We use variable-pressure neutron and X-ray diffraction measurements to determine the uniaxial and bulk compressibilities of nickel(II) cyanide, Ni(CN)$_2$. Whereas other layered molecular framework materials are known to exhibit negative area compres
sibility, we find that Ni(CN)$_2$ does not. We attribute this difference to the existence of low-energy in-plane tilt modes that provide a pressure-activated mechanism for layer contraction. The experimental bulk modulus we measure is about four times lower than that reported elsewhere on the basis of density functional theory methods [{it Phys. Rev. B} {bf 83}, 024301 (2011)].
The molecular framework Ag(tcm) (tcm$^-$ = tricyanomethanide) expands continuously in two orthogonal directions under hydrostatic compression. The first of its kind, this negative area compressibility behaviour arises from the flattening of honeycomb
-like layers during rapid pressure-driven collapse of the interlayer separation.
