Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 151

The important question that rises here is whether the Metal-Metal distances,
observed in the oxides can be taken as an internal barometer that gives insight
about the internal pressure at which cations are subjected.
Looking again at Figs. 9 and 11 , it can be ensured that in Na 2 SO 3 , the Na 2 S
subarray is formed by blocks of anti-fluorite, although coexisting with an incipient
Ni 2 In-type structure. Even in an exhaustive dissection of the structure, one is able to
recognize fragments of a cotunnite-type structure.
We must emphasize, however, that the internal pressure exerted by three O atoms
is not enough to stabilize the high-pressure phases of Na 2 S, and, hence, the Ni 2 Al-
type structure appears as an intermediate stage in the transitions path of Na 2 S.

3.4 Na 2 SO 2 : A Hypothetical Compound with the Anti-Fluorite
Structure?

In Scheme 1 , it can be seen that the number of oxides with an anti-fluorite-related
structure is scarce. Examples are the HT phases of both Li 2 SO 4 and CsLi(CrO 4 ).
This can be interpreted as if the insertion of O atoms into anti-fluorite structure of
the alloys produced an internal pressure, difficult to overcome at high temperature.
As is readily seen, all the oxides collected therein contain either three or four
oxygens per formula, and the subsequent question that arises is whether a com-
pound with a lower O-content, such as Na 2 SO 2 , could exist and whether it could
maintain the anti-CaF 2 -type structure for the Na 2 S subarray.

4 The Complete Journey from Fluorite to the Laves Phases

We have seen that the TiSi 2 -type structure (Na 2 S subarray in thenardite V-Na 2 SO 4 )
might well be an intermediate step in the Ni 2 In MgCu 2 transition [ 14 ]. Their

structural relationships were discussed in Sect. 2.2 and were based on the fact that
TiSi 2 possesses structural fragments of both the initial and the final phases. We
must also recall that this double transition has only been reported for the oxide
Na 2 MoO 4 [ 15 ] (see Scheme 1 ) .
In the same manner, it has been discussed that the Ni 2 Al-type structure, found
in Na 2 SO 3 , should be considered as an intermediate step in the CaF 2 ! Ni 2 In
transition, so that our model forces the Na 2 S subarray of Na 2 SO 3 (Ni 2 Al type) to fit
into the set of structures and phase transitions quoted in Scheme 1 , as it will be
explained below.
We have already justified and described that phenacite could also be an interme-
diate phase in the fluorite

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Ni 2 In transition as deduced from the data reported for
Li 2 SO 4 . Finally, as it will be discussed later, the Fe 2 P-type structure might be the
fourth missing link in the series collected in Scheme 1 .
Thus, the four structures we have just discussed, i.e. phenacite, Ni 2 Al, Fe 2 P
and TiSi 2 , are meaningful and could serve as the thread that connects other

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