Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 178

atoms form 4 4 planar nets (Fig. 26c ), and if we were able to extract the monolayers
of Si atoms, the TiSi structure (Fig. 25b ) would be reconstructed!, a similar
“exercise” to that practiced in Ca 2 Sn (Fig. 25 ) . The difference with Ca 2 Sn resides
in that in the FeB-type structure, the insertion of an additional atom entails that the
trigonal prisms are converted into square prisms, a pattern that results more difficult
to identify as a derivation of the FeB structure. However, in TiSi 2 , the permanence
of the TiSi-blocks (CrB type) makes their relationship more visible (Fig. 26 ).
The can also account for the TiSi 2 structure. If the Ti atom donates one

electron to one Si atom, the pseudo-formula Si 1 (

EZKC

-ScSi) is formed. This pseudo-
structure corresponds, in fact, to the real ScSi compound [ 109 ], which is just B 33
(CrB type). Again, an impressive coincidence! The unit cell constants of ScSi:
a

C

¼ 3.66 ˚ ¼ 3.99, b ¼ 9.88, c are comparable to those of TiSi 2 : a ¼ 3.61,

¼ 3.65 ˚ .
The relevance of this example is twofold: on the one hand, the strong relationship
existing between CrB- and FeB-type structures; on the other hand, the already
mentioned polymorphism (CrB and FeB type) of both ZrSi and ZrGe [ 107 ] that
helps to understand the Ca 2 SnO 4 structure. In view of this, the

b ¼ 13.79, c

Ca, Sr, Ba)
compounds might also stabilize the FeB-type structure under the appropriate con-
ditions of temperature and/or pressure. In fact, in the Ca 2 Sn subarray, the stabiliza-
tion of a TiSi structure (FeB type) is induced by the “ foreign ” Ca atom [ 3 , 110 ].
An interesting observation that gives coherence to all these structures and phase
transitions is the existence of the rocksalt carbide TiC [ 111 ]. The occurrence of this
phase fits with the fact that TiSi, ZrSi and ZrGe are of the CrB type. Thus, the
structures formed by heavier elements can be formed by the lighter ones under
compression. Because the NaCl-type structure undergoes, under pressure, the
transitions NaCl

M Sn ( M ¼

CsCl, the rocksalt TiC would stabilize, by compression,
any of these structures. Similarly, TiSi, ZrSi, ZrGe and even CaSn, SrSn and BaSn
(all of them CrB type) could produce either by oxidation or compression, the CsCl-
type structure.
This double transition has been observed in many compounds such as the
pressure-induced CrB

! CrB !

!

CsCl transition in BaSn [ 1 , 2 ] , a point which will be
discussed more extensively in the next sections.
Our conclusion is that the cation subarray in Ca 2 SnO 4 is not a rare arrangement.
Even if no isostructural alloy has been found so far, its interpretation in terms of the
EZKC , as well as its fitting into the general scheme of the related structures,

indicates its strong relationship to the ZrSi 2 type.
The explanation of the Ca 2 Sn subarray in terms of the can be extended to

the Mn 2 Ge subarray of Mn 2 GeO 4 . The compounds Mn 0.5 Co 0.5 B[ 112 ], Fe 0.5 Mn 0.5 B
[ 113 ] and MnB [ 114 ] are of the FeB type. Now, if we write Mn 0.5 Co 0.5 BasMn
(CoB 2 ) and postulate, following the

EZKC

, that the Co atom transfers one electron
to each B atom, the formula becomes Mn(

EZKC

-MnC 2 ), equivalent to MnC and
isoelectronic with the MnGe subarray of the oxide. The same result is obtained if
we start from Mn 2 Ge. The transfer of one electron from the Ge atom to one Mn
atom would lead to the formula

C

C -Fe(MnAl), where the MnAl moiety is isoelec-

tronic to MnB (also FeB type).