  Zn(CN)2 is known to have an isotropic negative thermal expansion (NTE)
coefficient (about -51 x 10-6 K-1) over 10-370 K that is twice as large as that
of ZrW2O8. We have measured the pressure dependence of the phonon spectra up to
30 meV from a polycrystalline sample of Zn(CN)2 at pressures of 0, 0.3, 1.9 and
2.8 kbar at temperatures of 165 and 225 K. The measurements enabled us to
estimate the energy dependence of the ratios Gamma/B (Gamma are Gruneisen
parameters as a function of phonon energy Ei at ambient pressure and B is the
bulk modulus), which reflect the anharmonicity of phonons. We conclude that the
phonon modes of low energy below 15 meV play an important role in the
understanding of the NTE behavior in Zn(CN)2 and the measured anharmonicity can
quantitatively explain the NTE.
