Unusual Anti-Thermoplastics and Low Thermal Expansion in 2D Metal Halide Crystals

Zero thermal expansion (ZTE) materials are critically important in numerous scientific and technical applications due to their versatile physical properties. However, these materials are predominantly inorganic oxides and metal alloys, while the intrinsically ZTE molecule-based single crystals remain exceptionally scarce. This scarcity arises from the lack of a comprehensive understanding of molecular deformation and spatial orientation, which are governed by the intricate coupling between molecular dynamics and intermolecular interactions. Herein, we have presented the linear ZTE and unusual anti-thermoplastics in a two-dimensional metal-halide crystal of G₃Sb₂Br₉ (GSB, where G is guanidinium). GSB exhibits linear ZTE along the a-axis and b-axis (α_a  = 1.77 × 10^-6 K^-1, α_b  = 1.76 × 10^-6 K^-1) between 150 and 400 K, being comparable with several heterogeneous metal alloys (e.g., LaFe₅₄Co_3.5Si_3.35). This behavior can be attributed to the wine-rack motions, which produce compensatory effects between angular contraction of the inorganic framework and bond elongation. More interestingly, the crystal exhibits anti-thermoplastic above 420 K, with both its hardness and Young's modulus increasing significantly. 2D metal-halide molecular crystals combining ultralow in-plane thermal expansion and anti-thermoplastics are rarely reported. This work expands the scope of ZTE-active materials and provides a material platform for studying thermal-stiffening in soft metal-halide frameworks.