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Melting Curves and Phase Transitions of the Ammonium Halides to 40 kbar

1969; American Institute of Physics; Volume: 50; Issue: 3 Linguagem: Inglês

10.1063/1.1671208

1520-9032

Carl W. F. T. Pistorius,

Chemical Thermodynamics and Molecular Structure

The phase diagrams of the ammonium halides were studied to 40 kbar by means of differential thermal analysis. The present work confirms the recent piston-displacement study of the phase diagram of NH4F. The NH4F II–IV–liquid triple point is probably located at 16.8 kbar, 309°C. The melting curve of NH4F III rises to 529°C at 40 kbar. The present λ-transition curves of NH4Cl and ND4Cl are in good agreement with earlier hydrostatic work at low pressures. There are indications that the λ transition may change into a first-order transition above ∼ 30 kbar. A pure octopole–octopole interaction appears to be insufficient to predict the course of the transition line at higher pressures. NH4Cl, NH4Br, and NH4I melt at 520°, 523°, and 495°C, respectively, at ∼ 0.2 kbar. The melting curves rise slowly to the II–I–liquid triple points, and rapidly thereafter.