Chemistry Reference
In-Depth Information
a
b
c
Na
S
O
Fig. 2 (a) The structure of V-Na
2
SO
4
(thenardite) showing its similarity with that of TiSi
2
(b).
In thenardite, the Na and S atoms are represented by
green
and
yellow spheres
, respectively. The O
atoms (
red spheres
) are bonded to S atoms to identify the SO
4
groups. In TiSi
2
, the Si and Ti atoms
are represented by
grey
and
green spheres
, respectively. The Ti atoms, connected by
red lines
,
Purple
and
yellow spheres
are Rb and S atoms, respectively. The adamantane structure of S atoms
is drawn with
green lines
of V-Na
2
SO
4
and Rb
2
S (all of them
ddd).
The structural coincidence of thenardite (V-Na
2
SO
4
) and Rb
2
S provides a new
example of how HP phases of lighter elements appear as stable phases, under
ambient conditions, for heavier elements of the same group. Thus, if the TiSi
2
-
type structure is predicted for Rb
2
S at relatively low pressures, the same structure
should be obtained at higher pressures for Na
2
S. Consequently, the Na
2
S subarray
of thenardite might well be regarded as a Na
2
S alloy (of the TiSi
2
type) which is
stabilized by the pressure exerted by the O atoms.
F
2.2 Two Parallel Transitions: Olivine
!
Thenardite
!
Spinel
and Ni
2
In
!
TiSi
2
!
MgCu
2
The discussion in the above subsection indicates that the TiSi
2
-type structure might
well be an intermediate step in the Ni
2
In
MgCu
2
transition in the same manner
that thenardite could be an intermediate phase in the olivine
!
!
spinel transition.
olivine has been
ambient conditions, transforms to thenardite at 873 K which then converts into
Interestingly, the double transition spinel
!
thenardite
!
Ni
2
In
transitions have not been reported so far. However, theoretical calculations are in
progress to see whether this double transition could take place in any binary
AB
2
compound.
TiSi
2
!
