Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 168

related to the Sr 2 PbO 4 type, will not be considered. Our discussion will commence
with the isostructural Ca 2 SnO 4 compound.

8.1 The Structure of Ca 2 SnO 4 and Related Compounds

Ca 2 SnO 4 belongs to the Sr 2 PbO 4 type ( nma) [ 87 , 88 ] . As seen in Table 4 , this

structure remains stable, under compression, up to its decomposition into the
perovskite CaSnO 3 and the rocksalt CaO phases, i.e. no phase transition has been
observed so far.
Another interesting compound is Mn 2 GeO 4 [ 18 ] . It is spinel at ambient condi-
tions but transforms into the Sr 2 PbO 4 type under pressure, decomposing at higher
pressures. Two decompositions have been reported [ 16 ] , i.e. MnGeO 3 (ilmenite)

P

þ

CaO (rocksalt) and the alternative GeO 2 (fluorite) 2 MnO (rocksalt) (Table 4 ) .

On the other hand, Sr 2 SnO 4 has four polymorphs (

þ

C mca, P 4 2 /ncm, P ccn, and

I mmm). All these phases, closely related between them, are slight distortions of the

Ba 2 SnO 4 -type structure ( 4/mmm) [ 89 ] which at more elevated pressures decom-

pose into perovskite + rocksalt (Table 5 ). The data discussed above can be sum-
marized by saying that

I

the M 2 X O 4 oxides can follow the pressure-induced

transitions Al 2 MgO 4 (spinel) Ba 2 SnO 4 . At higher pressures, they

decompose in different ways, as summarized in Table 4 . The new structural types
Sr 2 PbO 4 and Ba 2 SnO 4 have been added to Scheme 1 .
Next, we will focus on the description of the Ca 2 SnO 4 structure which is
orthorhombic (

! Sr 2 PbO 4 !

3.26 ˚ , Z 2 and is pro-

jected along the shortest c axis in Fig. 21 . The Ca 2 Sn subarray is drawn in Fig. 21a .
At first glance, the structure can be described as a set of almost regular tetragonal
prisms of Ca atoms (green), filled by Sn atoms (grey). The prisms share faces
forming columns along the c axis. One of these columns is at the centre of the unit
cell of Fig. 21a and has been isolated in Fig. 21d . The dimensions of these Ca
prisms (3.18

P bam), with a ¼ 5.64, b ¼ 9.69, c ¼ ¼

5.27 ˚ ) acquire a special importance to understand the
structure. It is worth mentioning that we have not found any binary alloy isostruc-
tural to the Ca 2 Sn subarray. This important exception will be discussed later.
Figure 21a clearly shows that the columns of prisms are tilted around the c axis.
This tilting gives rise to additional Ca-Ca contacts that form fragments with the

3.26

Table 5 The unit cell dimensions (in ˚ ) of the BaSnO 3 and BaO fragments co-existing in
Ba 2 SnO 4 . They are compared with the corresponding values in the pure phases, i.e. the BaSnO 3
perovskite, the HP phase of the BaSn alloy (CsCl type) and the BaO (NaCl type). These fragments
are depicted in Fig. 20
Ba 2 SnO 4 BaSnO 3 BaSn BaO

4.05 ˚

BaSnO 3 4.14 4.14 3.8 a ¼ 4.11 a ¼ a 5.72

d Ba-Ba

¼

mean 4.05 ¼ 4.08

4.10 ˚

BaSn a ¼ 4.12 a ¼ 4.11 a ¼ -

BaO a ¼ 5.75 a ¼ 5.81 - a 5.53

d Ba-Ba

¼

¼ 3.91