Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 171

The correlation between CaSnO 3 and the bixbyite type finds support in the two
facts that CaSnO 3 is also obtained as an ilmenite-type polymorph [ 91 ] , and that, at
the same time, In 2 O 3 undergoes the bixbyite

corundum transition at 65 kbar and
800-1,300 K [ 93 ] (note the structural similarities between ilmenite and corundum
structures). Thus, it is not surprising that CaSnO 3 , when forming part of the
Ca 2 SnO 4 compound, adopts one of the stable structures of the isoelectronic
In 2 O 3 , i.e. in this case, fragments of the bixbyite-type structure.
If Ca 2 SnO 4 had adopted the ideal structure of Fig. 21c , with alternate columns of
both empty and Sn-filled prisms of Ca atoms, the empty prisms would produce the
new Ca-Ca contacts with the adjacent prisms. This would be achieved by tilting the
filled prisms, producing a denser packing of the Ca atoms as illustrated by the
sequence of structures represented in Figs. 21a, b, c and 22 .
When the O atoms are considered we obtain Fig. 23a , which shows how the Sn
atoms are coordinated octahedrally by six O atoms. The SnO 6 octahedra share
edges, forming chains parallel to the c axis (perpendicular to the projection plane).
These octahedral chains are really fragments of the rutile-like structure of SnO 2 ,
represented in Fig. 23b , and are arranged as if the columns of Ca 2 SnO 4 would be
ready to reconstruct the rutile structure in case the Ca 2 O 2 fragments disappear.
In Fig. 23a , we see that O atoms are located at the middle point of the longest
edges of the SnCa 8 prisms. These O atoms have a double role. On the one hand, they
form Ca-O-Ca edges like in the rocksalt CaO structure (Fig. 22 ) . On the other hand,
they contribute to form the fragments of the rutile-like SnO 2 structure. Taking this
feature into account, it is easy to imagine that if the Ca-O-Ca edges could be
eliminated, the structure would collapse producing the rutile-like network depicted
in Fig. 23b . The impression therefore is that the Ca 2 SnO 4 structure would be “ well
prepared ” to reconstruct the SnO 2 (rutile-type) structure.

!

a b

Fig. 23 (a) The structure of Ca 2 SnO 4 , viewed along the c axis, in which the O atoms ( red spheres )
have been drawn. The SnO 6 octahedra share edges forming chains perpendicular to the projection
plane. (b) The structure of the rutile-type SnO 2 showing the similarity of the SnO 6 octahedra in
both compounds. The tetragonal unit cell of SnO 2 has dimensions of a 3.19 ˚ and is ¼ 4.74, c ¼

projected on the ab plane