Negative linear compressibility and unusual dynamic behavior of NaB3

Abstract

First-principles calculations reveal that sodium boride (NaB3) undergoes a phase transition from a tetragonal P4/mbm phase to an orthorhombic Pbam phase at ∼16 GPa, accompanied by a counterintuitive lattice expansion along the crystallographic a 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 P4/mbm and Pbam 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 NaB3 compound sheds light on a material with a combination of NLC and ion transportation at extreme conditions. © 2021 American Physical Society.