Soft modes and negative thermal expansion in Zn(CN)2 from Raman spectroscopy and first-principles calculations

We have studied Zn(CN)(2) at high pressure using Raman spectroscopy and report Gruneisen parameters of the soft phonons. The phonon frequencies and eigenvectors obtained from ab initio calculations are used for the assignment of the observed phonon spectra. Out of the 11 zone-center optical modes, 6 modes exhibit negative Gruneisen parameter. The calculations suggest that the soft phonons correspond to the librational and translational modes of C N 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.