MELTING CURVES AND PHASE TRANSITIONS OF AMMONIUM HALIDES TO 40-KBAR

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 NH 4F 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 X-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-Iiquid triple points, and rapidly thereafter.