Powder neutron diffraction and molecular dynamics (MD) simulations
have been used to investigate the structural behaviour of
silver sulfide, Ag2S, at elevated temperatures. Above
~450 K Ag2S adopts the β phase in which the
S2- possess a body-centred cubic arrangement.
Analysis of the neutron diffraction is in good agreement with
the previously proposed structural model in which the Ag+
predominantly reside within the tetrahedral interstices. At
~865 K Ag2S transforms to the α phase in
which the anion sublattice adopts a face-centred cubic
arrangement. Structural refinements of this phase indicate that
the cations are distributed predominantly in the tetrahedral
cavities but with a significant fraction in the octahedral
holes. MD simulations, using established
potentials for this compound, confirm the stability of the two
high-temperature superionic phases and show good agreement with
the measured Ag+ distribution within the unit cell.
The nature of the
superionic transition in Ag2
HgI4
and Cu2
HgI4
has been investigated using temper dependent powder neutron diffraction and impedance spectroscopy techniques. In the case of Ag2
HgI4
, the superionic
transition occurs at Tc
= 326(2) K and is accompanied by a 50-fold increase in the ionic conductivity. In the Cu+
analogue, which has a lower conductivity for a given temperature, the corresponding values are Tc
= 338(4) K and
/
~ 6. The ambient temperature crystal structures of the two compounds are different (space group I
for
-Ag2
HgI4
and I
2m
for
-Cu2
HgI4
) but, in contrast to the most recent study, the high temperature polymorphs are found to be isostructural (space group F
3m
). Possible explanations for the different behaviour of the ionic conductivity of the two compounds are given.
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