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

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.