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Lanthanide hexaborides (LnB₆) 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 SmB₆(mixed valent, with a peak in specific heat, and pressure induced magnetic phase transitions), we present a unifying dynamic bonding model for LnB₆that explains simultaneously EuB₆(possessing an anomalous peak in specific heat at low T, magnetic phase transitions, and no mixed valency), YbB₆(mixed valent topological insulator), and rather ordinary LaB₆. We show that Ln can engage in covalent bonding with boron, and, in some members of the LnB₆family, 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.

Lanthanide hexaborides (LnB₆) 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 SmB₆(mixed valent, with a peak in specific heat, and pressure induced magnetic phase transitions), we present a unifying dynamic bonding model for LnB₆that explains simultaneously EuB₆(possessing an anomalous peak in specific heat at low T, magnetic phase transitions, and no mixed valency), YbB₆(mixed valent topological insulator), and rather ordinary LaB₆. We show that Ln can engage in covalent bonding with boron, and, in some members of the LnB₆family, 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.

Samarium hexaboride is an anomaly, having many exotic and seemingly mutually incompatible properties. It was proposed to be a mixed‐valent semiconductor, and later a topological Kondo insulator, and yet has a Fermi surface despite being an insulator. We propose a new and unified understanding of SmB₆centered on the hitherto unrecognized dynamical bonding effect: the coexistence of two Sm−B bonding modes within SmB₆, corresponding to different oxidation states of the Sm. The mixed valency arises in SmB₆from thermal population of these distinct minima enabled by motion of B. Our model simultaneously explains the thermal valence fluctuations, appearance of magnetic Fermi surface, excess entropy at low temperatures, pressure‐induced phase transitions, and related features in Raman spectra and their unexpected dependence on temperature and boron isotope.

Samarium hexaboride is an anomaly, having many exotic and seemingly mutually incompatible properties. It was proposed to be a mixed‐valent semiconductor, and later a topological Kondo insulator, and yet has a Fermi surface despite being an insulator. We propose a new and unified understanding of SmB₆centered on the hitherto unrecognized dynamical bonding effect: the coexistence of two Sm−B bonding modes within SmB₆, corresponding to different oxidation states of the Sm. The mixed valency arises in SmB₆from thermal population of these distinct minima enabled by motion of B. Our model simultaneously explains the thermal valence fluctuations, appearance of magnetic Fermi surface, excess entropy at low temperatures, pressure‐induced phase transitions, and related features in Raman spectra and their unexpected dependence on temperature and boron isotope.