Unraveling low thermal expansion of CrBO3 via atomic scale analysis: Flexible CrO6 octahedron vs rigid BO3 triangle group

Abstract

Borates are pivotal in optical technology owing to their exceptional optical performance, yet their thermal expansion property, critical for optical device stability, remains inadequately understood at the atomic level. While prior studies have correlated the macroscopic thermal expansion coefficient (α) with the lattice dynamic or Grüneisen parameter, the atomically resolved contribution to α has never been quantified, limiting precise material design. Herein, a pioneering strategy combining topological basin analysis with the quasi-harmonic approximation was conducted to decode the atomic scale origin of thermal expansion in borates. Focusing on hexagonal CrBO3 with isolated [BO3] group, a relatively low positive α (15 × 10−6 K−1 at 300 K) was predicted via density functional theory. Beyond conventional Grüneisen and elastic modulus analyses, the first quantitative partitioning of thermal expansion contribution from individual atoms Cr, B, and O through topological basin derived atomic volume and compressibility was achieved. This approach reveals a thermal expansion ratio of 1.33:1.00:1.73 for Cr, B, and O atoms, respectively. The [CrO6] octahedron is identified as the primary thermal expansion driver, contrasting with the rigid [BO3] triangle, a finding validated by bond strength analysis and temperature-dependent elastic moduli. This work establishes a paradigm: topological basin analysis bridges macroscopic α to atomic flexibility/rigidity, offering a universal tool for precise engineering thermal expansion in functional crystals.