Localized Symmetry Breaking for Tuning Thermal Expansion in ScF 3 Nanoscale Frameworks.

The local symmetry, beyond the averaged crystallographic structure, tends to bring unusual performances. Negative thermal expansion is a peculiar physical property of solids. Here, we report the delicate design of the localized symmetry breaking to achieve controllable thermal expansion in ScF3 nanoscale frameworks. Intriguingly, an isotropic zero thermal expansion is concurrently engineered by localized symmetry breaking, with a remarkably low coefficient of thermal expansion of about +4.0 × 10-8 /K up to 675 K. This mechanism is investigated by the joint analysis of atomic pair distribution function of synchrotron X-ray total scattering and extended X-ray absorption fine structure spectra. A localized rhombohedral distortion presumably plays a critical role in stiffening ScF3 nanoscale frameworks and concomitantly suppressing transverse thermal vibrations of fluorine atoms. This physical scenario is also theoretically corroborated by the extinction of phonon modes with negative Grüneisen parameters in rhombohedral ScF3 . The present work opens an untraditional chemical modification route to achieve controllable thermal expansion by breaking local symmetries in materials.