Low Thermal Expansion in Niobate Borate NbBO₄ Enabled by Compensated Grüneisen Parameters of NbO₈ Polyhedra

Precise control of thermal expansion in functional borates is critical for optical device applications, yet the underlying structure-property relationships remain underexplored. Herein, we employ first-principles calculations to predict that NbBO4-a borate featuring isolated [BO4] tetrahedra-exhibits low thermal expansion ( = 11.48 × 10-6 K-1 at 300 K), comparable to that of predicted near-zero expansion compound Zn4B6O13. Elastic constant calculations confirm its exceptional quasi-harmonic behavior and low linear compressibility. Through lattice dynamics analysis, the phonon spectrum and atomic Grüneisen parameters decomposition demonstrate that [NbO8] polyhedra dominate the thermal expansion. Specifically, part strong anharmonic vibrations along the ab-plane generate negative Grüneisen parameters, which counteract positive contribution and reduce the thermal expansion. This work establishes a mechanistic framework for designing borates with tailored thermal expansion via selective polyhedral control.