Chapter 19: Metallic Structures III: Rare Earth–Transition Metal Systems

The fundamental basis for understanding these materials lies in the significant atomic size differences between large rare earth elements and smaller transition metals, which constrains the formation of discrete stoichiometric line compounds rather than extensive solid solution ranges. The text begins with cubic Laves phase structures, including the MgCu2 and MgZn2 prototypes, and discusses the exceptional magnetostrictive behavior displayed by Terfenol-D compositions. Progression continues through derived cubic structures such as UNi5, Th6Mn23, and LaCo13, where specific coordination geometries including icosahedral polyhedra determine the overall lattice symmetry. A critical portion addresses hexagonal phases that govern magnetic coercivity performance, particularly the SmCo5 structure derived from the CaCu5 archetype, where rare earth atoms occupy distinct crystallographic sites. The chapter details the dumbbell substitution mechanism, a process in which pairs of transition metal atoms progressively replace individual rare earth atoms, systematically converting 1:5 stoichiometric compounds into the hexagonal Th2Ni17 and rhombohedral Th2Zn17 structures with 2:17 composition ratios. The tetragonal Nd2Fe14B phase receives substantial coverage due to its technological dominance in contemporary permanent magnet applications, alongside discussion of monoclinic 3:29 phases that exhibit hybrid atomic stacking patterns. The concluding material focuses on interstitial engineering strategies, wherein small atoms such as nitrogen or carbon occupy interstitial lattice positions, generating lattice parameter expansion and substantially augmenting magnetic performance metrics including Curie temperature elevation and increased saturation magnetization values.