Uniaxial Negative Thermal Expansion in γ-LiBO2 with a Closed-Framework Diamond-Like Structure

https://pubs.acs.org/doi/full/10.1021/acs.chemmater.4c01205

Negative-thermal-expansion (NTE) materials violate the common knowledge of “thermal expansion and cold contraction” in solids and embrace various physical mechanisms. In most phonon-driven NTE materials, an open-framework structure is necessary to accommodate the spatially anisotropic phonon excitations of the bridged atoms, but such a structural feature may result in structural instability at a high temperature. Herein, we focus on γ-LiBO₂ with a closed-framework diamond-like structure and identify its uniaxial NTE behavior over the largest temperature range (100−850 K) among this structural family. As the temperature increases, the synergetic structural modification of the constituent structural groups, i.e., the stretching and bending of the Li−O bonds in floppy [LiO₄] and the tension or rotation in the [BO₄] group, accounts for NTE along the c-axis. Our study unveils that, apart from the anisotropic phonon excitations of individual atoms, the preferred phonon excitations of structural groups are also able to generate NTE, which would update the understanding of the NTE mechanism and guide the further exploration of phonon-driven NTE materials.