  The orthovanadates are useful as host matrices for immobilization of
radioactive wastes. The thermodynamic stability of these materials is crucial
for their applications in high pressure and temperatures environment. It is
necessary to investigate the phonons in the entire Brillouin zone, beyond the
zone-centre phonons accessible in previous Raman and infrared experiments. We
have carried out extensive neutron inelastic scattering experiments to derive
the phonon dispersion relation of YVO4 up to high energy transfer of 65 meV
using a single crystal, which are perhaps reported for the first time in any
orthovanadate compound. The measured phonon dispersion relation is in good
agreement with our first principles density functional theory as well as shell
model calculations. The calculated pressure dependence of phonon modes in the
zircon and scheelite phases shows unstable modes and violation of the Born
stability criteria at high pressure, which may be lead to instability in YVO4
at high pressures. We also calculate large anisotropy in the thermal expansion
behavior which arises from difference in anisotropic elasticity and mode
Gr\"uneisen parameters.
