Title: Thermal Expansion of the Heavy-fermion Superconductor PuCoGa$_{5}$,
Abstract: We have performed high-resolution powder x-ray diffraction measurements on a
sample of $^{242}$PuCoGa$_{5}$, the heavy-fermion superconductor with the
highest critical temperature $T_{c}$ = 18.7 K. The results show that the
tetragonal symmetry of its crystallographic lattice is preserved down to 2 K.
Marginal evidence is obtained for an anomalous behaviour below $T_{c}$ of the
$a$ and $c$ lattice parameters. The observed thermal expansion is isotropic
down to 150 K, and becomes anisotropic for lower temperatures. This gives a
$c/a$ ratio that decreases with increasing temperature to become almost
constant above $\sim$150 K. The volume thermal expansion coefficient
$\alpha_{V}$ has a jump at $T_{c}$, a factor $\sim$20 larger than the change
predicted by the Ehrenfest relation for a second order phase transition. The
volume expansion deviates from the curve expected for the conventional
anharmonic behaviour described by a simple Grüneisen-Einstein model. The
observed differences are about ten times larger than the statistical error bars
but are too small to be taken as an indication for the proximity of the system
to a valence instability that is avoided by the superconducting state.