Abstract: The red shift under pressure in optical transitions of layered compounds with CuCl64- 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 CuCl64- 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 CuCl64- 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.

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