Two-Dimensional Negative Thermal Expansion in a Crystal of LiBO₂

Negative thermal expansion (NTE), violating the common sense of “thermal expansion and cold contraction” effects, is a novel temperature-responding behavior of great scientific and technical significance. Herein, we report a two-dimensional (2D) NTE behavior in a crystal of LiBO2, which is constructed by graphite-like [LiBO2]∞ layers. This intriguing thermal property originates from the synergistic effect of the distortion of in-plane [LiO3] bases in [LiO4] tetrahedra and the rotation of [BO3] triangles in the [LiBO2]∞ layer, driven by the force perpendicular to the layer owing to the large interlayer separation as temperature increases. Remarkably, the in-plane and out-of-plane Li–O bonds within the [LiO4] tetrahedra have nearly the same bond strength and exhibit the similar variation with respect to temperature, and this is quite different from the common sense on the 2D NTE behavior in layered structures that the intralayer atomic interaction must be much stronger than the interlayer ones. Our study deepens the understanding of the 2D NTE mechanism and would promote the exploration for NTE materials.