Lattice relaxation study of the 4f-5d excitation of Ce3+-doped LaCl3, LaBr3, and NaLaF4:: Stokes shift by pseudo Jahn-Teller effect

The local geometry of La3+ in the crystals LaCl3, LaBr3, and LaNaF4 is that of a tricapped trigonal prism with C-3h symmetry. If the crystals are doped with cerium, Ca3+ replaces some of the La3+ ions. The local geometry changes slightly when Ca3+ is in the ground state (4f). However, in the 5d state, the relaxation of the lattice is quite different because the 5d levels experience a pseudo Jahn-Teller coupling with certain lattice displacements. This is because the lowest two 5d states (a singlet and a doublet, neglecting spin-orbit interaction) are very close in energy. We found that the Ca3+ ion moves away from the centered position, accompanied by a strong deformation of the prism. One of the anions of the triangle of caps moves outward, also because of the pseudo Jahn-Teller distortion, and four anions of the prism move inward. The result is the occurrence of a 0.6-1.0 eV large Stokes shift in the 5d-4f emission. We analyzed the off-center movement of cerium in detail. For this study, we employed a widely used band structure code based on density functional theory. Ionic cluster calculations using the Hartree-Fock method confirm the results obtained and give additional information about the dynamics of the relaxation process.