Aramburu, J. A.4; Garcia-Fernandez, P.4; García Lastra, Juan Maria3; Barriuso, M. T.4; Moreno, M.4
1 Department of Physics, Technical University of Denmark2 Universidad de Cantabria3 Department of Energy Conversion and Storage, Technical University of Denmark4 Universidad de Cantabria
Seeking to understand why the cubic centre in MgO:Cr3+ has the same 10Dq value as emerald, ab initio cluster and periodic supercell calculations have been performed. It is found that the equilibrium Cr3+-O2- distance, R, in MgO:Cr3+ is equal to 2.03 Å and thus 0.06 Å higher than that measured for the emerald. Calculations carried out on the isolated CrO69- complex at R = 2.03 Å give 10Dq = 14 510 cm-1, which is 10% smaller than the experimental figure for MgO:Cr3+. Nevertheless, when the internal electric field, ER(r), due to the rest of the lattice ions is also taken into account, the calculated 10Dq = 16 210 cm-1 coincides with the experimental value. Accordingly, the colour shift for different oxides doped with Cr3+ can be well understood on the basis of this extrinsic contribution to 10Dq usually ignored in a ligand field description. The calculated electrostatic potential, VR(r), related to ER(r), is found to be attractive when the electronic density is lying along 〈110〉 directions and |r| > 1 Å driven by the first shell of twelve Mg2+ ions. The action of VR(r) upon the CrO69- complex slightly decreases the energy of t2g(xy, xz, yz) orbitals with respect to that for eg(3z2-r2, x2-y2) orbitals, thus enhancing the 10 D q value by 0.2 eV. However, the addition of VR(r) induces very small changes in the electronic density, a relevant fact that is related to the 2E(t2g3)→4A2(t2g3) emission energy being nearly independent of the host lattice along the series of Cr3+-doped oxides.
Journal of Physics: Condensed Matter, 2013, Vol 25, Issue 17