Aramburu JA, García-Fernández P, García-Lastra JM, Barriuso MT, Moreno M. Colour due to Cr3+ ions in oxides: a study of the model system MgO:Cr3+.
JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2013;
25:175501. [PMID:
23552342 DOI:
10.1088/0953-8984/25/17/175501]
[Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
Seeking to understand why the cubic centre in MgO:Cr(3+) has the same 10Dq value as emerald, ab initio cluster and periodic supercell calculations have been performed. It is found that the equilibrium Cr(3+)-O(2-) distance, R, in MgO:Cr(3+) is equal to 2.03 Å and thus 0.06 Å higher than that measured for the emerald. Calculations carried out on the isolated CrO(6)(9-) complex at R = 2.03 Å give 10Dq = 14,510 cm(-1), which is 10% smaller than the experimental figure for MgO:Cr(3+). 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 Cr(3+) 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 Mg(2+) ions. The action of VR(r) upon the CrO(6)(9-) complex slightly decreases the energy of t2g(xy,xz,yz) orbitals with respect to that for eg(3z(2) - r(2),x(2) - y(2)) 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 (2)E(t(2g)(3)) −> (4)A(2)(t(2g)(3)) emission energy being nearly independent of the host lattice along the series of Cr(3+)-doped oxides.
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