Neutron diffraction study of structural changes in ammonium halides under high pressure

Structural changes in deuterated ammonium halides ND4Cl and ND4Br at pressu res up to 40 kbar and ND4I at pressures up to 86 kbar have been studied by means of neutron diffraction at room temperature. The equations of state an d the pressure dependencies of the deuterium position parameter were obtain ed. It was found that the order-disorder II-IV phase transition from the di sordered CsCl - type cubic phase TT into the ordered CsCl-tppe cubic phase IV occurs ar equal critical values of the deuterium position parameter u(cr ) = 0.152(2) in ND4Br and ND4Cl. For ND4I, no II-IV phase transition was ob served. A phase transition into the high-pressure phase V was detected in N D4I at 80 < P < 86 kbar. Structure of ND4I(V) was found to be the same as t he structure of the low-temperature phase ND4I(III) - tetragonal one with a ntiparallel ordering of ammonium ions, space group P4/nmm. The critical val ue of the position parameter for the II-V phase transition in ND4I u = 0.14 8(2) is close to the u(cr) for II-IV transition in ND4Cl and ND4Br. (C) 199 9 Elsevier Science B.V. All rights reserved.