Interplay between WO₆ Octahedra Rigidity and Li Sub-Lattice Flexibility in Triclinic Li₂W₂O₇: Raman, DFT, Hirshfeld...

We report a combined experimental and theoretical investigation of the structural, electronic, elastic, and vibrational properties of triclinic Li₂W₂O₇. The crystal structure was confirmed through Rietveld refinement, in good agreement with the reported triclinic model. Density functional theory (DFT) calculations within the DFT-GGA/PBE framework reproduce the experimental lattice parameters with deviations below 5%. Bader charge analysis reveals predominantly ionic Li-O interactions (Li ≈ +0.90e) combined with significant W-O covalency (W ≈ +2.98e), and electronic structure calculations show a wide O 2p → W 5d charge-transfer band gap characteristic of d⁰ tungstates. Elastic constant calculations confirm mechanical stability with moderate anisotropy. Raman spectroscopy supported by DFT phonon calculations enables reliable mode assignment, revealing pronounced Li atomic motion in many WO₆ vibrations. Hirshfeld surface analysis indicates that Li···O/O···Li contacts dominate the crystal packing (≈55.7%), with a 22.15% void fraction in the framework. High-pressure Raman measurements up to 9.3 GPa show predominantly positive pressure coefficients and clear spectral modifications between 6.3 and 7.5 GPa, providing evidence for a pressure-induced structural phase transition driven by octahedral tilting and symmetry reduction. The pressure response is governed by the interplay between rigid WO₆ octahedra and a more compressible Li-O sublattice.