Chemistry Reference
In-Depth Information
Fig. 9 (a) The structures of Na
2
SO
3
, projected on (110). The Na
2
S subarray is identical to that of
the Ni
2
Al alloy represented in (b). (b) The structure of the Ni
2
Al alloy, projected on (110).
Purple
and
grey spheres
represent Ni and Al atoms, respectively
unique compound adopting this structural type. K
2
SO
3
and the high-temperature
phase of Cs
2
SO
4
(stable above 993 K), adopt also the same cation array. These
compounds will be discussed later. For the moment, the reader can visualize their
3.2 A Mechanism for the CaF
2
!
Ni
2
Al Transition
In the above section, we have briefly discussed the direct structural relationship
between both the CaF
2
and Ni
2
Al structures. The connection was established at a
qualitative level, by considering the Ni
2
Al array as a filled anti-fluorite structure.
However, both structures could be more strongly related if we were able to deduce a
plausible transition mechanism leading to the Na
2
S subarray (Ni
2
Al type) through a
simple displacement of atomic layers in the anti-fluorite Na
2
S.
T
h
e comparison betw
e
en both arrays is facilitated if we put both structures, CaF
2
(Fm3m) and Na
2
SO
3
(P3), on a common hexagonal frame. It is
we
ll known that a
fcc
-network contains implicit a primitive rhombohedral cell (
R
3m). In the anti-
4.61
˚
,
a
¼
60
, with S atoms at
(1
a
): (0, 0, 0) and Na atoms
a
t (2
c
): (1/4, 1/4, 1/4; 3/4, 3/4, 3/4). The corresponding
threefold hexagonal cell (
fluorite Na
2
S, the
R
cell has dimensions of
a
¼
11.29
˚
, with S
R
3m) has dimensions of
a
¼
4.61,
c
¼
atoms at (3
a
): (0, 0, 0) and Na atoms at (6
c
): (0, 0,
z
), with
z
¼
1/4. The anti-fluorite
structure referred to this
cell is represented in Fig.
10
.
Following the group-subgroup rela
ti
onship
s,
the sym
m
etry of the anti-fluorite
Na
2
S
can be lowered in the sequence
R
R
3m
!
P
3m1
!
P
3
!
P
3. Because Na
2
SO
3
is
P
3, the transformation Na
2
S (anti-fluorite)
!
Na
2
S (Ni
2
Al) will be carried out in
