Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 154

pressure, which establishes that, within a group of the Periodic Table, the
ambient pressure structures of heavier elements are stabilized by the lighter
ones at high pressures. As it has been discussed above, the Ni 2 Al alloy could
transform into an Ni 2 In type under pressure, in the same manner that, at high
temperature, it might undergo the Ni 2 Al

CaF 2 transition. Although these
experiments have never been carried out, the behaviour of the Li 2 S, Na 2 S and
K 2 S alloys [ 6 - 8 ], as well as the same subarrays in the corresponding oxides,
strongly support these hypotheses.
3. The third reason is connected to the equivalence between oxidation and pressure
and its relationship with the oxygen contents. In this respect, the structures
of Na 2 SO 3 will be compared with those of Na 2 SO 4 . We have seen that the
transition path in Na 2 S under pressure is anti - fluorite

!

! anti - cotunnite ! Ni 2 In

type [ 7 ].

When four O atoms are inserted into the Na 2 S matrix, they exert a pressure
which exceeds that needed to form the Ni 2 In-type subarray, but insufficient to reach
the MgCu 2 -type structure. We have seen that the cation subarray (TiSi 2 type), in
thenardite (V-Na 2 SO 4 ), is an intermediate step in the olivine spinel transition

(Fig. 3 ), and that, when temperature increases, the pressure is partly released and
the Ni 2 In-type subarray is recovered. This means that the Na 2 S subarray follows, by
heating, an opposite pathway to that followed under pressure (Fig. 1 ) .
In the case of Na 2 SO 3 , with a lower O-content, the Na 2 S subarray is Ni 2 Al type
[ 41 ] (Fig. 9 ). We have mentioned previously how small displacements (

!

0.5 ˚ )of
both the Na (3) and the S atoms would lead to the Ni 2 In-type structure, an idea that
is in agreement with recent studies on iron silicides [ 49 ] . One of them, Fe 2 Si,
extremely difficult to obtain as a pure phase was synthesized [ 50 ] by rapid quench-
ing from very high temperatures. Its structure is represented in Fig. 13b , to show
that the atomic coordinates of the corresponding Fe and Si atoms are intermediate
between those of Ni 2 In and Ni 2 Al types.

Fig. 13 The structures of the three compounds intimately related: (a)Ni 2 Al, (b)Fe 2 Si and (c)
Ni 2 In (also HP-Na 2 S). The figures suggest that a possible Ni 2 Al Ni 2 In phase transition might

occur by a continuous displacement of both Ni(Fe) atom and the Al(Si), following the sequence
(a)

!

! (b) ! (c), up to reach the most symmetrical Ni 2 In (HP-Na 2 S) structure