Reflections on Sixty Years of Solid State Chemistry

In celebration of Rudolf Hoppe's 90th birthday, three developments in solid state chemistry that have occurred over his tenure are personally recalled: (1) The recognition that long‐range orbital order‐disorder transitions introduce cooperative electron‐lattice interactions has (a) accounted for numerous phase transitions in solids, (b) led to formulation of the rules for the sign, ferromagnetic vs. antiferromagnetic, of interatomic spin‐spin interactions, and (c) provides a rationale for single‐ion magnetostriction and magnetocrystalline anisotropy in transition‐metal compounds. Moreover, the further recognition that short‐range cooperative orbital ordering extends to temperatures well above the long‐range orbital ordering temperature or critical concentration of orbitally degenerate cations has (a) led to the introduction of chemical inhomogeneties into a single crystallographic phase to achieve a technical objective and (b) consideration of their role in determining the sign of spin‐spin interactions in a paramagnetic phase containing orbital fluctuations. (2) Exploration of the transition from localized to itinerant 3d‐electron behavior in transition‐metal oxides showed it to be first‐order, which has allowed interpretation of (a) charge‐density waves in single‐valent systems and (b) dynamic phase segregations in mixed‐valent systems to be manifestations of a different type of long‐range vs. short‐range cooperative electron‐lattice coupling. (3) The study of Li‐insertion chemistry has led (a) to Li‐ion rechargeable batteries, (b) an appreciation of the influence of counter cations in solids on d ‐electron redox energies, and (c) the concept of Fermi energy pinning and its consequences as the energy of a d‐ electron couple in a transition‐metal compound descends across the top of the anion‐ p bands.