Pressure Enhancement of Negative Thermal Expansion Behavior and Induced Framework Softening in Zinc Cyanide

The pressure-dependent structure and functionality of the coordination framework material zinc cyanide, Zn(CN) 2, has been explored using in situ neutron powder diffraction. A third-order Birch−Murnaghan equation of state fit to variable pressure (0−0.6 GPa) data collected at ambient temperature ( K 0 = 34.19(21) GPa, K 0 ‘ = −6.0(7)) shows that, contrary to behavior observed for typical materials, the Zn(CN) 2 framework becomes more compressible at higher pressures. Variable temperature (50−300 K) data collected at 0.2 and 0.4 GPa indicate that the negative thermal expansion effect in Zn(CN) 2 becomes more pronounced at pressure with the coefficient of thermal expansion (α = d T /𝓁d𝓁) varying by ca. −1 × 10 -6 K -1 per 0.2 GPa applied pressure up to an average (50−300 K) value of −19.42(23) × 10 -6 K -1 at 0.4 GPa. Both these unusual phenomena have been linked to increased framework flexibility at high pressure.