Designing Elastic Softening Under Compression via Dipole-Dipole Interactions: The Case of Na₂TiSiO₅

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 Na₂TiSiO₅, synthetic paranatisite, as a model compound for pressure-induced softening. Its bulk modulus B₀ decreases sharply from 94(2) GPa (Pmc2₁ phase) to 55.4(1.1) GPa (Pca2₁ phase) above approximately 6.0 GPa. High-pressure structural analyses reveal substantial shortening of the head-to-tail TiO₅ 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.