Fluoridation of HfO2

Fluoridation of HfO2 was carried out with three commonly used solid-state fluoridation agents: PVDF, PTFE, and NH4HF2. Clear and reproducible differences are observed in the reaction products of the fluoropolymer reagents and NH4HF2 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-NH4HF2 reaction. Synchrotron XRD, 19F 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-UO3 structures such as Zr7O9F10. Comparison with the hydrolysis of β-HfF4 under identical conditions depicts that the NH4HF2 route produces the oxyfluoride with greater selectivity and at lower temperatures. Thermodynamic calculations were employed to explain this result. Potential reaction pathways for the NH4HF2 fluoridation of HfO2 are discussed.