based counterparts, yielding HSE+SOC+vdW values of 1.29-1.88 eV across the composition series. At the spin-orbit-coupled level, the computed band-edge splittings contain an artificial ferroelectric component arising from the ordered molecular orientations of the finite supercells; they are therefore not interpreted as intrinsic bulk Rashba parameters and are noted only as a qualitative, methodology-controlled comparison between the two A-site chemistries. Optical spectra computed within the PBE+SOC+Delta framework demonstrated strong and broad visible-light absorption with a reduced blue-shift compared to MA-based analogs, confirming that SbH+4 substitution preserves spectral coverage despite bromide incorporation. Among all investigated compositions, SbH4PbBr2I emerges as a computationally promising candidate warranting experimental investigation, combining an HSE+SOC+vdW band gap of 1.71 eV in the useful range for wide-gap photovoltaic absorbers with crystalline integrity within the simulation timescale up to 500 K and strong visible-light absorption. These results computationally demonstrate that SbH4PbBrxI3-x represents a candidate hydride-based perovskite family that, if it proves synthetically accessible, could help address the stability-efficiency trade-off inherent to iodide-rich compositions.