Neutron scattering study of structure and dynamics of ammonium halides under high pressure

Complete text of publication follows. Structural changes in ammonium halides ND4Cl, ND4Br and ND4F at pressures up to 40 kbar and ND4I at pressures up to 86 kbar have been studied bz means of neutron diffraction at room temperature. The pressure dependencies of the lattice parameter a and the deuterium position parameter u were obtained. It was found that the order - disorder II-IV phase transition in ND4Br and ND4Cl occurs at equal critical values of u, ucr=0.152(2). For ND4F, u is initially higher than ucr and only the ordered CsCl type phase III exists at high pressure. For ND4I, no II-IV phase transition was observed. A phase transition into the recently discovered high pressure phase V was detected in ND4I at 80 < P < 86 kbar. The structure of ND4I(V) was found to be the same as the structure of the low temperature phase ND4I(III) - tetragonal one with antiparallel ordering of ammonium ions, space group P4/nmm. Vibrational spectra of NH4I and HN4F have been studied by means of incoherent inelastic neutron scattering at pressure up to 40 kbar. Vibration and transverse optical (TO) modes frequencies as functions of pressure were obtained. Both frequencies increase under pressure. (author)