Interplay between WO ₆ Octahedra Rigidity and Li Sub-Lattice Flexibility in Triclinic Li ₂ W ₂ O ₇ : Raman, DFT, Hirshfeld Surface, and High-Pressure Studies

High Resolution Image Download MS PowerPoint Slide We report a combined experimental and theoretical investigation of the structural, electronic, elastic, and vibrational properties of triclinic Li 2 W 2 O 7 . 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 ≈ +0.90 e ) combined with significant W–O covalency (W ≈ +2.98 e ), and electronic structure calculations show a wide O 2 p → W 5 d charge-transfer band gap characteristic of d 0 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 WO 6 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 WO 6 octahedra and a more compressible Li–O sublattice.