Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 180

a b c d

C 11b

Fig. 28 (a) The tetragonal C 11b structure (I4/mmm) of MoSi 2 . The layers of tetragonal prisms
centred by Mo atoms ( green spheres ) alternate with fragments of a similar Si bct net. Both
fragments are outlined in (b). The structure (pseudo- C 11b) of the TiSi 2 (Fmmm) is projected in
(c) where the body-centred tetragonal (bct) prisms become now almost cubic, as shown in (d)

From the data collected in Table 5 , we can extract an important conclusion, i.e.
the topology and distances of the basic units BaO and BaSn remain unaltered in the
more complex compounds. Recall that this was reported for BaSn and BaSnO 3 [ 1 ]
and can now be extended to Ba 2 SnO 4 . This means that in a given compound, both
the structural and metric requirements of all its components are satisfied simulta-
neously. This assertion was evident in the Ca 2 SnO 4 structure and enhances the
concept of multiple resonance structures [ 33 ] .
Unlike Ca 2 SnO 4 , whose Ca 2 Sn subarray has no similarity with any alloy struc-
ture, the Ba 2 Sn subarray of Ba 2 SnO 4 is of the Mo 2 Si type ( 4/mmm), also iso-

structural to the related silicides CrSi 2 and WSi 2 , among other compounds [ 116 ].
The Mo 2 Si structure is projected in Fig. 28a . It consists of alternate layers of
tetragonal prisms of Si, centred by Mo atoms, and fragments of body-centred
tetragonal prisms of Si atoms. The fragment represented in Fig. 28b contains both
structural elements to be compared with Fig. 27b . The dimensions of the MoSi 8
prisms are a

I

2.67 ˚ , a cell related to the cation array of the rutile-like
oxides which, on the other hand, are formed by all the elements involved in these
alloys (Cr, Mo, Si, etc.). As an example, we can cite the stishovite (SiO 2 )(

¼ 3.20, c ¼

P 4 2 /

2.64 ˚ ) expanded in the ab plane by the insertion of oxygen.
Another interesting feature related to the MoSi 2 structure is the existence of a
related polymorph of TiSi 2 , denoted as pseudo- C 11b. This phase is orthorhombic, S.
G. Fmmm (a subgroup of I4/mmm) and is represented in Fig. 28c . It is metastable
[ 117 ] and has been isolated during the C 49 to C 54 transition of TiSi 2 [ 118 ]. Recall
that the C 49 structure was discussed on the occasion of the relationship between FeB
and Mn 2 Ge and corresponds to the ZrSi 2 type represented in Fig. 26c . This structure
is also a metastable phase of TiSi 2 , which has only been stabilized in thin layers
[ 119 ]. Interestingly, this phase transforms later into the stable C 54, corresponding to
the cation array of thenardite , represented in Fig. 2 . Note that in TiSi 2 , the TiSi
prisms are almost cubic (3.25

mnm, a

¼

4.10, c

¼

3.23 ˚ ) (compare Fig. 28b, d ) .
An interesting property of the MoSi 2 structure (Fig. 28a ) is that it remains stable
up to 2,170 K, undergoing a transition to the CrSi 2 -type structure, which is the

3.25