Origin of Negative Thermal Expansion in CubicZrW2O8Revealed by High Pressure Inelastic Neutron Scattering

Abstract

Isotropic negative thermal expansion has been reported in cubic ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ over a wide range of temperatures (0–1050 K). Here we report the direct experimental determination of the Grüneisen parameters of phonon modes as a function of their energy, averaged over the whole Brillouin zone, by means of high pressure inelastic neutron scattering measurements. We observe a pronounced softening of the phonon spectrum at $P=1.7\mathrm{}\mathrm{kbar}$ compared to that at ambient pressure by about 0.1–0.2 meV for phonons of energy below 8 meV. This unusual phonon softening on compression, corresponding to large negative Grüneisen parameters, is able to account for the observed large negative thermal expansion.