Melting curve and high-pressure polymorphism of NH 4HF 2

The melting curve of NH ₄HF ₂ I rises from 125.2°C at atmospheric pressure to a triple point II/I/liquid at 9.3 kbar, 220°C. The I/II phase boundary is terminated at a triple point III/I/II at ∼45 kbar, 295°C. The melting curve of the new phase NH ₄HF ₂ II passes through a broad maximum at ∼39 kbar, 306°C, and is terminated at a triple point III/II/liquid at 46.3 kbar, 301°C. The melting curve of NH ₄HF ₂ III rises with pressure. The NH ₄HF ₂ III may be a dense hydrogen-bonded phase. Liquid NH ₄HF ₂ appears to be anomalous in several respects, and has a high compressibility relative to the solid phases.