Towards a Single Chemical Model for Understanding Lanthanide Hexaborides.

Lanthanide hexaborides (LnB ) have disparate and often anomalous properties, from structurally homogeneous mixed valency, to superconductivity, spectral anomalies, and unexplained phase transitions. It is unclear how such a diversity of properties may arise in the solids of identical crystal structures and seemingly very similar electronic structures. Building on our previous model for SmB (mixed valent, with a peak in specific heat, and pressure induced magnetic phase transitions), we present a unifying dynamic bonding model for LnB that explains simultaneously EuB (possessing an anomalous peak in specific heat at low T, magnetic phase transitions, and no mixed valency), YbB (mixed valent topological insulator), and rather ordinary LaB . We show that Ln can engage in covalent bonding with boron, and, in some members of the LnB family, also easily access alternative bonding states through the electron-phonon coupling. The accessibility, relative energetics, and bonding nature of the states involved dictate the properties.