Origin of negative thermal expansion in cubic ZrW2O8 revealed by high pressure inelastic neutron scattering

Isotropic negative thermal expansion has been reported in cubic ZrW2O8 over a wide range of temperatures (0-1050 K). Here we report the direct experim ental determination of the Gruneisen parameters of phonon modes as a functi on of their energy, averaged over the whole Brillouin zone, by means of hig h pressure inelastic neutron scattering measurements. We observe a pronounc ed softening of the phonon spectrum at P = 1.7 kbar compared to that at amb ient pressure by about 0.1 - 0.2 meV for phonons of energy below 8 meV. Thi s unusual phonon softening on compression, corresponding to large negative Gruneisen parameters, is able to account for the observed lame negative the rmal expansion.