Double ReO₃-type fluorides exhibit greater structural flexibility and elemental tunability than their single ReO₃-type counterparts, drawing widespread interest in applications involving negative thermal expansion (NTE) and optics. However, their applications are extremely constrained by the scarcity of the materials themselves, the harsh preparation conditions, and the high cost of the requisite raw materials. In this work, we have successfully obtained the cubic phase CaTiF₆ more facilely with inexpensive CaCl₂ and H₂TiF₆. The ultraviolet–visible (UV–vis) absorption spectrum indicates a bandgap of 3.6 eV and semiconducting properties. Raman spectroscopy analysis reveals the structure from rhombic to cubic transformation around 138 K, accompanied by a significant isotropic NTE in the cubic phase, with a coefficient of α_l = −7.26 × 10⁻⁶ K⁻¹ (175–475 K). Theoretical calculations based on first principles indicate that the semiconducting properties originate from the hybridization of the 3d orbit of Ti and the 2p orbit of F. The driving force for NTE in CaTiF₆ comes from the transverse thermal vibrations of fluorine atoms, as confirmed by density functional theory (DFT) calculation. This research provides a novel, facile, and cost-effective synthetic way and reveals in depth the electronic properties and NTE mechanism of CaTiF₆, which dramatically promotes the development of Ti-based fluorides.

Due to the high structural flexibility and controllable thermal expansion, cubic double ReO₃-type negative thermal expansion (NTE) fluorides provide a solution for solving the prominent phenomenon of thermal expansion mismatch between materials. However, the expensive raw materials and complex synthesis steps limit its practical application. In this work, we have designed a more advantageous method for the synthesis of NTE material CaZrF₆, and it is expected to be generalized to the synthesis of other double ReO₃-fluorides. Intriguingly, a new orthorhombic phase CaZrF₆ has been synthesized via this method in a lower temperature. Unlike the strong isotropic NTE of the cubic phase CaZrF₆, the orthorhombic phase shows the strong anisotropic positive thermal expansion (PTE). The combined analysis of temperature-dependent X-ray diffraction (XRD), Raman spectra, and first-principles calculations shows that the low frequency phonon vibration mode with negative Grüneisen parameter in cubic CaZrF₆ are strongly correlated with the transverse thermal vibration of F atoms and dominates the NTE of the material.

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.

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 ( $R\overline{3}$) to cubic ( $Fm\overline{3}m$) structure at about 200 K, and a strong isotropic NTE (α_v = −15.78 × 10⁻⁶ K⁻¹) appears in the cubic phase. Lattice dynamics calculations from first-principles illustrate that the NTE is due to the transverse vibration of fluorine atoms excited by low-frequency phonons. This work not only broadens the NTE family of fluorides, but also provides a new facile and low-cost fabricati on method for the preparation of NTE fluorides.