Synthesis, electronic property, and negative thermal expansion in cubic CaTiF₆

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.