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.