Compounds Containing Tetragonal Cu2⁺ Complexes: Is the d_x^2–_y²–d_3z²–r² Gap a Direct Reflection of the Distortion?

It is widely assumed that the gap, Δ, between dx2–y2 and d3z2–r2 orbitals in fluorides and oxides containing tetragonal Cu2+ or Ag2+ complexes directly reflects the tetragonal distortion in the MX6 complex (M = d9 ion; X = F–, O2–). This assumption on that relevant quantity is shown to be not correct through the study of pure K2CuF4-, KCuF3-, and Cu2+-doped KZnF3 and K2ZnF4 model compounds.

Indeed, ab initio calculations prove that Δ in these insulating materials also depends on the internal electric field created by the rest of lattice ions on active electrons confined in a given CuF64– complex. This internal field, especially important for layered compounds, is shown to explain all puzzling experimental facts on the d–d transitions of the studied systems and is of interest in the search of new Cu2+ and Ag2+ superconducting materials where a strong correlation between Δ and the transition temperature, Tc, has been conjectured.