Fig. 2. Schematic phase diagrams for strongly correlated f-electron intermetallics.

Temperature T versus control parameter δ phase diagram for the compounds (A) CeT₂X₂ (5, 6), (B) YbT₂X₂ (5, 6), and (C) EuT₂X₂ (36, 37), where T = transition metal and X = p-block element. Here, δ is the chemical composition x or pressure P. The trends for the magnetic ordering temperatures T_N,C, superconducting transition temperatures T_c, valence change transition temperatures T_V, and Fermi liquid crossovers T_FL that are observed in these phase diagrams are described in the text. The solid lines represent phase boundaries, whereas the dotted lines represent crossover regions: e.g., temperature ranges over which a system enters a Fermi liquid or intermediate valence (IV) ground state. The f-electron valences are also indicated (3+ or 4+ for cerium, 2+ or 3+ for ytterbium, and 2+ or 3+ for europium). Note that these diagrams focus on the critical regions near δ_c, but the more general Doniach phase diagram includes a region where the ordering temperatures increase with δ as the RKKY interaction strengthens, before collapsing toward T = 0 at δ_c (14). This trend is seen in the ordering temperatures presented in Fig. 3.