Optical spectroscopy of (C2H5NH3)2CdCl4:Cu2+ under pressure: Study of Cu2+ local structure from theoretical calculations

The variations experienced by the energy Eu(π) of the eu(π)→b1g (∼x2–y2) charge‐transfer transition of (C2H5NH3)2CdCl4:Cu2+ upon pressure in the 0‐ to 40‐kbar range have been measured at room temperature by means of a sapphire anvil cell. These data reveal that Eu(π) undergoes a red shift of 1400 cm−1 on passing from ambient pressure to 40 kbars. To understand this puzzling result theoretical calculations of ∂Eu(π)/∂Req and ∂Eu(π)/∂Rax have been performed where Req and Rax mean the equatorial and axial Cu2+–Cl− distances of the elongated CuCl64− complex, respectively. All results indicate that ∂Eu(π)/∂Req and ∂Eu(π)/∂Rax for Req=228 pm and Rax=297 pm are indeed negative. Moreover ab initio complete active space self‐consistent field (CASSCF/CASPT2) and density functional calculations lead to ∂Eu(π)/∂Rax values, which are about 10 times smaller than those of ∂Eu(π)/∂Req. From the ensemble of experimental and theoretical results, it is concluded that a pressure of 40 kbars gives rise to a decrement of ≈25 pm of the axial distance and at the same time to an increase of ≈7 pm of the equatorial one. It is stressed that the present study on a diluted Jahn–Teller impurity lies far beyond the current possibilities of X‐ray absorption structure techniques.