Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 133

However, this amazing outcome suggested the need of new HP experiments to
understand the physical meaning of this challenging phenomenon.
New high-pressure experiments were carried out by in situ synchrotron X-ray
diffraction techniques for Li 2 S[ 6 ] , Na 2 S[ 7 ] and K 2 S[ 8 ]. Our aim was to corrobo-
rate whether the HP phases of these sulphides were isostructural to the cation
subarrays of their corresponding oxides (sulphites and sulphates). Na 2 S undergoes
several phase transitions at HP [ 7 ], and Na 2 SO 4 has several HT polymorphs [ 9 ] .
This pair of compounds serves us as an illustrative guide, although other oxides,
such as Na 2 SO 3 , will also be considered.
Under ambient conditions, Na 2 S is anti-fluorite, undergoing the double transition
anti-fluorite (CaF 2 )

! !

Ni 2 In type at high pressures
[ 7 ]. This sequence follows the trend of other fluorite-like compounds [ 10 ]. The
importance of this result is that the final HP phase (Ni 2 In type) corresponds to the
cation-array structure of the HT phases of Na 2 SO 4 , which in turn has the olivine-
type structure.
It should be emphasized here that in agreement with the topological analysis
carried out by Blatov and co-workers [ 11 - 13 ] , the cation array of olivine (

anti-cotunnite (anti-PbCl 2 )

P nma) is

erroneously assigned to the Ni 2 In type ( 6 3 /mmc). In fact, of the three HT phases of

Na 2 SO 4 [ 9 ], only one is strictly olivine-like (

P

P nma) and only one is strictly Ni 2 In

type ( 6 3 /mmc). These similarities have been discussed in several articles [ 4 , 7 , 14 ]

and will also be discussed below. They are significant because they connect with the
well-known olivine

P

spinel transition, occurring at very high pressures, for
compounds like Mg 2 SiO 4 and Fe 2 SiO 4 , among others. It should be emphasized
that the equivalent transition Ni 2 In

!

! Cu 2 Mg has never been reported for the

alloys.
The olivine

spinel transition has also been reported for Na 2 MoO 4 , a com-
pound which, like Na 2 CrO 4 , is isostructural to the olivine-like, HT phase of Na 2 SO 4
(

!

nma). The importance of these relationships is greater if it is noted that Na 2 MoO 4
does not undergo the direct olivine

P

spinel transition but undergoes a double
transition involving an intermediate phase which is isostructural to the room-
temperature phase of Na 2 SO 4 (

!

ddd), known as the mineral thenardite. Thus,
Na 2 MoO 4 undergoes the transitions spinel

F

! !

thenardite olivine at tempera-

tures of 873 K and 913 K, respectively [ 15 ] .
Because the intermediate phase (thenardite type) is coincident with the room
temperature phase of sodium sulphate (V-Na 2 SO 4 ), Na 2 MoO 4 may be viewed as a
bridge connecting the binary structures CaF 2 and MgCu 2 , as well as oxides like
HT-Li 2 SO 4 (anti-fluorite) with the Na 2 MoO 4 (spinel-type) structure. It should be
added that the Na 2 S subarray of thenardite (V-Na 2 SO 4 ) was first identified as TiSi 2
type [ 11 - 13 ], a phase which has not yet been found for the sulphide Na 2 S[ 14 ] .
This means that, in the same manner that the thenardite-type structure is the
intermediate step in the olivine ! spinel transition, a similar transformation might

occur in the Ni 2 In ! Cu 2 Mg transition through the intermediate TiSi 2 , i.e. Ni 2 In !

TiSi 2 ! Cu 2 Mg. It has also been reported [ 16 ] that, at more elevated pressures,

spinels transform into other structure types, like Ca 2 SnO 4 and Ba 2 SnO 4 , which
finally decompose into a mixture of phases.