Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 170

Fig. 22 Stereopair of a fragment of the Ca 2 SnO 4 structure. It coincid e s with a unit cell of CaO,
which has been truncated by the (110) plane. Both the (001) and (001) faces are centred by Ca
atoms, whereas both the (100) and (010) are centred by Sn atoms. The latter have dimensions of
4.55

5.27 ˚ (mean, 4.91 ˚ ). Note that some O atoms are displaced from the Ca-Ca edges
towards the Sn atoms, to satisfy the Sn-O distance

(Fig. 21c ) should be, in principle, unstable and should convert, through a concerted
rotation, into the real structure of Fig. 21a , forming in this way the additional Ca-Ca
contacts.
The coexistence of the SnCa 8 prisms and the Ca blocks suggests that the Ca 2 Sn
subarray might be formulated as the sum of CaSn + Ca. If the O atoms are taken into
account, Ca 2 SnO 4 could then be contemplated as an intergrowth of CaSnO 3 + CaO.
This “conceptual decomposition” is based on both the topology and the distances in
the Ca blocks represented in both Figs. 21d, e and 22 .
The partial formula CaSnO 3 , and hence the cation subarray CaSn, has the
stoichiometry of a perovskite, such as the real one CaSnO 3 [ 90 ], a compound that
has also been obtained as ilmenite type [ 91 ]. Surprisingly, the SnCa 8 prisms in
Ca 2 SnO 4 (Fig. 21d ) are neither perovskite- nor ilmenite-like.
In view of this discrepancy, it is important to consider again the dimensions of
the prisms (3.18 5.27 ˚ ), which can here be averaged to tetragonal

3.26

5.27 ˚ . The important outcome is that these dimen-
sions are comparable to the unit cell of elemental Indium (

prisms with a ¼ 3.22 and c ¼

I 4/mmm, a ¼ 3.25,

4.95 ˚ ). An analysis of the cubic In 2 O 3 structure [ 92 ] has shown that the
structure of elemental In remains almost unchanged in the bixbyite-type oxide
In 2 O 3 , where there exit distorted tetragonal prisms of In with dimensions of
3.35

c ¼

5.06 ˚ (compare with the CaSn prisms).
This coincidence is by no means casual. It becomes physically meaningful if
we assign to the CaSn pair (II-IV) of atoms the same role as the isoelectronic In
(III) or Sc atoms ( C -Sc 2 O 3 is also bixbyite type). In fact, if we apply the EZKC [ 14 ] ,
the transfer of one electron from Sn to Ca would convert them into

3.35

C -In and C -Sc,

respectively. Then, the CaSnO 3 moiety could be formulated as [ -In]O 3 and

regarded as a fragment of the bixbyite-type oxides Sc 2 O 3 and In 2 O 3 . Recall that in
these sexquioxides, the structure of the parent metal is preserved [ 92 ].
On the other hand, we mentioned above that the columns of SnCa 8 prisms are
also fragments of an AuCu-type structure (

C -Sc C

4/mmm) (Fig. 22 ). This structure is
found in similar compounds such as InMg (3.24

P

4.48 ˚ ), Ga 0.4 Mn 1.6 , FeSe,

4.80 ˚ ), and surprisingly in CaPb (3.62 4.49 A ˚ ) and

AlTi, CsI, BiNa (3.46

4.32 A ˚ )!

in InSc itself (3.20