Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 144

a b

Fig. 7 (a) The trigonal structure (P3) of Na 2 SO 3 , projected on (110) showing the distorted
trigonal prisms of Na atoms ( violet spheres ) and the SO 3 groups, displaced from the centre of
the trigonal prisms. (b) The hexagonal structure (P6 3 /mmc) of I-Na 2 SO 4 (Ni 2 In type) to see the
analogies and differences with Na 2 SO 3

subarray is neither cotunnite type nor Ni 2 In type (Fig. 1 ), and the identification of
this intermediate is needed.
Alternatively, the analysis of double alkali sulphates shows that LiNaSO 4
crystallizes, at ambient conditions, in the CaLiPO4-type structure which, in turn,
has a cation subarray of the BaCl 2 (Fe 2 P) type. Thus, the fact that LiNaSO 4 trans-
forms at 818 K into a cubic (supposedly) stuffed blende-type structure together with
the Ni 2 In PbCl 2 transition undergone by LiKSO 4 at HT [ 30 , 35 ] leads to assume

that the Fe 2 P-type (anti-BaCl 2 ) structure might well be another intermediate phase
in this part of the journey. This alternative structure will be discussed in detail later.
Now we will focus on the analysis of the Na 2 S subarray of Na 2 SO 3 to see
whether it fits into the general scheme of t ransitions depicted in Scheme 1 . The
structure of Na 2 SO 3 [ 36 , 37 ] is trigonal,

!

2) and is represented in Fig. 7a .
The Na atoms occupy the sites Na(1) at 1 a (0, 0, 0), Na(2) at 1 b (0, 0, 1/2) and Na(3)
at 2 d (1/3, 2/3, 0.6667), and the S atoms are situated at 3 d (1/3, 2/3, 0.173). A first
inspection of the structure reveals that its cation array does not correspond to any of
the expected phases (Fe 2 P, filled-wurtzite or CaF 2 ).

P 3( Z ¼

3.1 Na 2 SO 3 and the Ni 2 Al Alloy

The Na 2 SO 3 structure admits several descriptions. We will start with that which
relates Na 2 SO 3 to the high-temperature phase I-Na 2 SO 4 ( 6 3 /mmc). Both structures

are represented in Fig. 7 . To highlight their similarity in Fig. 7a we have drawn
only those Na-Na contacts that form distorted Na 6 tricapped trigonal prisms
(TCTPs), which are equivalent to those of I-Na 2 SO 4 (Fig. 7b ) . The distortion of

P