Dynamical Bonding Driving Mixed Valency in a Metal Boride

Paul J Robinson; Julen Munarriz; Michael E Valentine; Austin Granmoe; Natalia Drichko; Juan R Chamorro; Priscila F Rosa; Tyrel M McQueen; Anastassia N Alexandrova

doi:10.1002/anie.202000945

Dynamical Bonding Driving Mixed Valency in a Metal Boride

Angew Chem Int Ed Engl. 2020 Jun 26;59(27):10996-11002. doi: 10.1002/anie.202000945. Epub 2020 Apr 6.

Authors

Paul J Robinson^{1

2}, Julen Munarriz¹, Michael E Valentine³, Austin Granmoe³, Natalia Drichko³, Juan R Chamorro^{3

4}, Priscila F Rosa⁵, Tyrel M McQueen^{3

4

6}, Anastassia N Alexandrova^{1

7}

Affiliations

¹ Department of Chemistry and Biochemistry, University of California Los Angeles, Los Angeles, CA, 90095, USA.
² Current Address: Department of Chemistry, Columbia University, New York, NY, 10027, USA.
³ Institute for Quantum Matter, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD, 21218, USA.
⁴ Department of Chemistry, The Johns Hopkins University, Baltimore, MD, 21218, USA.
⁵ Los Alamos National Laboratory, Los Alamos, NM, 87545, USA.
⁶ Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD, 21218, USA.
⁷ California NanoSystems Institute, Los Angeles, CA, 90095, USA.

PMID: 32202032
DOI: 10.1002/anie.202000945

Abstract

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 SmB₆ centered 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 SmB₆ from 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.

Keywords: Raman spectroscopy; chemical bonding; mixed valency; pressure induced phase transitions; samarium hexaboride.

Abstract

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