Optical and vibrational properties of Rb 2 CuCl 4 ( H 2 O ) 2 : Pseudo Jahn-Teller distortion

This work investigates the electronic and vibrational structures of the pseudo Jahn-Teller (PJT) compound Rb₂CuCl₄ ({H₂O) }₂, along with their pressure dependence using optical absorption and Raman spectroscopy. The rhombic (D₂₇) local structure of the CuCl₄ ({H₂O) }₂^2- complex undergoes a progressive pressure-induced transformation, in which the two short and two long Cu-Cl bonds (perpendicular to the H₂O-Cu-OH₂ axis) converge in length, ultimately suppressing the PJT distortion at 10 GPa. This yields a tetragonal (D₄₇) symmetry with four equivalent Cu-Cl bonds. The spectra reveal the crystal-field electronic structure of the complex in both D₄₇ and PJT-distorted D₂₇ geometries. Their energies, calculated via the angular overlap model (AOM), accurately describe the ground state and the excited states. Notably, the first crystal-field transition---between PJT-active levels a₁₆ (3z^2-r^2) and b₁₆ (x^2-y^2) of Cu^2+---occurs at 1. 32 eV in the PJT-distorted (D₂₇) phase but shifts to 1. 09 eV in the undistorted D₄₇ phase, directly quantifying the PJT-induced splitting. The vibrational modes exhibit unusual pressure dependence: The frequency associated with the long Cu-Cl bond in D₂₇ shows a large pressure shift, while the short Cu-Cl bond mode shifts slightly. These converge at the transition pressure (Pₓ = 10 GPa), confirming the progressive suppression of the PJT distortion (from Q₁₁₆^0=0. 7 at ambient pressure to Q₁₁₆^0=0 at Pₓ). A Gr\"uneisen-type model correlates the pressure-dependent Cu-Cl stretching frequencies with bond-distance variations, providing an alternative method to track PJT distortion under pressure. The local volume V (P) of the complex, derived from this approach, follows a Murnaghan equation of state with a bulk modulus B₋₎₂=22 (2) GPa, consistent with the x-ray diffraction value (B₂ₑₘₒₓ=20. 5 GPa).