The Prediction of Vibrational Spectroscopy for ZrCl4 in Molten LiCl-KCl Eutectic: A Deep Potential Molecular Dynamics Study

Abstract

In this work, first-principles molecular dynamics and deep potential molecular dynamics are combined to investigate the local coordination chemistry and vibrational spectroscopic responses of Zr4+ in molten LiCl−KCl−ZrCl4. The simulations identify a statistically dominant 6-fold Zr4+−Cl− local coordination motif, denoted as [ZrCl6]2−, with an approximately octahedral geometry and a characteristic Zr4+−Cl− distance of 2.46 Å. This Zr-centered chloride cage exhibits picosecond-scale dynamical persistence, in clear contrast to the rapidly relaxing Li+−Cl− and K+−Cl− coordination environments. The Zr4+-centered subsystem also displays structured vibrational features across 0−10 THz, with the strongest high-frequency contribution located around 8−9 THz, reflecting constrained Zr4+−Cl− relative motion within the local coordination cage. On this structural and dynamical basis, local infrared and Raman spectra are evaluated from DPMD trajectories. The local infrared spectrum exhibits two dominant bands at 147 and 277 cm−1, assigned to the parent-symmetry ν4 (T1u) bending/deformation mode and ν3 (T1u) antisymmetric Zr4+−Cl− stretching mode, respectively. The local Raman spectrum displays two principal bands at 155 and 314 cm−1, corresponding to Eg/T2g nontotally symmetric deformation and A1g symmetric breathing/stretching modes, respectively. These results establish a direct structure−dynamics−spectroscopy relationship for the dynamically persistent 6-fold Zr4+−Cl− coordination motif in molten chlorides.