Electronic Structure Calculation of Cr3+ and Fe3+ in Phosphor Host Materials Based on Relaxed Structures by Molecular Dynamics Simulation

The electronic structures of the luminescent center ions Cr3+ and Fe3+ in the deep red phosphors LiAl5O8:Cr3+, alpha-Al2O3:Cr3+, and gamma-LiAlO2:Fe3+ were calculated by the DV-Xc method, in which the local distortion induced by the replacement of Al3+ sites in the host crystals by the luminescent center ions was reproduced by classical molecular dynamics (MD) simulation. The MD simulations based on classical dynamics allowed for the handling of more than 1000 atoms for the lattice relaxation calculations, which was advantageous to simulate situations in which a small number of foreign atoms (ions) were dispersed in the host lattice as in phosphors, even when typical periodic boundary conditions were applied. The relaxed lattices obtained after MD indicated that the coordination polyhedra around Cr3+ and Fe3+ expanded in accordance with the size difference between the luminescent center ions and Al3+ in the host crystals. The overall profiles of the partial density of states (p-DOSs) of the isolated Cr3+ and Fe3+ 3d orbitals were not significantly affected by the lattice relaxation, whereas the widths of the energy splitting of the 3d orbitals were reduced. The electronic structure calculations for Fe-Fe pairs in gamma-LiAlO2 showed that the antiferromagnetic interactions with antiparallel electron spins between the Fe3+ ions were preferred, especially when the Fe-Fe pair was on the first-nearest neighboring cation sites.