Measurement of anharmonicity of phonons in negative thermal expansion compound Zn(CN)2 by high pressure inelastic neutron scattering

TL;DR

This study investigates how phonon anharmonicity influences the negative thermal expansion in Zn(CN)2 by measuring phonon spectra under pressure, revealing low-energy phonons' critical role in NTE behavior.

Contribution

It provides the first quantitative analysis linking phonon anharmonicity to NTE in Zn(CN)2 through high-pressure inelastic neutron scattering.

Findings

Low-energy phonons below 15 meV are key to NTE.

Measured anharmonicity explains the magnitude of NTE.

Phonon spectra shift with pressure, indicating anharmonic effects.

Abstract

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.