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Samarium hexaboride is an anomaly, having many exotic and seemingly mutuallyincompatible properties. It was proposed to be a mixed-valent semiconductor,and later - a topological Kondo insulator, and yet has a Fermi surface despitebeing an insulator. We propose a new and unified understanding of SmB$_6$centered on the hitherto unrecognized dynamical bonding effect: the coexistenceof two Sm-B bonding modes within SmB$_6$, corresponding to different oxidationstates of the Sm. The mixed valency arises in SmB$_6$ from thermal populationof these distinct minima enabled by motion of B. Our model simultaneouslyexplains the thermal valence fluctuations, appearance of magnetic Fermisurface, excess entropy at low temperatures, pressure-induced phasetransitions, and related features in Raman spectra and their unexpecteddependence on temperature and boron isotope.

Dynamical Bonding Driving Mixed Valency in a Metal Boride