Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 142

2.4 The Opposite Way: From Olivine to Fluorite

Until here, we have dealt with the olivine spinel transition path. Now we will

discuss the phases located on the upper row of Scheme 1 , i.e. the phases existing in
the olivine

!

fluorite path.
We have seen that Na 2 SO 4 has three HT polymorphs [ 9 ] . Although the three
phases, i.e. III-(

!

C mcm, olivine-related) (463 K), II-( P nma, olivine-like) (493 K)

and I-Na 2 SO 4 ( 6 3 /mmc) (521 K), are all related to olivine [ 4 , 7 , 14 ], we will

restrict our comments to the hexagonal phase I-Na 2 SO 4 whose Na 2 S subarray is
isostructural to the hexagonal HP phase of Na 2 S itself (Ni 2 In type) [ 7 ] (compare
Fig. 1c and Fig. 5b ) .
As stated above, the pressure-induced thenardite

P

! spinel transition involves the

simultaneous TiSi 2 ! MgCu 2 -type transition in the cation arrays. This transforma-

tion clearly illuminates that the structure of thenardite is midway between olivine
and spinel [ 14 ] . For this reason, when thenardite is heated, the internal pressure
exerted by oxygens is released, giving rise to the olivine-related structures of the III,
II and I phases, with the simultaneous TiSi 2 ! Ni 2 In transition of the Na 2 S

subarray. The exact coincidence of both the structure of I-Na 2 SO 4 and that of
HP-Na 2 S (both P6 3 /mmc) is shown in Fig. 6 .
In this context, one would expect that, at higher temperatures (equivalent to
lower pressures), I-Na 2 SO 4 could further transform into a

-K 2 SO 4 type (with the
anti-cotunnite structure) (Fig. 1b ), following the inverse pathway of Na 2 S under
pressure (Fig. 1 ) . There is at present no experimental evidence of this transition in
Na 2 SO 4 , but the transformation occurs in the ternary sulphate KLiSO 4 above
948 K [ 29 ] (see Scheme 1 ) . A comparison of the K 2 SO 3 and K 2 SO 4 structures is
expected to provide new insights about these transitions. We discuss it below in
Sect. 5 .

b

a b

a

Na

c

b

Fig. 6 The structures of I-Na 2 SO 4 (a) and that of the HP-Na 2 S(b) projected on the (110) plane of
the hexagonal (P6 3 /mmc) cells, showing the identical arrangement of both Na 2 S arrays. The figure
emphasizes the Na 6 trigonal prisms which are filled by S atoms ( yellow spheres )inNa 2 S(b) and
by (SO 4 ) groups in Na 2 SO 4 (a), where the rotational disorder is made evident