  We have studied the temperature dependence of crystal and magnetic structures
of the Jahn-Teller distorted transition metal difluorides CrF$_2$ and CuF$_2$
by neutron powder diffraction in the temperature range 2-280 K. The lattice
parameters and the unit cell volume show magnetoelastic effects below the
N\'eel temperature. The lattice strain due to the magnetostriction effect
couples with the square of the order parameter of the antiferromagnetic phase
transition. We also investigated the temperature dependence of the Jahn-Teller
distortion which does not show any significant effect at the antiferromagnetic
phase transition but increases linearly with increasing temperature for CrF$_2$
and remains almost independent of temperature in CuF$_2$. The magnitude of
magnetovolume effect seems to increase with the low temperature saturated
magnetic moment of the transition metal ions but the correlation is not at all
perfect.
