Negative linear compressibility and unusual dynamic behavior of NaB 3

First-principles calculations reveal that sodium boride $({\mathrm{NaB}}_{3})$ undergoes a phase transition from a tetragonal $P4/mbm$ phase to an orthorhombic $Pbam$ phase at $\ensuremath{\sim}16$ 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 ${\mathrm{NaB}}_{3}$ compound sheds light on a material with a combination of NLC and ion transportation at extreme conditions.