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.

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.