Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 182

decomposition products “already existed” in Ba 2 SnO 4 . Moreover, the impression is
that the process begins in the previous step (the Ca 2 SnO 4 phase, Sr 2 PbO 4 type).
We have seen (Figs. 21 - 23 ) that this structure admitted several descriptions. In
one of them, we have outlined the similarity of the CaSnO 3 -moiety with the
isoelectronic C -In 2 O 3 and how these fragments are mixed with fragments of the
rocksalt CaO. The outcome we wish to emphasize here is that the CaSn prisms
(Fig. 21d ) are, in fact, fragments of a fcc -array like the one formed by cations in the
C -In 2 O 3 (Sc 2 O 3 ). Stated differently, the Ca 8 Sn prisms are, in fact, a fragment of a
hypothetical CaSn alloy with the rocksalt structure.
Because the rocksalt structure undergoes the pressure-induced transitions NaCl

! !

CsCl, this sequence is consistent with the phases observed in the binary
alloys CaSn, BaSn, SrPb, etc. All these compounds, CrB type at ambient condi-
tions, should convert into NaCl type at high temperature, but they transform to a
CsCl type at high pressure. It should be remarked that unlike the CrB

CrB

! CsCl

transition, occurring in BaSn [ 2 ] , the HT CrB NaCl transition has never been

reported. The main consequences of this behaviour are as follows:

!

1. The CaSn ( fcc ) array should convert into a bcc array when pressure increases.
Although the data of Tables 4 and 5 do not include such a transition, the structural
differences between Ca 2 SnO 4 and Ba 2 SnO 4 are consistent with this approach. It
should be recalled that the heavier elements within a group of the Periodic Table
stabilize the high-pressure phases of the lighter ones, so that the fcc sub-structure
in CaSn should necessarily be of the CsCl type in the BaSn, as it really occurs!
2. Ca 2 SnO 4 and Ba 2 SnO 4 are previous stages to the real separation, which takes
place at higher pressures. The moieties BaSnO 3 þ

rocksalt)
pre-exist in both structures and the compound is well prepared for this final
separation.

BaO (perovskite þ

Although we have already mentioned the experiments concerning the spinel !

Sr 2 PbO 4 transition at high pressure, a complete study concerning the spinel !

Sr 2 SnO 4 ! Ba 2 SnO 4 ! decomposition has not been reported so far. On the

the partial pressure-induced transitions Mn 2 GeO 4 !

contrary, Sr 2 PbO 4 and

Ca 2 GeO 4 !

Ba 2 SnO 4 are known and will be discussed in the next subsection.
An experimental or theoretical study of these processes should be desirable.

9.2 The Phases of CaGe, Ca 2 Ge and the CaGeO 3
and Ca 2 GeO 4 Oxides

In the Ca:Ge system, the phases CaGe and Ca 2 Ge are of special interest in our
discussion. CaGe is CrB type at ambient conditions [ 124 ]. Although it is expectable
a CrB CsCl transition at HP, as in BaSn [ 2 ] , no HT/HP transition has been

reported for CaGe. On the other hand, Ca 2 Ge is, at ambient conditions, PbCl 2 type,
like the related Ca 2 Si and Ca 2 Sn compounds [ 94 ].

!