Red shift in optical excitations on layered copper perovskites under pressure: role of the orthorhombic instability

The red shift under pressure in optical transitions of layered compounds with CuCl₆^4- units is explored through first-principles calculations and the analysis of available experimental data. The results on Cu²⁺-doped (C₂H₅NH₃)₂CdCl₄, that is taken as a guide, show the existence of a highly anisotropic response to pressure related to a structural instability, driven by a negative force constant, that leads to an orthorhombic geometry of CuCl₆^4- units but with a hole displaying a dominant 3z²-r² character (z being the direction perpendicular to the layer plane). As a result of such an instability, a pressure of only 3 GPa reduces by 0.21 Å the longest Cu²⁺-Cl^- distance, lying in the layer plane, while leaving unmodified the two other metal-ligand distances. Owing to this fact, it is shown that the lowest d-d transition would experience a red shift of 0.34 eV while the first allowed charge transfer transition is also found to be red shifted but only by 0.11 eV that reasonably concurs with the experimental value. The parallel study on Jahn-Teller systems CdCl₂:Cu²⁺ and NaCl:Cu²⁺ involving tetragonal elongated CuCl₆^4- units shows that the reduction of the long axis by a pressure of 3 GPa is three times smaller than that for the layered (C₂H₅NH₃)₂CdCl₄:Cu²⁺ compound. Accordingly, the optical transitions of such systems, which involve a positive force constant, are much less sensitive to pressure than in layered compounds. The origin of the red shift under pressure undergone by the lowest d-d and charge transfer transitions of (C₂H₅NH₃)₂CdCl₄:Cu²⁺ is discussed in detail.