Cr3+ in layered perovskites: do the electron paramagnetic resonance parameters only depend on the impurity-ligand...

The actual value of axial, R(ax), and equatorial, R(eq), impurity-ligand distances for Cr(3+) embedded in tetragonal K(2)MgX(4) (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 K(2)MgF(4):Cr(3+) R(eq) and R(ax) are found to be coincident within only 0.5 pm. When the g tensor of K(2)MgF(4):Cr(3+) is derived considering only the CrF(6)(3-) unit in vacuo at the calculated equilibrium geometry the g(⊥)-g(||) quantity fails to reproduce the experimental value by one order of magnitude. In contrast, when the active electrons localized in the CrX(6)(3-) 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, (4)T(2), 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 K(2)MgX(4):Cr(3+) (X = F, Cl) are not mainly due to a local deformation of the CrX(6)(3-) 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.