Negative linear compressibility and unusual dynamic behaviors of NaB3

TL;DR

This study uses first-principles calculations to reveal negative linear compressibility and unusual ion transport behaviors in NaB3 under high pressure and temperature, highlighting its potential as a superionic conductor.

Contribution

It uncovers the phase transition, negative linear compressibility, and unique ion exchange phenomena in NaB3, providing new insights into its extreme condition behaviors.

Findings

NaB3 exhibits negative linear compressibility during phase transition.

NaB3 becomes a superionic conductor at high temperature.

Na cation pairs show rare local exchange behaviors.

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 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.