Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 186

extended by O'Keeffe and Hyde [ 5 ] and Blatov et al. [ 11 - 13 ] and which has found
physical meaning in the works of Vegas et al. [ 3 , 4 , 14 , 33 , 63 , 64 ].
Although no compound has been identified which follows a transit along the
complete series of structures described in our work, when the structures of the
alloys and the oxides are considered simultaneously, one obtains a complete vision,
“a movie”, of how the initial structure is continuously transformed, producing a
complete series of structures whose sequence is chemically meaningful. The iden-
tical behaviour of the alloys and the cation arrays helps the identification of the
“ missing links ”. When taken together the structures define a transition which is
understandable and physically sound.
The whole series of transitions, collected in Scheme 1 , can be thought of as
resulting from small pieces (short steps) which connect to form a longer chain that
we have named Structural Journey . These small pieces correspond to transitions
which connect, for example, the olivine spinel structures in

Na 2 MoO 4 [ 15 ] . However, a change in the composition, e.g. in LiCsCrO 4 [ 81 ] ,
makes the transition go in the opposite direction, i.e. a temperature-induced,
transition olivine

! thenardite !

Cs-filled zincblende takes place, whereas the
double sulphate KLiSO 4 (LiCaPO 4 type), with a cation array of the Fe 2 P type,
transforms at HT into a stuffed-wurtzite. Both cation networks zincblende and
wurtzite transform each other in the compounds KLiMoO 4 and CsLiSO 4 . Thus,
KLiMoO 4 , wurtzite type (

! cotunnite !

6 3 mc) at ambient conditions, transforms into a cubic
filled zincblende-type structure at 701 K [ 129 ] .
Also interesting is the behaviour of K 2 SO 4 , which has a cotunnite-type structure
at ambient conditions, which transforms into a Ni 2 In at HT and goes, by decreasing
the O content, up to the Ni 2 Al-type structure in K 2 SO 3 (isostructural to Na 2 SO 3 ).
The manganese germanate Mn 2 GeO 4 provides an illustrative example of the
transition which connects the olivine-spinel pathway with the lower part of
Scheme 1 . It shows how spinels can transform under compression into the
Sr 2 PbO 4 -type structure. Another germanate Ca 2 GeO 4 shows us more drastic transi-
tions, i.e. the room temperature, olivine-like phase transforms at elevated tempera-
tures, into a Ca-filled wurtzite (the starting point of the walk), whereas, under
pressure, the same compound takes up the Ba 2 SnO 4 -type structure, the last step
before decomposition. The surprising behaviour of this compound is that both the
olivine

P

Ba 2 SnO 4 transitions take place without the
stabilization of the intermediate phases. The reason for these jumps could be that
the intermediate structures could have a very narrow range of stability, a feature
that could also occur for other expectable structures in the frame of Scheme 1 ,but
which has never been observed. They could be described as “small stability range
structures”.
The above examples illustrate that all these pieces of structural information
overlap completing the walk for the Structural Journey . It seems improbable that
a single compound could adopt the complete set of structures of the Structural
Journey .Ca 2 GeO 4 comes closest. We have seen that to pass a step between two
pieces, it is often necessary to change some of the atoms involved in the com-
pounds. The equivalence between the effects of oxidation and pressure makes that

! wurtzite and the olivine !