Competing Hydrostatic Compression Mechanisms in Nickel Cyanide

TL;DR

This study investigates the compression mechanisms of nickel cyanide using neutron and X-ray diffraction, revealing unique in-plane tilt modes that influence its compressibility and contrasting with other layered materials.

Contribution

It provides experimental measurements of Ni(CN)$_2$'s uniaxial and bulk compressibilities, highlighting the role of low-energy tilt modes and challenging previous DFT-based predictions.

Findings

Ni(CN)$_2$ does not exhibit negative area compressibility.

Experimental bulk modulus is about four times lower than DFT predictions.

Low-energy in-plane tilt modes facilitate layer contraction under pressure.

Abstract

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 compressibility, 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)].