Efficient Calculation of the Negative Thermal Expansion in ZrWO.

We present a study of the origin of the negative thermal expansion (NTE) on ZrWO by combining an efficient approach for computing the dynamical matrix with the Lanczos algorithm for generating the phonon density of states in the quasi-harmonic approximation. The simulations show that the NTE arises primarily from the motion of the O-sublattice, and in particular, from the transverse motion of the O atoms in the W-O and W-O-Zr bonds. In the low frequency range these combine to keep the WO tetrahedra rigid and induce internal distortions in the ZrO octahedra.