Abstract: The actual value of axial, Rax, and equatorial, Req, impurity?ligand distances for Cr3+ embedded in tetragonal K2MgX4 (X = F, Cl) lattices has been explored by means of density functional theory (DFT) calculations on clusters involving up to 69 ions using two different functionals. For K2MgF4:Cr3+ Req and Rax are found to be coincident within only 0.5 pm. When the g tensor of K2MgF4:Cr3+ is derived considering only the CrF63? unit in vacuo at the calculated equilibrium geometry the quantity fails to reproduce the experimental value by one order of magnitude. In contrast, when the active electrons localized in the CrX63? complex (X = F, Cl) are allowed to feel the anisotropic electric field coming from the rest of the lattice ions the splitting in the first excited state, 4T2, increases by one order of magnitude. The present results thus show that the g tensor anisotropy and the zero-field splitting constant, D, observed for K2MgX4:Cr3+ (X = F, Cl) are not mainly due to a local deformation of the CrX63? octahedron but to the action of the internal electric field, often ignored when seeking the microscopic origin of electronic properties due to impurities in insulating lattices. Accordingly, serious doubts on the validity of the superposition model are cast by the present work.

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