Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 137

with the hexagonal HP phase of Na 2 S (Ni 2 In type). Both structures (sulphide and
sulphate) belong to the space group 6 3 /mmc [ 7 ] .

The thermal behaviour of V-Na 2 SO 4 has been interpreted [ 14 ] in the light of the
oxidation-pressure relationship [ 1 , 24 ]. According to this idea, the pressure exerted
by oxygens could depend on the O contents. Thus, in thenardite, with four O atoms
inserted into Na 2 S, cations adopt the TiSi 2 -type structure. It is worthy of mention
that the TiSi 2 structure contains fragments and features of cubic Laves phases [ 4 , 7 ] .
It seems that four O atoms produce an internal pressure which is not high enough to
attain the MgCu 2 -type structure, characteristic of spinels.
Consequently, the thenardite structure could be regarded as an intermediate step
in the olivine

P

!

spinel transition [ 14 ] . Thus, when thenardite is heated, the pressure
is partly released and the cations adopt the structures of lower pressure phases, e.g.
those found in the olivine-like structure (Ni 2 In type) of III-, II- and I-Na 2 SO 4 .
The lack of coincidence of the cation array of thenardite with any known
structure of Na 2 S was interpreted by Vegas et al. [ 14 ] as an indicative of the
formation of an intermediate structure between cotunnite and Ni 2 In type. However,
such an assumption requires the previous identification of the structure type that
forms the cations in thenardite, to rationalize its structure in the framework of our
concept relating oxidation and pressure [ 1 , 24 ].

2.1 The Cation Array of Thenardite (V-Na 2 SO 4 )
and TiSi 2 : The First “Missing Link”

A wide search in the crystallographic databases allowed us to find that the structure
of the Na 2 S subarray of thenardite V-Na 2 SO 4 (

ddd) was of the TiSi 2 type [ 14 ]. At
that time, we were not aware that this structural similarity had been previously
reported by Blatov et al. [ 11 - 13 ].
An interesting question is whether the TiSi 2 -type structure, formed by cations in
the oxides Na 2 SO 4 ,Na 2 MoO 4 and Na 2 CrO 4 , could also be stable for the alkali
sulphides. This phase was not observed in the high-pressure experiments carried out
by us on Na 2 S[ 7 ]. However, further theoretical calculations [ 23 ] predicted the
existence of a stable Fddd phase for the analogue Rb 2 S. Although this phase was
not identified, the coincidence with the space group of thenardite led us to identify

F

Scheme 1 (continued) altogether the “structural journey” described along this chapter. The
journey begins in the fluorite structure ending in the MoSi 2 -type structure which decomposes at
more elevated pressures. Column 1 contains the structure types involved in the transitions. In
column 2 are collected all the binary compounds AB 2 that undergo the partial transitions. Columns
3 and 4 contain the corresponding oxides AB 2 O 4 whose cation arrays undergo transitions similar to
those of column 2. Column 2, on the one hand, and columns 3 and 4, on the other hand, build the
two concurrent pathways of the phase transitions followed by alloys and oxides and which we have
denoted as “Structural Journeys”