Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 134

All these results show that, up to now, all these phase transitions, either in alloys
or in oxides, have been regarded as partial, isolated processes (involving only one to
two steps) and also that the behaviour of the alloys M 2 X has not been correlated
with that of the corresponding M 2 X O 4 oxides. However, when all the structures are
viewed from a wider perspective, one can discover unexpected bridges, especially
when the transitions of alloys and oxides are examined in a concerted way.
The aim of this study is to travel through all the structures, starting in the fluorite
and finishing in the decomposition process. The structures will be analysed in the
light of the oxidation-pressure equivalence, considering at the same time that
the HT is opposite to the application of pressure and that opposite transitions can
also occur. In some cases, the cation arrays will be explained in terms of the EZKC
[ 14 ] to put all the structures under a common basis.
Such a complete study, including the whole transitions, has not been reported
previously. It is really a hard task. However, the discovery of the TiSi 2 structure in
thenardite [ 11 , 14 ] as well as the structure-type formed by the Na 2 S subarray in
Na 2 SO 3 induced us to think that these arrays could be the missing links in the
complete structural route of Na 2 S under pressure and alerted us to the possibility
that other structures could appear. This allows the assembling of the pieces to form
the complete chain going from the CaF 2 type to the MgCu 2 type, or even to the
cation array of the final ternary phase Ba 2 SnO 4 .
Many of the compounds described in this work were the object of individual
studies. Others were analysed in the frame of phase transition studies, such as the
HT study on Na 2 SO 4 [ 9 ] and the double transition spinel

olivine in
Na 2 MoO 4 [ 15 ] . We must add the HT, HP studies on Mn 2 GeO 4 which inform us
about the opposite transitions spinel

! thenardite !

!
Sr 2 PbO 4 -type transition at HP [ 17 - 19 ]. And finally, we will examine the compound
Ca 2 GeO 4 [ 20 , 21 ], which at HT transforms into a filled wurtzite, whereas under
pressure converts into the Ba 2 SnO 4 type, a transition that also conforms to the
principles discussed here.
When all these partial transitions are put together, and the “missing links” are
inserted in the path, the apparent lack of connection between some phase transitions
disappears, obtaining a global view of the process which, beginning either in the
wurtzite or fluorite structures, ends with a “programmed” decomposition of the final
Sr 2 PbO 4 - and Ba 2 SnO 4 -type phases [ 16 ] .

! olivine at HT and also of the spinel

2 From Fluorite to Spinel: The Structures of Na 2 S
and Their Oxides, Na 2 SO 3 and Na 2 SO 4

At ambient conditions, Na 2 S is anti-fluorite [ 22 ] , undergoing the reversible transi-
tions anti-fluorite Ni 2 In type at about 7 and

16 GPa, respectively [ 7 ]. The three experimentally observed structures of Na 2 S
are illustrated in Fig. 1 .

! anti-cotunnite (PbCl 2 ) type !