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 ucr=0.1530.002
in ND4Br and ND4Cl. For ND4J,
the position parameter do not reach critical value ucr
and this phase transition was not observed. For ND4F,
the position parameter is initially higher than ucr
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.