Structural study of ND4I at high pressures and low temperatures

Structure, positional, and thermal parameters of ND4I were studied at high pressures up to 90 kbar and low temperatures down to 10 K using time-of-fli ght neutron diffraction. The phase transition from a disordered CsCl-type c ubic phase ND4I(II) into a recently discovered high pressure phase ND4I(V) was observed at P = 80(5) kbar. Surprisingly, the structure of the high pre ssure phase V was found to bear a strong resemblance to that of the ambient pressure, low-temperature phase III - tetragonal structure with an antipar allel ordering of ammonium ions, space group P4/nmm. The critical value of the deuterium positional parameter corresponding to the II-V transition is close to the one for the phase transition between the disordered and ordere d CsCl-type cubic phases II and IV in other ammonium halides.