LARGE LOW-TEMPERATURE SPECIFIC-HEAT IN THE NEGATIVE THERMAL-EXPANSIONCOMPOUND ZRW2O8

The specific heat C(T) of ZrW2O8 is measured from 1 to 300 K. Below 50 K, C(T) is dominated by an Einstein term of energy (h) over bar omega (0) = 5 meV, which represents 6.5% of the total lattice degrees of fre edom. This concentration of phonon spectral weight at low energies is unusual in materials of cubic structure and is a possible indication o f ''rigid unit modes'' discussed for underconstrained crystal structur es. The relationship between C(T) and the large negative thermal expan sion in:this compound is discussed.