A comparative theoretical study on the vibrational spectra of V₂O₅·nH₂O

The structural and vibrational properties of hydrated vanadium pentoxide (V₂O₅·nH₂O) were investigated using first-principles density functional theory (DFT) calculations. Comparative analysis revealed characteristic Raman peaks arising from interlayer water molecules and elucidated the evolution of the Raman spectra as a function of the hydration level. Notably, V₂O₅·H₂O does not adopt a strictly monoclinic lattice with C2/m symmetry. Instead, it exhibits an in-plane disordered structure that can be approximated by a triclinic unit cell closely resembling C2/m symmetry. In contrast to anhydrous α-V₂O₅, hydrated V₂O₅·nH₂O phases display distinct vibrational spectral signatures, including a Raman-active peak at approximately 760 cm^-1, corresponding to V-O₃-V stretching modes, and a pronounced enhancement near 890 cm^-1, associated with water-related modes. In partially hydrated systems (V₂O₅·0.5H₂O and V₂O₅·1.5H₂O), the framework disorder induces peak splitting. Molecular dynamics simulations, employing a machine learning-based force field applied to a supercell comprising 2560 atoms, demonstrate that the vibrational density of states (VDOS) for water molecules shifts from approximately 400 cm^-1 to 900 cm^-1 with increasing hydration. This study provides a comprehensive analysis of the vibrational modes in bilayer V₂O₅·nH₂O, offering critical insights into the vibrational spectra relevant to experimental studies of V₂O₅-based electrode materials.