Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 146

87.07 ), formed by only the
Na(1) and Na(3) atoms. These two representations of the structure are drawn in
Fig. 8a, b for comparison. However, the rhombohedra can be better observed in
Fig. 8c, d , where it can be seen that two-thirds of these Na rhombohedra are
occupied by the SO 3 groups, whereas one-third is filled by the Na(2) atoms.
If the O atoms are omitted, the Na and S atoms compose fragments of an anti-
fluorite-type structure with the S atom centering the distorted cube (compare Fig. 8c
and Fig. 1 ) . As discussed above, the remaining cubes (1/3), centred by the Na(2)
atoms, are in fact fragments of a bcc -Na structure (Fig. 8c ). In summary, the cation
array in Na 2 SO 3 is, in fact, an anti-fluorite structure whose empty cubes have been
filled by Na atoms, and where anti-fluorite- and bcc -Na-fragments coexist.
It should be outlined that this almost bcc type of environment for both S and
Na(2) atoms was recognized by Blatov [ 13 ].
This type of arrangement is related to the BiF 3 -type structure, formed by
compounds such as Cs 3 Bi [ 38 ] and Fe 3 Si [ 39 ] . Both types of structures coincide
in the existence of the respective Na 8 (Fe 8 ,Cs 8 ) cubes, forming a simple cubic ( sc )
array. However, the different stoichiometry makes BiF 3 and Na 2 S differ in that, in
Na 2 SO 3 , only one-third of the cubes are centred by S atoms. On the contrary, in
Fe 3 Si (Cs 3 Bi), alternate cubes are occupied by either Fe(Cs) or Si(Bi) atoms. The
result is that each Fe 8 Si cube is surrounded by six unlike cubes, whereas in Na 2 SO 3
each Na 8 (SO 3 ) cube is surrounded by three like and three unlike cubes (Fig. 8c ).
Because in both anti-fluorite and Na 2 SO 3 the Na:S ratio is 2:1, it is clear that the
Na 2 SO 3 array is also related to fluorite, as if the empty Na 8 cubes of fluorite were
now filled by Na and S atoms. Thus, the filling of the empty cubes leads necessarily
to a denser structure, in agreement with the concept relating the equivalence
between oxidation and pressure [ 1 , 4 ] . Thus, the insertion of three O atoms in the
anti-fluorite array of Na 2 S would provoke a more compact structure, although
the pressure is not enough to reach the already mentioned PbCl 2 -, Fe 2 P- and
Ni 2 In-type arrays. The most important outcome, however, is that the structure of
Na 2 SO 3 provides a very valuable information, namely the fluorite

rhombohedrally distorted simple cubic network ( a ¼

cotunnite
transition takes place through an intermediate and, up to now, unknown step .
At this point, the crucial question is whether the cation array in Na 2 SO 3
corresponds to the structure of a new alloy or, on the contrary, it can be regarded
as a mere distortion of the Ni 2 In-type structure. This question is, by no means banal
because the possible new alloy must necessarily fit into the general scheme of
compounds and phase transitions of the alloys quoted in Scheme 1 . Although the
structural similarity between Na 2 SO 3 and Ni 2 In had already been pointed out
[ 4 , 40 ], it is only now, through our research results, that the corresponding alloy
has been identified, unveiling the unknown quoted in Scheme 1 , so that

!

? ¼ Ni 2 Al

I n summary, the Na 2 S subarray, in Na 2 SO 3 , is isostructural to the Ni 2 Al alloy

( 2) [ 41 ], as shown in Fig. 9a, b . Therefore, the Ni 2 Al-type structure

becomes the second missing link of Scheme 1 . Nevertheless, Na 2 SO 3 is not the

P 3m1, Z ¼