Thermal Expansion of the Heavy-fermion Superconductor PuCoGa5

We have performed high-resolution powder x-ray-diffraction measurements on a sample of $^{242}\mathrm{PuCoGa}_{5}$, the heavy-fermion superconductor with the highest critical temperature of ${T}_{c}=18.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The results show that the tetragonal symmetry of its crystallographic lattice is preserved down to 2 K. Marginal evidence is obtained for an anomalous behavior 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 $\ensuremath{\sim}150\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The volume thermal expansion coefficient ${\ensuremath{\alpha}}_{V}$ has a jump at ${T}_{c}$, a factor $\ensuremath{\sim}20$ 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 behavior described by a simple Gr\"uneisen-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.