It is pointed out that interactions between octahedral-site cations are cation-anion-cation interactions if the cation-occupied octahedra share a common corner, but may be primarily cation- -cation (no anion intermediary) if the cation-occupied octahedra share either a common edge or a common face. It is further pointed out that cation- -cation interactions may be naturally classified as "strong" or "weak" since there is a critical cation separation below which the interacting electrons are best described by a collective-electron model, above which by a Heitler-London model. The characteristics of the different interactions under varying conditions are summarized. In the case of strong cation- -cation interactions, covalent-type bonds may be formed at low temperatures. The resulting phase transitions are marked by the following features: (1) the transitions may be noncooperative (isolated cation-cation pairing introducing no symmetry change to the structure) and extend over a considerable temperature interval ($\Delta T\sim 100°$C), or cooperative, occurring at a definite temperature and exhibiting thermal hysteresis. (2) Bonded cations are displaced from the center of symmetry of their anion interstice (in contrast to Jahn-Teller or spin-orbit distortions). (3) Bonding electrons are spin-paired so that they make no contribution to the atomic moment unless localized, unpaired $d$ electrons are simultaneously present to weaken the covalent-type bond via intra-atomic exchange. (4) Bonding electrons cannot contribute to metallic-type conductivity so that if the bond-forming phase ties up all of the outer $d$ electrons in covalent-type bonds, the transition is semiconducting ⇌ metallic. The physical properties of several compounds that illustrate the importance of the cation- -cation interactions are discussed.

• Received 29 September 1959

DOI:https://doi.org/10.1103/PhysRev.117.1442