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We report ab-inito density functional theory calculation of phonons in cubicphase of ZrW2O8 in the entire Brillouin zone and identify specific anharmonicphonons that are responsible for large negative thermal expansion (NTE) interms of translation, rotation and distortion of WO4 and ZrO6. We have useddensity functional calculations to interpret the experimental phonon spectra asa function of pressure and temperature as reported in literature. We discoverthat the phonons showing anharmonicty with temperature are different from thoseshowing anharmonicity with pressure although both are of similar frequencies.Only the latter phonons are associated with NTE. Therefore the cubic and/orquadratic anharmonicity of phonons is not relevant to NTE but just the volumedependence of frequencies. The calculations are able to reproduce the observedanomalous trends, namely, the softening of the low frequency peak at about 5meV in the phonon spectra with pressure and its hardening with temperature,while both the changes involve a compression of the lattice.

Negative Thermal Expansion in cubic ZrW2O8: Role of Phonons in Entire Brillouin Zone From ab-inito Calculations