Fluoridation of HfO 2 .

Fluoridation of HfO ₂ was carried out with three commonly used solid-state fluoridation agents: PVDF, PTFE, and NH ₄ HF ₂ . Clear and reproducible differences are observed in the reaction products of the fluoropolymer reagents and NH ₄ HF ₂ with the latter more readily reacting in air. Strong evidence of distinct, previously unreported hafnium oxyfluoride phases is produced by both reactions, and efforts to isolate them were successful for the air-NH ₄ HF ₂ reaction. Synchrotron XRD, ¹⁹ F NMR, and elemental analysis were employed to characterize the phase-pure material which appears to be analogous to known Zr-O-F phases with anion-deficient α-UO ₃ structures such as Zr ₇ O ₉ F ₁₀ . Comparison with the hydrolysis of β-HfF ₄ under identical conditions depicts that the NH ₄ HF ₂ route produces the oxyfluoride with greater selectivity and at lower temperatures. Thermodynamic calculations were employed to explain this result. Potential reaction pathways for the NH ₄ HF ₂ fluoridation of HfO ₂ are discussed.