  First-principles calculations reveal that sodium boride (NaB3) undergoes a
phase transition from a tetragonal P4/mbm phase to an orthorhombic Pbam phase
at about 16 GPa, accompanied by 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 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 (or rotation) behavior, which may be originated from the
asymmetric energy barriers of different diffusion paths. The study of NaB3
compound sheds new light on a material with the combination of NLC and ion
transportation at extreme conditions.
