Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 181

CrSi 2 C 49 C 11b C 54

Quartz

2123 K

CrB+Si CsCl+Si Thenardite

P 6 2 22 C mcm I 4/mmm F ddd

Scheme 3 Evolution of the TiSi 2 structures by increasing temperature. The central transition
C 49 ß C 11b shows how the TiSi moiety undergoes the expected CsCl to CrB transformation

structure of the Si atoms in quartz [ 63 ] . This new relationship is, in our opinion, of
greatest interest because it reflects, in the alloys, features that take place in the
oxides. Thus, in the same manner that quartz transforms at high pressure into
stishovite (rutile-like SiO 2 )[ 120 ] , the MoSi 2 structure ( C 11b) should recover, at
very high temperatures (lower pressures), the quartz-like arrangement. And it
occurs so! This transition [ 121 ] indicates that the quartz rutile transition,

occurring in SiO 2 , GeO 2 and GaAsO 4 , also takes place in the oxygen-free alloys.
In Scheme 3 are summarized the above transitions, which indicate that more
structural types can be added to the “ structural walk ”. Thus, if we take thenardite
(TiSi 2 ) as the starting point, the atoms can follow an alternative way, forming the
metastable C 11b structure (TiSi 2 )(Fig. 28c ) . This phase, which is an intergrowth of
TiSi + Si in the forms of CsCl + bcc -Si, finally forms the C 49 structure (Fig. 26c ) ,
which is also an intergrowth of TiSi + Si but in the forms of CrB + bcc -Si. That is, in
TiSi 2 , the TiSi subnet undergoes its own CrB

!

CsCl transition .Itshouldbe
remarked that the CrB-type structure has not been found in the isolated phase TiSi.
Instead, it forms the strongly related phase FeB type, discussed in connection with
Ca 2 SnO 4 . Remember also that ZrSi exists in both polymorphs, i.e. CrB and FeB.
As already mentioned, the rutile

!

quartz transition occurs directly from C 11b to
CrSi 2 . This feature should be connected with the transitions sequence zincblende

!

!

Quartz CsCl, observed in some compounds, such as ZnTe

[ 122 ] . Note the coincidence of these transitions with those quoted in Scheme 3 .
The behaviour of the Si layers, intercalated between the TiSi-blocks, also merits
discussion. In TiSi 2 , the Si layers form 4 4 planar nets with Si-Si distances of
2.54 ˚ . If we assume they are part of a fcc cell, the unit cell parameter ( a

! NaCl ! CrB (FeB) !

3.61 ˚ )

¼

3.34 ˚
and the Si-Si distances of 2.36 ˚ [ 123 ]. At the transition pressure, the values in
TiSi 2 and fcc -Si would become closer.

would be comparable to that of fcc -Si, stable above 79 GPa. At 87 GPa, a ¼

9.1 The Ca 2 SnO 4 - and Ba 2 SnO 4 -Type Structures
and the Decomposition Processes

In the light of the dissection of the structures of Ca 2 SnO 4 and Ba 2 SnO 4 , and
considering the experimental data on their decompositions at higher pressures
(Tables 4 and 5 ), we can conclude that such decompositions seem to obey a
mechanism that “is written” in the last steps of the structural walk .
Thus, the decomposition of Ba 2 SnO 4 in the BaSnO3 perovskite and the rocksalt
BaO is expectable in view of the structures drawn in Fig. 27 , which means that the