Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 145

the TCTP, in the sulphite, occurs because the Na(3) atom (at z ¼ 0.6667) and the S

atom (at z ¼ 0.1730) are displaced from the ideal positions, at z ¼ ¾ and z ¼ ¼,

respectively, of the Ni 2 In-type structure.
Thinking of this array as a distortion of the Ni 2 In type, one could affirm that the
insertion of three O atoms almost reaches the Ni 2 In-type structure, but that the
pressure is not high enough as to stabilize the regular P 6 3 /mmc structure. Small

0.5 ˚ ) of both Na (3) and S atoms, in opposite directions along
the c axis (horizontal axis in Fig. 8a ), would produce the Ni 2 In-type array. The
impression is that the Na and S atoms are in the right way, moving towards the
expected positions .
A second approach to the structure is obtained when only the Na-Na contacts of
3.77 ˚ are drawn. In this case, the contacts, represented in Fig. 8b , define a slightly

displacements (

a b

a

c

b

d

c

c a

a

b

c

b

Fig. 8 (a) The trigonal structure (P3) of Na 2 SO 3 , projected on (110) emphasizing the description
based in the distorted trigonal prisms. (b) The same projection rotated clockwise around the
projection axis. The Na-Na contacts outline the distorted Na 8 cubes which are centred either by Na
atoms or by SO 3 groups. This arrangement is seen in perspective in (c). (d) An alternative view of
the structure showing an isolated Na 8 cube, filled by a SO 3 group, to show the complete coordina-
tion of the S atoms (8 + 6)