Neutron diffraction study of structural changes in ammonium halides ND4X (X = F, Br, Cl, I) under high pressure

Structural changes in CsCl - type cubic phases of ammonium halides ND4X, (X = F, Cl, Br, I) at pressures up to 40 kbar have been studied by means of neutron diffraction. The equations of state and the pressure dependencies of the deuterium position parameter were obtained. It was found that the order-disorder phase transition from the phase II in which ammonium ions are randomly distributed over two equivalent positions (CsCl - type cubic structure, space group Pm3m) into the phase IV in which ammonium ions are oriented in parallel (CsCl-type cubic structure, space group P (4) over bar 3m), occures at equal critical values of the position parameter u(cr)=0.153+/-0.002 in ND4Br and ND4Cl. For ND4I, the position parameter does not reach the critical value u(cr) and this phase transition was not observed. For ND4F, the position parameter is initially higher than u(cr) and only the ordered CsCl - type cubic phase exists at high pressure. Partial compressibilities of halogen ions were determined.