Atomic Linkage Flexibility Tuned Isotropic Negative, Zero, and Positive Thermal Expansion in MZrF₆ (M = Ca, Mn, Fe, Co, Ni, and Zn)

The controllable isotropic thermal expansion with a broad coefficient of thermal expansion (CTE) window is intriguing but remains challenge. Herein we report a cubic MZrF₆ series (M = Ca, Mn, Fe, Co, Ni and Zn), which exhibit controllable thermal expansion over a wide temperature range and with a broader CTE window (-6.69 to +18.23 × 10^-6/K). In particular, an isotropic zero thermal expansion (ZTE) is achieved in ZnZrF₆, which is one of the rarely documented high-temperature isotropic ZTE compounds. By utilizing temperature-dependent high-energy synchrotron X-ray total scattering diffraction, it is found that the flexibility of metal···F atomic linkages in MZrF₆ plays a critical role in distinct thermal expansions. The flexible metal···F atomic linkages induce negative thermal expansion (NTE) for CaZrF₆, whereas the stiff ones bring positive thermal expansion (PTE) for NiZrF₆. Thermal expansion could be transformed from striking negative, to zero, and finally to considerable positive though tuning the flexibility of metal···F atomic linkages by substitution with a series of cations on M sites of MZrF₆. The present study not only extends the scope of NTE families and rare high-temperature isotropic ZTE compounds but also proposes a new method to design systematically controllable isotropic thermal expansion frameworks from the perspective of atomic linkage flexibility.