Jahn-Teller ions in cubic crystals explored through DFT calculations

The behaviour of Jahn-Teller ions in cubic insulating lattices has been explored by means of cluster calculations within the density functional theory (DFT) framework. In systems with octahedral coordination parameters like the Jahn-Teller energy, EJT, or the angular frequency, ωE, have been derived from the Qθ (≈3z2-r2) dependence of total energy. Results for NaCl:Rh2+ have been obtained for (3z2-r2)1, (x2-y2)1, and also (3z2-r2)0.5(x2-y2 )0.5 electronic configurations. In agreement with experimental data for NaCl:Rh2+ the present calculations lead to a strong Jahn-Teller effect (EJT/ℏωE≈6) and to an elongated octahedron as equilibrium geometry characterised by Qθ0=0.30 A ̊. For a compressed geometry the corresponding energy minimum is found to be about 0.03 eV above and characterised by Qθ0=-0.22 A ̊. As salient feature these facts are shown to come essentially from the anharmonicity involved in the Eg vibrational mode and not from the quadratic coupling between this mode and the ground electronic state. The value EJT/ℏωE=0.65 obtained for MgO:Cu2+ is consistent with the observation of electron paramagnetic resonance (EPR) spectra displaying a cubic angular pattern. The EJT/ℏωE ratio is shown to increase by a factor of three on passing from MgO:Cu2+ to SrO:Cu2+. The origin of this important variation, consistent with the observation of static EPR spectra for SrO:Cu2+, are briefly discussed. Finally the present results on SrCl2:Cu2+ support the existence of a big off-centre motion of Cu2+ along 〈0 0 1〉 directions. This conclusion stresses the importance of theoretical calculations for cases where model Hamiltonians based in truncated series expansion can hardly be meaningful. © 2003 Elsevier B.V. All rights reserved.