Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 135

a b c

a

S
Na

S
Na

c

b

Fig. 1 The three structures of Na 2 S. S ( yellow ), Na ( red ). (a) The anti-fluorite structure stable at
ambient conditions. (b) The anti-cotunnite (anti-PbCl 2 ) structure, stable at 7 GPa. (c) The Ni 2 In-
type structure obtained at 16 GPa. In (b) and (c), blue lines connect the Na atoms forming the
TCTP centred by the S atoms

As it has been discussed above, it has been suggested [ 4 , 14 ] that the transitions
Fluorite Ni 2 In might continue onward to reach the structure of the

cubic Laves phases ( Ni 2 In

! Cotunnite !

MgCu 2 ). The reason for this assumption is that, even
if never observed in alloys, this transition is implicit in the well-known olivine
(Ni 2 In-related)

!

spinel (MgCu 2 ) transition. The only theoretical study on this
possible Na 2 S phase predicts [ 23 ] the unstability of the MgCu 2 -type structure, even
at pressures of 1 Mbar.
The relationships between the phases of Na 2 S and Na 2 SO 4 have been widely
discussed [ 4 , 7 , 14 ] . However, we intend to delve more deeply into these relation-
ships by discussing them in the framework of the whole set of structures involved in
the complete pathway CaF 2 type

!

MgCu 2 type. All these structures are collected
in Scheme 1 (see below) which contains the structures of the binary alloys as well as
those of corresponding oxides. Our discussion will begin with the structure of
thenardite (V-Na 2 SO 4 ) which is the stable phase under ambient conditions.
Within the concept that relates oxidation and pressure, probably, the most
unexpected feature is that the Na 2 S subarray of thenardite (TiSi 2 type) does not
correspond to any of the three HP phases of Na 2 S[ 7 ] . However, it has been reported
[ 9 ] that, by increasing temperature, thenardite (V-Na 2 SO 4 ) undergoes the following
transitions:

!

Thenardite V- F ddd ð Þ! III- C mcm ð Þ! II- P nma ð Þ! I- P 6 3 = ð mmc Þ

TiSi 2 CrVO 4 -type olivine-type Ni 2 In-type

The three HT phases of Na 2 SO 4 are related to the olivine structure and, in the
same manner, their Na 2 S subarrays are related to the Ni 2 In alloy [ 5 ], although it
should be remarked that only the II-phase is olivine-like and also that only the
hexagonal I-phase is strictly of the Ni 2 In type. This feature is especially important
because the I-phase is the only phase whose Na 2 S substructure is fully coincident