NEUTRON DIFFRACTION STUDY OF STRUCTURAL CHANGES IN AMMONIUM HALIDES ND₄X (X=F, Cl, Br, J) UNDER HIGH PRESSURE

A.M.Balagurov¹, V.P.Glazkov², D.P.Kozlenko¹, B.N.Savenko¹, V.A.Somenkov², S.Sh.Shilstein²

¹ JINR, Frank Laboratory of Neutron Physics, 141980 Dubna, Russia.
² Russian Scientific center "Kurchatov Institute", 123182, Moscow, Russia.

Structural changes in ammonium halides ND₄X, (X = F, Cl, Br, J) 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 in which the ammonium tetrahedra are randomly oriented (CsCl-type cubic structure, space group Pm3m) into the phase in which the ammonium tetrahedra are oriented in parallel (CsCl-type cubic structure, space group P43m), occures at equal critical values of the position parameter u_cr=0.1530.002 in ND₄Br and ND₄Cl. For ND₄J, the position parameter do not reach critical value u_cr and this phase transition was not observed. For ND₄F, the position parameter is initially higher than u_cr and only fully ordered CsCl type phase exists at high pressure.

The work was supported by Russian Foundation of Basic Research, grants N 97-02-16622 and N 97-02-17587.