First-Principles Prediction of Anisotropic Infrared Optical Properties in Orthorhombic LaMnO₃: The Crucial Role of Four-Phonon Scattering

LaMnO₃-based perovskites are crucial for advanced thermal management due to their tunable infrared optical properties, which are fundamentally governed by infrared-active phonons. However, the anisotropic infrared optical properties of their parent material, LaMnO₃, remain insufficiently understood. Here, we systematically investigate the anisotropic infrared optical properties of orthorhombic LaMnO₃ using a parametrized Lorentz oscillator model based on first-principles calculations. We employ PBEsol + U (U = 4.5 eV) to compute the lattice dynamic properties, yielding all infrared-active transverse optical (TO) phonon modes and the corresponding dielectric function. Crucially, phonon damping is accurately determined from anharmonic lattice dynamics by incorporating both three- and four-phonon scattering processes, which produces damping factors consistent with available experimental data. Using these parameters, the anisotropic infrared dielectric function and emissivity along the crystallographic directions [010], [001], and [100] are predicted. This work provides a comprehensive theoretical framework and a reliable basis for understanding the infrared phonon behavior and anisotropic optical response of orthorhombic LaMnO₃ and provides physical guidance for predicting orientation-dependent mid-infrared emissivity in LaMnO₃-based infrared functional materials.