Rare‐Earth‐Doped CaAlSiN ₃ : A GGA+U+SOC Study of Electronic, Optical, Magnetic, and Mechanical Properties

Wide‐bandgap nitrides are promising for advanced‐energy applications due to their structural stability and tunable optical features. This study employs density functional theory (DFT) with GGA+U and spin–orbit coupling (SOC) in WIEN2k to investigate the electronic, magnetic, piezoelectric, and optical properties of pristine and Er‐doped CaAlSiN 3 . Substitution of Er at 8.5% and 17% introduces localized 4f states near the Fermi level, producing spin‐polarized bands, narrowed bandgaps, and asymmetric density of states. Simulated x‐ray absorption spectra display clear pre‐edge structures and redshifted edges with higher Er content, confirming f‐electron transitions. Dielectric analysis indicates enhanced polarizability, while optical absorption shows a redshift toward lower photon energies and enhanced near‐ultraviolet/ultraviolet activity due to Er‐4f‐related impurity states and f–d/N‐2p transitions. Elastic constants and electron localization function reveal robust mechanical stability with moderate softening and stronger charge confinement. Er incorporation further amplifies piezoelectric performance through lattice distortion and spin‐related polarization, positioning Er‐doped CaAlSiN 3 as a multifunctional candidate for optoelectronics and piezoelectric energy harvesting.