Local Bond Engineering in LiMBO 4 (M = Si, Ge): Synergistic Negative Grüneisen Parameter and Bond Stiffening for Reduced Thermal Expansion

Precise control of the thermal expansion is paramount for the stability of optical crystals in advanced photonic devices. Diamond-like borates, featuring rigid frameworks of corner-sharing BO4 tetrahedra, are promising candidates. However, achieving ultralow or negative thermal expansion remains a significant challenge. This work demonstrates a rational strategy of local bond engineering within the LiSiBO4 prototype to dramatically suppress thermal expansion. Strategic substitution of Si4+ with larger Ge4+ yields LiGeBO4, reducing its volumetric thermal expansion coefficient from approximately 30.2 × 10–6 K–1 to 24.5 × 10–6 K–1. Structural analysis reveals that the diamond-like framework possesses exceptional rigidity, characterized by nearly invariant B–B separation within the interconnected BO4 network, which establishes a dimensionally stable scaffold. Within this constrained scaffold, Li+ and M4+ (M = Si or Ge) cations occupy distinct interstitial sites. Crucially, the incorporation of larger Ge4+ introduces a longer, weaker Ge–O bond, inducing pronounced distortion in GeO4 tetrahedra. This distortion generates negative Grüneisen parameters (γ), inherently counteracting lattice expansion. Concomitantly, the expanded GeO4 tetrahedron severely compresses the spatial environment of the adjacent LiO4 polyhedron. This spatial confinement forces the Li–O bond to contract and stiffen, significantly restricting their thermal vibrational amplitudes. The synergistic interplay between the negative γ contribution arising from the distorted GeO4 and the bond stiffening within the spatially compressed LiO4 collectively minimizes the overall thermal expansion of the lattice. This study establishes targeted local bond engineering within rigid, geometrically constrained frameworks as a powerful and generalizable paradigm for the design of next-generation optical materials with ultralow or potentially negative thermal expansion.