Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 179

The conclusion is that in both Ca 2 SnO 4 and Mn 2 GeO 4 , the insertion of an
additional Ca (Mn) atom into a hypothetical CaSn (MnGe) subarray stabilizes the
TiSi (FeB type) structures of their respective CaSn and MnGe moieties.

9 The Structure of Ba 2 SnO 4

Ba 2 SnO 4 and the isostructural Ca 2 GeO 4 compounds are the final step in the long
journey discussed in this work (Table 5 ). The aristotype is K 2 NiF 4 ( 4/mmm) and

its structure is drawn in Fig. 27a . In this section, we will focus on Ba 2 SnO 4
(Fig. 27a ), leaving the description of the Ca 2 GeO 4 polymorphs for a later section
(Sect. 9.2 ) .
The classical description of this structure consists of an intergrowth of perov-
skite-like blocks of composition BaSnO 3 and fragments of BaO (NaCl type). Both
blocks composing the structure are drawn together in Fig. 27b , whereas the BaO
fragment is outlined in a unit cell of the rocksalt BaO structure in Fig. 27c . These
BaO fragments differ completely from the CaO fragments in Ca 2 SnO 4 (Fig. 22 ) .
The other component of the structure, the BaSnO 3 -blocks, are formed by slightly
distorted cubes of Ba atoms (4.14

I

3.88 ˚ ), centred by the Sn atoms. The
O atoms form octahedra around the Sn atoms. This is, in fact, a distortion of the
BaSnO 3 perovskite [ 115 ] and hence of the HP phase (CsCl type) of BaSn [ 1 , 2 ]
(Table 5 ) .

4.14

Fig. 27 (a) The structure of Ba 2 SnO 4 (I4/mmm), viewed along the a axis, showing the perovskite-
like blocks of composition BaSnO 3 alternating, along c, with fragments of the BaO (rocksalt)
structure. The atoms are Ba ( green ), Sn ( grey ) and O ( red ). (b) A view of the two fragments. The
upper side shows a BaSnO 3 perovskite-like fragment. The lower part is a fragment of the BaO
represented in (c). (c) A unit cell of the rocksalt BaO structure. The blue lines remark the structural
fragments forming part of Ba 2 SnO 4 . The Ba-Ba distances in the BaO fragment are 4 4.14 and

4.02 ˚ (mean 4.08 ˚ ) 4