Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 160

coordinated by nine Fe atoms, three of which belong to the surrounding prisms.
These three additional contacts are highlighted with blue lines in Fig. 15b .
Next, the structural differences will be remarked. Thus, in Fe 2 P (Fig. 15b ) , each
TCTP is surrounded by another six. Three of them, formed by the Fe atoms
represented as brown spheres in Fig. 15b , are at the same height, and the other
three, formed by the Fe atoms represented by medium green spheres, are displaced
c/2 (Fig. 15b ) . On the contrary, in LaCl 3 (Fig. 15a ), each TCTP is surrounded by
only three columns, also displaced c/2, as it can be seen in Fig. 15a .
The most significant outcome of this structural dissection of BaCl 2 is that the Ba
(2)Cl(1) 3 subarray reproduces fragments of the LaCl 3 structure [ 62 ] . This feature is,
by no means, a coincidence, for it can also be rationalized in terms of the EZKC

[ 14 , 33 , 63 , 64 ], as it was the Ba 2 Cl 3 subarray (Fig. 14c ). Thus, if the Ba 2 Cl 3 moiety
transferred one electron to the Ba(1) atom, the former would become ( C -Cs)BaCl 3 ,
whereas the latter, formed by Ba(1)Cl(2) 3 , would convert into (BaCl 3 ) 1 . This
pseudo-anion can be written as -LaCl 3 and is just the subarray represented in

Fig. 14d . Because Ba and La are contiguous elements, the application of the

C

EZKC
results, in this special case, in a marvellous coincidence between the structures of
C -LaCl 3 and LaCl 3 itself! This justifies the use of LaCl 3 as a referent isostructural

compound.
Another relevant consequence of this analysis is that the hexagonal BaCl 2
structure is the result of two structural species that are interwoven , i.e. BaCl 3
(LaCl 3 type) and Ba 2 Cl 3 , the fitting of the two entities being achieved through the
common atom, Ba(1) at (0, 0, 0), which serves as a “ joker ” for both the CoSn (NiIn-
type) and LaCl 3 -type substructures.
The dimensions of both unit c el ls illuminate this feature. LaCl 3 ( P 6 3 /m) has

4.35 ˚ . They are almost
identical, but their contents differ from La 2 Cl 6 to Ba 3 Cl 6 .Thus,La 2 Cl 6 needs
one additional La atom to match the stoichiometry of BaCl 2 (BaCl 3 þ

4.37 ˚ and BaCl 2 (

a ¼ 7.48, c ¼ P 62m) has a ¼ 7.60, c ¼

BaCl ¼

Ba 2 Cl 4 ¼ BaCl 2 ). This lacking atom is just the one drawn as a red sphere in Fig. 15a .

The important conclusion is that both substructures are interwoven. It seems that
a sort of mutual (feedback) exchange of information occurs between them. We must
emphasize that the insertion of the BaCl 3 prisms into the hexagonal tunnels
(Fig. 15b ) is not a question of space-filling. The BaCl 3 substructure needs the
arrangement of the BaCl (CoSn type) subnet to extend its structure (Fig. 15 ) .
They necessitate each other, and it is by means of their coupling that both entities
can fulfil their own structural requirements as much as possible.

6.3 The Ba Subarray in Hexagonal BaCl 2

If the Cl atoms are neglected, the resulting Ba subnet corresponds to that of the -Ti

phase [ 65 ] , which is a variant of the AlB 2 -type structure discussed in Sect. 2.3
(Fig. 5 ) . This Ba subarray is represented in Fig. 16a and can be compared with the

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