Abstract
First-principles calculations reveal that sodium boride undergoes a phase transition from a tetragonal phase to an orthorhombic phase at GPa, accompanied by a counterintuitive lattice expansion along the crystallographic axis. This unusual compression behavior is identified as negative linear compressibility (NLC), which is dominantly attributed to the symmetry breaking of the boron framework. Meanwhile, the and phases form superionic conductors after undergoing a peculiar swap state at high temperature. Specifically, under “warm” conditions the Na cation pairs exhibit a rare local exchange (rotation) behavior, which may be originated from the asymmetric energy barriers of different diffusion paths. The study of the compound sheds light on a material with a combination of NLC and ion transportation at extreme conditions.
- Received 28 September 2020
- Revised 27 January 2021
- Accepted 22 February 2021
DOI:https://doi.org/10.1103/PhysRevMaterials.5.035002
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