Fluoridation of HfO2

Abstract

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.