  We investigate detailed lattice dynamics of copper halides CuX (X=Cl, Br, I)
using neutron inelastic scattering measurements and ab-initio calculations
aimed at a comparative study of their thermal expansion behavior. We identify
the low energy phonons which soften with pressure and are responsible for
negative thermal expansion. The eigenvector analysis of these modes suggests
that softening of the transverse-acoustic modes would lead to NTE in these
compounds. The calculations are in very good agreement with our measurements of
phonon spectra and thermal expansion behavior as reported in the literature.
Our calculations at high pressure further reveal that large difference in
negative thermal expansion behavior in these compounds is associated with the
difference of the unit cell volume.
