Near-zero thermal expansion coordinated with geometric flexibility and π⋯π interaction in anisotropic [Zn8(SiO4)(m-BDC)6]n

Abstract

Zero thermal expansion (ZTE), a phenomenon in which a lattice structure remains constant upon heating, has attracted considerable interest owing to its potential applications. To date, in molecular materials, intrinsic single-phase ZTE or near-ZTE materials, such as isotropic Prussian blue analogues, are very rare. Herein, we report a near-ZTE compound, [Zn₈(SiO₄)(m-BDC)₆]_n, which has an anisotropic framework structure with 1D helical chains along the crystallographic 4₂ axis. High-resolution synchrotron powder X-ray diffraction and single crystal diffraction investigations reveal a positive thermal expansion (PTE) α_a = +4.72 × 10⁻⁶ K⁻¹ and a negative thermal expansion (NTE) α_c = −13.77 × 10⁻⁶ K⁻¹, with a near-ZTE volumetric behaviour α_V = −4.35 × 10⁻⁶ K⁻¹ in the temperature range 100–475 K. Raman spectra and density functional theory (DFT) calculations demonstrate that spring-like geometric deformation results in the anisotropic thermal expansion behaviour. Further temperature-dependent structural analysis and reduced density gradient (RGD) calculations evidence that the π⋯π repulsion leads to the contraction of the building linkage. The synergistic effect of the geometric deformation and π⋯π interaction gives rise to the overall volumetric near-ZTE behaviour in [Zn₈(SiO₄)(m-BDC)₆]_n.