Optical and vibrational properties of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mi>Rb</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mrow> <mml:mi>CuCl</mml:mi> </mml:mrow> <mml:mn>4</mm

This work investigates the electronic and vibrational structures of the pseudo Jahn-Teller (PJT) compound ${\mathrm{Rb}}_{2}{\mathrm{CuCl}}_{4}{({\mathrm{H}}_{2}\mathrm{O})}_{2}$, along with their pressure dependence using optical absorption and Raman spectroscopy. The rhombic $({\mathrm{D}}_{2\mathrm{h}})$ local structure of the ${\mathrm{CuCl}}_{4}{({\mathrm{H}}_{2}\mathrm{O})}_{2}^{2\ensuremath{-}}$ complex undergoes a progressive pressure-induced transformation, in which the two short and two long Cu-Cl bonds (perpendicular to the ${\mathrm{H}}_{2}\mathrm{O}\text{\ensuremath{-}}\mathrm{Cu}\text{\ensuremath{-}}{\mathrm{OH}}_{2}$ axis) converge in length, ultimately suppressing the PJT distortion at 10 GPa. This yields a tetragonal $({\mathrm{D}}_{4\mathrm{h}})$ symmetry with four equivalent Cu-Cl bonds. The spectra reveal the crystal-field electronic structure of the complex in both ${\mathrm{D}}_{4\mathrm{h}}$ and PJT-distorted ${\mathrm{D}}_{2\mathrm{h}}$ 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 ${\mathrm{a}}_{1\mathrm{g}}(3{z}^{2}\ensuremath{-}{r}^{2})$ and ${\mathrm{b}}_{1\mathrm{g}}({x}^{2}\ensuremath{-}{y}^{2})$ of ${\mathrm{Cu}}^{2+}$---occurs at 1.32 eV in the PJT-distorted $({\mathrm{D}}_{2\mathrm{h}})$ phase but shifts to 1.09 eV in the undistorted ${\mathrm{D}}_{4\mathrm{h}}$ phase, directly quantifying the PJT-induced splitting. The vibrational modes exhibit unusual pressure dependence: The frequency associated with the long Cu-Cl bond in ${\mathrm{D}}_{2\mathrm{h}}$ shows a large pressure shift, while the short Cu-Cl bond mode shifts slightly. These converge at the transition pressure (${P}_{t}$ = 10 GPa), confirming the progressive suppression of the PJT distortion (from ${Q}_{{\mathrm{b}}_{1\mathrm{g}}}^{0}=0.7$ \AA{} at ambient pressure to ${Q}_{{\mathrm{b}}_{1\mathrm{g}}}^{0}=0$ at ${P}_{t}$). 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 ${B}_{\mathrm{loc}}=22(2)$ GPa, consistent with the x-ray diffraction value $({B}_{\mathrm{cryst}}=20.5 \mathrm{GPa})$.