  Negative thermal expansion in ZrW2O8 was investigated using a flexibility
analysis of ab-initio phonons. It was shown that no previously proposed
mechanism adequately describes the atomic-scale origin of negative thermal
expansion in this material. Instead it was found that NTE in ZrW2O8 is driven,
not by a single mechanism, but by wide bands of phonons that resemble
vibrations of near-rigid WO4 units and Zr-O bonds at low frequency, with
deformation of O-W-O and O-Zr-O bond angles steadily increasing with increasing
NTE phonon frequency. It is asserted that this phenomenon is likely to provide
a more accurate explanation for NTE in many complex systems not yet studied.
