Thermal Signatures of the Kondo Volume Collapse in Cerium

X-ray diffraction measurements of cerium in the vicinity of the isostructural γ-α transition have been performed with high precision and accuracy from room temperature to almost 800 K. The disputed location of the critical point has been found to occur at 1.5±0.1 GPa and 480±10 K. The data are well fit by the Kondo volume collapse model plus a quasiharmonic representation of the phonons. The resultant free energy is validated against data for the thermodynamic Gr\"uneisen parameter and, beyond the dominant spin-fluctuation contribution, indicates a dramatic change in the lattice Gr\"uneisen parameter across the transition.