Designing Elastic Softening Under Compression via Dipole–Dipole Interactions: The Case of Na ₂ TiSiO ₅

The bulk modulus (B), a key indicator of material compressibility, is generally expected to increase with pressure as interatomic repulsion intensifies. Conventional interatomic potentials are often approximated by nearly symmetric single‐well profiles. By contrast, dipole‐mediated interactions are directional and intrinsically asymmetric, allowing attractive interactions to strengthen during compression. This distinction suggests a chemical route to elastic softening that is not captured by conventional hardening models. Here, we identify Na 2 TiSiO 5 , synthetic paranatisite, as a model compound for pressure‐induced softening. Its bulk modulus B 0 decreases sharply from 94(2) GPa ( Pmc 2 1 phase) to 55.4(1.1) GPa ( Pca 2 1 phase) above approximately 6.0 GPa. High‐pressure structural analyses reveal substantial shortening of the head‐to‐tail TiO 5 alignment, while Born effective charge calculations indicate strengthened dipoles along the c axis. Consistently, Raman spectra show a pronounced redshift of the Ti–O stretching mode, indicating a lower Ti–O bond force constant. Together, these observations support a pressure‐driven rebalancing between Ti–O bonding and Ti–O···Ti dipolar interactions. Our results establish dipole‐interaction potentials as a design principle for compression‐softening materials and advance dipole‐driven elasticity engineering in polar solids.