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

Abstract

ABSTRACT:The variations experienced by the energy E u (π) of the e u (π) → b 1g (∼x 2 -y 2 ) charge-transfer transition of (C 2 H 5 NH 3 ) 2 CdCl 4 :Cu 2+ 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 E u (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 ∂E u (π)/∂R eq and ∂E u (π)/∂R ax have been performed where R eq and R ax mean the equatorial and axial Cu 2+ -Cl − distances of the elongated CuCl 6 4− complex, respectively. All results indicate that ∂E u (π)/∂R eq and ∂E u (π)/∂R ax for R eq = 228 pm and R ax = 297 pm are indeed negative. Moreover ab initio complete active space self-consistent field (CASSCF/CASPT2) and density functional calculations lead to ∂E u (π)/∂R ax values, which are about 10 times smaller than those of ∂E u (π)/∂R eq . 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.

Red Shift in Optical Excitations on Layered Copper Perovskites under Pressure: Role of the Orthorhombic Instability

“…The charge transfer spectrum of (C 2 H 5 NH 3 ) 2 CdCl 4 :Cu 2+ calculated in the range 2.8–4.2 eV for several pressures using the VASP code is displayed on Figure 6. That spectrum is dominated by a band peaked at 3.4 eV at zero pressure not far from the value E CT =3.12 eV measured experimentally [11] . Moreover, Figure 6 shows that the calculated band maximum is red shifted by 0.11 eV under a 3 GPa pressure, a value close to ΔE CT =−0.14 eV determined experimentally.…”

“…Bearing the results of the former subsections in mind this one is addressed to gain a better insight into the red shift under pressure observed [11] for the lowest charge transfer (CT) transition in (C 2 H 5 NH 3 ) 2 CdCl 4 :Cu 2 + in the range 0-3 GPa. If E CT means the energy of such a transition it is experimentally found dE CT /dP = À 0.04 eV/GPa.…”

Variation of the Jahn–Teller distortion with pressure in perovskite layers A2CuCl4. Influence on the charge‐transfer band

“…Besides the crystal structure and the diluted or concentrated nature of the (CuCl 6 ) 4-complex, the first CT band shifts to lower energies by 0.15-0.35 eV, depending on the compound, from ambient pressure to 10 GPa. The CT redshift has been initially explained in terms of changes of local structure around Cu 2+ according to expectations for CT shifts from electronic structure calculations [14,15]. The results indicate that CT redshifts induced by pressure in (CuCl 6 ) 4-are possible whatever the equatorial distance, R eq , increases upon pressure.…”

“…[4] for (C 2 H 5 NH 3 ) 2 CuCl 4 and in Refs. [8,15] for Cu progressive reduction of the (CuCl 6 ) 4-axial distortion with pressure. Recent XRD experiments correlate the dissappearance of Raman peaks with the reduction of the JT distortion at the orthorhombic-tomonoclinic phase transtion at 4 GPa [13].…”

Effects of Hydrostatic and Chemical Pressures on Impurities Determined through Optical Parameters

“…2) are often more sensitive than crystal field ones to DR variations. They have been used for exploring the effects of hydrostatic pressures upon elongated tetragonal CuCl 4À 6 units [23]. It has been proved by this procedure that a hydrostatic pressure always produces a decrement of the long bond but in some cases it leads to an increase of the short bond [23].…”