Rare-Earth-Doped CaAlSiN₃: 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 CaAlSiN₃. 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 CaAlSiN₃ as a multifunctional candidate for optoelectronics and piezoelectric energy harvesting.