Ground-state structure of sodium boride

Boron-rich solids are an important class of materials, combining high-temperature superconductivity, superhardness, ferromagnetism and quantum topological properties, which attracted broad interest all along owing to many interesting fundamental issues and huge potential applications. Binary borides had been a subject of extensive research. However, it is inconceivable that the exact compositions and crystal structures of sodium borides remained controversial even that sodium boride (Na2B30) was first synthesized about 50 years ago. Now an international team led by Profs. Xiang-Feng Zhou, Xiao Dong at Nankai University, and Prof. Artem R. Oganov at Skolkovo Institute of Science and Technology presented an instructive case of controversy in this “simple” system, which showed that very different structures can have very similar XRD and TEM patterns, making structure determination ambiguous, and in such cases input from theory is invaluable. In brief, we found unusual structure and novel properties in sodium borides (Na2B30 or Na2B29), that is,

1. Discovery of new ground state phase I212121-Na2B30with unusual open-framework boron structure, which may have potential advanced applications.

2. Rediscovery of old experimental phaseImma-Na2B30 with novel topological property, which possesses superior transport properties.

This work was published in Phys. Rev. B as Rapid Communication [Phys. Rev. B 97, 100102 (R) (2018)].

Fig. 1 (a) Crystal structure of I212121-Na2B30. (b) Crystal structure of I212121-B30. (c) Crystal structure of Imma-Na2B30. (d) Nodal rings of Imma-Na2B30 in the Brillouin zone.