Controllable Syntheses, Crystal Structure Evolution, and Photoluminescence of Polymorphic Zirconium Oxyfluorides.

Precise synthesis of polymorphic phases with similar components but distinct crystal structures is one of the key problems in inorganic chemistry. In this work, we report a fluorination method adopting ZrO2 as the starting material and NH4F as the fluoridation agent that can afford multiphases in the Zr-O-F system, including Zr7O9F10, Zr3O2F8, ZrO0.46F3.08, ZrO0.33F3.33, β-ZrF4, NH4Zr2F9, and NH4ZrF5. A preliminary phase formation diagram was established as a function of the fluorination temperature (T), reaction time (t), and F/Zr ratio after systematic optimization of the preparation conditions. Among the as-obtained phases, the detailed crystal structures of Zr7O9F10 and ZrO0.33F3.33 were refined based on the powder X-ray diffraction patterns. As the F/O ratio increases, the crystal structures of Zr-O-F phases transform gradually from an anion-deficient α-UO3-related structure of Zr7O9F10 to an anion-excess ReO3-related structure of ZrO0.33F3.33. At last, we also prepared Ti-doped ZrO2, Zr7O9F10, ZrO0.46F3.08, and ZrO0.33F3.33 to study the host-lattice-dependent photoluminescence properties of zirconium oxyfluorides. The four materials show distinct photoluminescence in the UV and visible regions due to different local coordination environments of Zr/Ti. This work demonstrates the low-temperature fluorination method as an efficient route to phase-selective polymorphic metal oxyfluorides, which can be employed in further structure-property relationship studies.