Colossal Negative Area Compressibility in the Ferroelastic Framework Cu(tcm)

Copper(I) tricyanomethanide, Cu(tcm), is a flexible framework material that exhibits the strongest negative area compressibility (NAC) effect ever observed─a remarkable property with potential applications in pressure sensors, artificial muscles, and shock-absorbing devices. Under increasing pressure, Cu(tcm) undergoes two sequential phase transitions (tetragonal → orthorhombic → monoclinic): It has an initial tetragonal structure (I4₁md) at ambient conditions, but this structure only persists within a narrow pressure range; at 0.12(3) GPa, a pressure-induced ferroelastic phase transition occurs, transforming Cu(tcm) into a low-symmetry orthorhombic structure (Fdd2). The orthorhombic phase has a NAC of -108(14) TPa^-1 in the b-c plane between 0.12(3) and 0.93(8) GPa. The NAC behavior is associated with framework hinge motion in a flexible framework with "wine-rack" topology. At 0.93(8) GPa, Cu(tcm) undergoes a second phase transition and transforms into a layered monoclinic structure (Cc) with topologically interpenetrating honeycomb networks. The monoclinic phase of Cu(tcm) exhibits a slight negative linear compressibility (NLC) of -1.1(1) TPa^-1 along the a axis and a zero area compressibility of K_ac = K_a + K_c = 0.0(4) TPa^-1 in the a-c plane over the pressure range of 0.93-2.63 GPa. In contrast to the orthorhombic phase, its mechanism is understood as the pressure-driven dampening of layer "rippling," which acts to increase the cross-sectional area of the layer at higher hydrostatic pressures. These findings have implications for understanding the underlying mechanism of NAC phenomenon in framework materials.