Phase Transitions in the Ammonium Halides

Abstract
High-pressure experiments, using the piston displacement technique have been performed at temperatures down to that of liquid nitrogen on NH4I, NH4Br, ND4Br, and NH4Cl. For the iodide two new phase transitions under pressure and a new triple point were observed. A new transition and a new triple point were observed in both the bromides; the existence of another transition was confirmed. The role of multipole interactions in causing the transitions is discussed, and on the basis of structural data and the present experiments a generalized phase diagram is presented which describes the gross behavior of the transitions in these substances.