Soft modes and NTE in Zn(CN)2 from Raman spectroscopy and first principles calculations

TL;DR

This study combines Raman spectroscopy and first-principles calculations to analyze soft phonon modes in Zn(CN)2, revealing their role in negative thermal expansion and lattice disordering under pressure.

Contribution

It provides detailed phonon mode assignments and Gruneisen parameters for Zn(CN)2, linking soft modes to thermal expansion behavior and high-pressure lattice disorder.

Findings

Six phonon modes have negative Gruneisen parameters.

Soft phonons are linked to librational and translational CN modes.

Lattice disordering occurs above 1.6 GPa.

Abstract

We have studied Zn(CN)2 at high pressure using Raman spectroscopy, and report Gruneisen parameters of the soft phonons. The phonon frequencies and eigen vectors obtained from ab-initio calculations are used for the assignment of the observed phonon spectra. Out of the eleven zone-centre optical modes, six modes exhibit negative Gruneisen parameter. The calculations suggest that the soft phonons correspond to the librational and translational modes of CN rigid unit, with librational modes contributing more to thermal expansion. A rapid disordering of the lattice is found above 1.6 GPa from X-ray diffraction.