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 (K₀ = 34.19(21) GPa, K₀" = -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 = dT/d) 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.