Lattice relaxation study of the 4f−5d excitation of Ce3+ -doped LaCl3 , LaBr3 , and NaLaF4 : Stokes shift by pseudo Jahn-Teller effect
Year: 2007
Type: article
Abstract: The local geometry of ${\mathrm{La}}^{3+}$ in the crystals $\mathrm{La}{\mathrm{Cl}}_{3}$, $\mathrm{La}{\mathrm{Br}}_{3}$, and $\mathrm{La}\mathrm{Na}{\mathrm{F}}_{4}$ is that of a tricapped trigonal prism with ${C}_{3h}$ symmetry. If the crystals are doped with cerium, ${\mathrm{Ca}}^{3+}$ replaces some of the ${\mathrm{La}}^{3+}$ ions. The local geometry changes slightly when ${\mathrm{Ca}}^{3+}$ 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 ${\mathrm{Ca}}^{3+}$ 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 $0.6--1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ large Stokes shift in the $5d\text{\ensuremath{-}}4f$ 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.
Source: Physical Review B
Authors: J. Andriessen, E. van der Kolk, P. Dorenbos
Institution: Delft University of Technology
Language: Latvian
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2138482134