Argentophilicity induced anomalous thermal expansion behavior in a 2D silver squarate

Abstract

Over the past decades, two dimensional (2D)-structured materials have attracted considerable interest due to not only their intrinsic aesthetic appeal but also their intriguing physical properties. Herein, a 2D bilayer silver squarate was synthesized, which showed combined colossal positive thermal expansion (PTE) along the packing direction and negative thermal expansion (NTE) within the layer. The combined analyses of high resolution synchrotron X-ray powder diffraction, atomic pair distribution function (PDF) and Raman spectra shed light on the anomalous thermal expansion behaviors being closely related to the interlayer argentophilicity and thermal flipping of the ligand. Furthermore, this compound exhibited an intrinsic moderate proton conductivity (1.2 × 10⁻⁴ S cm⁻¹ at 80 °C under 98% RH). We believe that this work can not only deepen our understanding of the thermal expansion properties of 2D materials but also provide guidance for the design and synthesis of novel functional materials.