The rapid progress of modern technologies has accelerated the prominence of thermal expansion mismatch between materials, and tunable thermal expansion materials will be a powerful safeguard against this challenge. Here, isotropic MHfF6 (M = Ca, Mn, Fe, and Co) compounds with tunable thermal expansion have been produced via a low-cost synthetic method and investigated. By utilizing temperature dependent X-ray diffraction (XRD) and Raman spectroscopy, combined with first principles calculations, it was revealed that the transverse thermal vibrations of the F atoms are dominated by low-frequency phonons with negative Grüneisen parameters and are therefore the origin of the negative thermal expansion (NTE). Very interestingly, with the increase of the M atomic number, the metal···F atomic linkages become stiffer, reducing the number of vibrational modes with negative Grüneisen parameters, so that the strong NTE can be gradually adjusted to moderate NTE and to near zero thermal expansion. The present study achieves the tunable thermal expansion in a new compound family and shed light on the internal mechanism from the perspective of lattice vibrational dynamics.
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Double ReO3-type fluorides have a great interest in the field of negative thermal expansion (NTE) and luminescent materials. However, their application is limited by the scarcity of quantity, expensive raw materials, and harsh preparation conditions. In this work we have found a new NTE material, CaSnF6, by applying the concept of the average atomic volume. More importantly, different from the previous solid-phase sintering and direct fluorination methods, the nano CaSnF6 has been synthesized for the first time by solvothermal method. The results of X-ray diffraction (XRD) and Raman spectroscopy show that a phase transition occurs from rhombohedral (