Chemistry Reference
In-Depth Information
a
b
Fig. 12 The [NaAlSi] subarray of the high temperature phase of NaAlSiO
4
(carnegieite). (a)
Drawing of the diamond-like skeleton formed by both Al (
light grey
) and Si (
dark grey
) atoms.
Brown spheres
represent Na atoms. (b) Drawing of the fluorite-like structure formed by the Na and
Al atoms when filling the tetrahedral voids of the fcc-Si array. Both Na and Al atoms form the
cubes which are filled alternately by the Si atoms
the formation of the NaAlSi structure. The result is that the Al atoms are forming,
like Fe in FeLiP, 4
4
planar nets of dimensions 2.92
˚
, similar to the (1 0 0) faces of
fcc
-Al, with Al-Al distances of 2.86
˚
, whereas the Na blocks form fragments
of
bcc
-Na, whose dimensions of
a
3.43
˚
, are in agreement with those
¼
4.13,
d
¼
The conclusion is that in the absence of any compatible NaSi phase, the Na
atoms form blocks of their own structure. Thus, NaAlSi is thus the result of the
simultaneous existence of Al metal, Na metal and the
of the elemental Na with
a
¼
4.21,
d
¼
-Si structures. On the
contrary, in FeLiP, Fe metal, LiCl and FeS co-exist in a “synergic” way.
The interpretation we have just made of the NaAlSi structure probably needs
additional support, which can be provided by the related oxide NaAlSiO
4
and,
specially, by its high temperature phase, the mineral carnegieite [
39
]. The structure
is represented in Fig.
12a
in which the O atoms have been omitted for clarity. The
structure is cubic (
C
7.30
˚
) and the AlSi subarray is forming an adaman-
tane skeleton with the O atoms midway between the Al and Si atoms. Thus, the
structure can be described as Na-stuffed cristobalite-like structure, with the [AlSi]
subarray forming a
P
2
1
3,
a
¼
C
.
In Fig.
12b
, we have represented the same structure but where its relationship
with the fluorite structure has been emphasized. The Na and Al atoms are filling all
the tetrahedral voids of the
fcc
-Si array, so that when the Na and Al atoms are
connected, they form the cubes characteristic of the CaF
2
structures.
Two features are worthy of mention, i.e. the first one is that this array is that of
the related Zintl phase LiAlSi. The second one is that high carnegieite is a
metastable phase which can be maintained up to 963 K. The structure was deter-
mined at 1,023 K [
39
].
The conclusion we can extract is that, like in NaAlSi, also in NaAl[SiO
4
], the Al
and Si atoms are forming the corresponding four-connected network. The alternative
interpretation, consisting in that the Na-Si pair of atoms forms a NaCl-type structure,
with Si at (0,0,0) and Na at (½, 0, 0) does not take place. Also in this case, the reason is
that the Al atoms are not capable of transforming the Si atoms into
-Si structure, as deduced from the
ZKC
C
-Cl. Instead, both
