The specific heat $C\left(T\right)$ of ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ is measured from 1 to 300 K. Below 50 K, $C\left(T\right)$ is dominated by an Einstein term of energy $ħ{\omega }_0\approx 5\mathrm{meV}$, which represents 6.5% of the total lattice degrees of freedom. This concentration of phonon spectral weight at low energies is unusual in materials of cubic structure and is a possible indication of “rigid unit modes” discussed for underconstrained crystal structures. The relationship between $C\left(T\right)$ and the large negative thermal expansion in this compound is discussed.

• Received 5 February 1998

DOI:https://doi.org/10.1103/PhysRevLett.80.4903