Vibrational Dynamics and Cation Mobility in the TTB Oxide K₃Li₂Nb₅O₁₅: A Combined Raman, AIMD, and QTAIM Study

The K3Li2Nb5O15 (KLN) compound, which crystallizes in a tetragonal tungsten bronze (TTB) structure, was synthesized and structurally resolved by the Rietveld refinement of X-ray diffraction (XRD) data, followed by characterization via Raman spectroscopy and theoretical investigation through an integrated computational approach. Density functional theory (DFT) calculations, incorporating the quasi-harmonic approximation and topological analysis based on the quantum theory of atoms in molecules (QTAIM), were employed to interpret vibrational features and to establish the crystal and electronic structures. Theoretical and experimental Raman spectra were compared, revealing significant discrepancies below 400 cm–1. These differences were attributed to the dynamic behavior of K+ and Li+ cations within the pentagonal and trigonal channels, respectively. Ab initio molecular dynamics simulations confirmed the high mobility of Li+ and the site-dependent behavior of K+, which directly impact the vibrational signatures. The combined vibrational and dynamic analyses suggest that the observed oxygen mobility, particularly in the channel regions, may influence the formation and migration of oxygen vacancies, with possible implications for ionic conductivity, gas sensing, and (photo)catalytic activity in functional oxides. This study provides a detailed atomistic understanding of the coupling between structural dynamics and vibrational spectra in complex TTB frameworks, contributing to deeper insight into phenomena rarely explored at this level.