Colour due to Cr3+ ions in oxides: a study of the model system MgO:Cr3

• Published in: Journal of physics. Condensed matter 2013-05, Vol.25 (17), p.175501-175501
• Main Authors: Aramburu, J A, García-Fernández, P, García-Lastra, J M, Barriuso, M T, Moreno, M
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Crystal and ligand fields; Electron paramagnetic resonance and relaxation; Electron states; Exact sciences and technology; Level splitting and interactions; Magnetic resonances and relaxations in condensed matter, mössbauer effect; Physics
• ISSN: 0953-8984; 1361-648X
• DOI: 10.1088/0953-8984/25/17/175501

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 and thus 0.06 higher than that measured for the emerald. Calculations carried out on the isolated complex at R = 2.03 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 driven by the first shell of twelve Mg2+ ions. The action of VR(r) upon the 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 10Dq 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 emission energy being nearly independent of the host lattice along the series of Cr3+-doped oxides.