Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 169

a b c

d e

b

c

a

Fig. 21 (a) The Ca 2 Sn subarray of the structure of Ca 2 SnO 4 , viewed along the c axis. The almost
tetragonal prisms of Ca atoms ( green spheres ) are centred by the Sn atoms ( grey spheres ). (b)
Artificial tilting of the prisms to see how the real structure could be derived from an ideal structure
represented in (c) where tilting has been completed. (d) Stereopair showing the blocks of SnCa 8 .
(e) Stereopair showing the Ca blocks which connect the different blocks of prisms. Note that the O
atoms have also been drawn to show the simultaneous existence of CaO fragments

topology of the fcc -Ca structure. Such a fragment is isolated in Fig. 21e , where we
have drawn the Ca-Ca contacts. The O atoms have been included to show that these
blocks are really fragments of a CaO (rocksalt) structure. Within these fragments,
the most significant Ca-Ca distances are 3.17, 3.26, 3.58 ˚ and 5.27 ˚ . The mean
value of the three shortest distances is 3.34 ˚ , which in the frame of a fcc structure
would correspond to a unit cell of a

4.72 ˚ . This value almost equals the unit

¼

3.40 ˚ ) (compare with 3.34 ˚ in
Ca 2 SnO 4 ). Because the fcc structure of elemental Ca is maintained in CaO, the
fragments connecting the prisms also conform to the fcc topology.
Taking as reference Fig. 21d , if the diagonals of the bases of prisms are drawn,
we obtain two almost perpendicular (1 0 0) faces of a CaO-like structure. This new
fragment is represented in Fig. 22 and corresponds to one half of a unit cell of CaO,
where two Ca atoms have been substituted by two Sn atoms. As seen in Fig. 22 , the
Sn atoms are located close to the centre of the two (1 0 0) faces of an ideal cubic
structure. The mean Ca-O-Ca distance is 4.91 ˚ , a value that compares well with
the unit cell of CaO (4.81 ˚ ). However, the Ca and the Sn atoms, rather than a
fcc -Ca unit cell, form one half of the unit cell of a CuAu-type alloy.
Coming again to Fig. 21a-c , it is clear that the tilting of the prisms is not
capricious but responds to the need of preserving the structure of CaO and,
hence, of fcc -Ca. Thus, the ideal structure formed with untilted Sn-filled prisms

4.81 ˚ , d Ca-Ca ¼ cell of the rocksalt CaO ( a

¼