Abstract: 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 (D2h) 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 (D4h) symmetry with four equivalent Cu-Cl bonds. The spectra reveal the crystal-field electronic structure of the complex in both D4h and PJT-distorted D2h 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 a1g(3z2-r2) and b1g(x2-y2) of Cu2+-occurs at 1.32 eV in the PJT-distorted (D2h) phase but shifts to 1.09 eV in the undistorted D4h phase, directly quantifying the PJT-induced splitting. The vibrational modes exhibit unusual pressure dependence: The frequency associated with the long Cu-Cl bond in D2h shows a large pressure shift, while the short Cu-Cl bond mode shifts slightly. These converge at the transition pressure (Pt = 10 GPa), confirming the progressive suppression of the PJT distortion (from Q0b1g = 0.7 & Aring; at ambient pressure to Q0b1g = 0 at Pt). A Gr & uuml;neisen-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 Bloc= 22(2) GPa, consistent with the x-ray diffraction value (Bcryst = 20.5 GPa).

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