  The thermal expansion behavior of metal fluorides can be tuned by choosing
appropriate metal cation. We present ab-initio lattice dynamical studies on the
metal fluorides (CaZrF6, MgZrF6 and SrZrF6) and identify the anharmonic phonon
modes responsible for the negative thermal expansion in these materials. These
phonons involve ZrF6 polyhedral rotational motion, which leads to large
transverse amplitude of the vibrations of the fluorine atom in the Zr-F-Zr
bond. The compounds with larger anisotropy in the thermal amplitude of the
fluorine atoms show larger NTE behaviour. This has enabled to understand the
large variation in thermal expansion behaviour of these compounds at high
temperature. The calculations also predict decrease of the frequency of these
anharmonic phonons with increasing temperature.
