The ground and excited state properties of the Cr³⁺ ion doped into the cubic host lattices Cs₂NaYCl₆ and Cs₂NaYBr₆ have been studied using density functional theory. A new symmetry based technique was employed to calculate the energies of the multiplets ⁴A_2g, ⁴T_2g, ²E_g, and ⁴T_1g. The geometry of the CrX^{3 -Â} ₆ cluster was optimized in the ground and excited states. A Madelung correction was introduced to take account of the electrostatic effects of the lattice. The experimental Cr–X distance in the ground state can be reproduced to within 0.01 Ã… for both chloride and bromide systems. The calculated d–d excitation energies are typically 2000–3000 cm^–1 too low. An energy lowering is obtained in the first ⁴T_2g excited state when the octahedral symmetry of CrX^{3 -Â} ₆ is relaxed along the eg Jahn–Teller coordinate. The geometry corresponding to the energy minimum is in excellent agreement with the ⁴T_2g geometry derived from high-resolution optical spectroscopy of Cs₂NaYCl₆:Cr³⁺. It corresponds to an axially compressed and equatorially elongated CrX^{3 -Â} ₆ octahedron.

The luminescence of [CrX₆]^3– X=Br^–, Cl^– has been studied through density functional theory (DFT) using both deMon and ADF codes. Multiplet energies⁴A₂,²E,⁴T₂, and⁴T₁ have been expressed as energies of non-redundant single determinants and calculated as in Ref. [1]. The influence of the metal ligand distance on the multiplet energies has been investigated. Of particular interest to this work is the Jahn-Teller effect distortion. We found that the system moves to a more stable geometry when the axial bond length is compressed and the equatorial one elongated in agreement with the experimental value.