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.
@article{arxiv.2102.08541,
title = {Negative linear compressibility and unusual dynamic behaviors of NaB3},
author = {Xin-Ling He and Shu-Ning Pan and Yue Chen and Xiao-Ji Weng and Zifan Wang and Dongli Yu and Xiao Dong and Jian Sun and Yongjun Tian and Xiang-Feng Zhou},
journal= {arXiv preprint arXiv:2102.08541},
year = {2021}
}