Universality, Tricriticality, and the Potts Transition in First-Stage Lithium-Intercalated Graphite

With use of ac calorimetry and neutron diffraction, the lithium sublattice melting transition in ${\mathrm{C}}_6$Li, which belongs to the universality class of the discrete three-dimensional, three-state Potts model, has been discovered and studied. Strong precursive effects surround the (weak) first-order transition at 715±2 K. Fitting power laws away from the discontinuous region yields $\alpha =0.50\mathrm{}\pm 0.05$ and $\beta =0.22\mathrm{}\pm 0.03$. These values are consistent with three-dimensional tricritical exponents and may reflect conjectured tricritical behavior.