Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 149

a b

c

Fig. 11 (a) The final structure of the Ni 2 Al type, after the displacement of the atomic sheets in the
fluorite-like Na 2 S. (b) The same view where the Na atoms located at the origin have been coloured
( green ) to outline ( red lines ) the denser new unit cell comparable to that of Na 2 SO 3 .(c) Stereopair
showing the filling of the cubes

R
unit cell which has served as a frame for the structural transformation (compare
with the cation array of Na 2 SO 3 shown in Fig. 8b ). In this figure, the contiguous
blocks are separated by voids as a consequence of the graphical process of creating
a denser fragment (Ni 2 Al) from a bigger unit cell. The reader can easily deduce that
these voids are non-existing in a real transformation.
The block represented in Fig. 11a contains 1.5 unit cells along c . Of course, in
the real physical process these blocks close up to form a continuous arrangement.
The unit cell dimensions of Na 2 S in the hypothetical Ni 2 Al-type structure would be
a

The structural block of Na 2 S, depicted in Fig. 11a , is inserted into a hexagonal

6.18 ˚ in
Na 2 SO 3 . This real unit cell of Na 2 S with the Ni 2 Al-type structure is outlined with
red lines in Fig. 11b . The final atomic coordinates are collected in Tables 2 and 3 .
Figure 11c is a 3D view of the Ni 2 Al-type structure to be compared with Fig. 8c .

5.63 ˚ , comparable to the values of a

¼ 4.61, c ¼ ¼ 5.46, c ¼

3.3 Can the Internal Pressure Be Estimated?

As stated above, in Na 2 SO 3 (Ni 2 Al type), two-thirds of the Na(1) 8 cubes are filled
by S atoms, forming so fragments of an anti-fluorite structure, whereas one-third of