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.