Understanding the relationship between structure and thermal expansion is of great fundamental scientific significance for revealing the nature of thermal expansion and achieving its precise control. This work reports a unique layered open-framework structure with a terminal CN group in $\mathrm{Zn}{\left[\mathrm{B}{\left(\mathrm{CN}\right)}_4\right]}_2$, which displays zero thermal expansion (ZTE) in plane over a wide temperature range (${\alpha }_{\mathrm{a}}=-0.66\times {10}^{-6}{\mathrm{K}}^{-1}$, 100–700 K). A joint study of variable-temperature synchrotron x-ray diffraction, Raman spectroscopy, and first-principles calculations has been performed to investigate the structure, thermal expansion, and mechanism. Our study reveals that the in-plane ZTE originates from two mechanisms: (i) the in-plane tension effect, which gives rise to phonon soft modes with negative Grüneisen parameters, primarily involving the coupled rotations of ${\mathrm{BC}}_4$ tetrahedra and ${\mathrm{ZnN}}_6$ octahedra; and (ii) the elastic effect, where the elastic constants act as a coupling coefficient that enhances the in-plane negative thermal expansion (NTE). This work reveals the origin mechanism of NTE in layered borate systems, emphasizing the synergistic effect between phonon contributions and elastic effects, providing insights into the physical nature of thermal expansion.

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• Elastic modulus
• Lattice dynamics
• Thermal expansion
• Density functional calculations
• Raman spectroscopy
• X-ray diffraction