Simple chemical synthesis and isotropic negative thermal expansion in MHfF₆ (M = Ca, Mn, Fe, and Co)

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.