Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 163

6.4 BaCl 2 : The Cotunnite-Type Phase and Its Relation
to the Fe 2 P-Type Structure

We have seen that at ambient conditions BaCl 2 is cotunnite type (Co 2 Si type)
( nma) [ 54 ] (recall that cotunnite is the mineral PbCl 2 ). Although this structure

has been drawn repeatedly in this chapter, it is represented again, in Fig. 18 , for the
sake of comparison with the Fe 2 P-type structure. We intend to highlight the
similarities between both phases, contributing in this way to a better understanding
of the transition Fe 2 P

P

Co 2 Si occurring between them [ 31 , 55 ] .
The BaCl 2 structure has been described, in previous sections, as related to
both CoSn- and LaCl 3 -type structures. In addition, we have remarked the peculiar
o

!

-Ti-type structure of the Ba subarray. However, a careful inspection of Fig. 18a
shows that, in the Fe 2 P-type structure, there are implicit blocks of the cotunnite
structure. Thus, the Fe 2 P structure can also be thought of as the condensation of the
zigzag chains of BaCl 6 trigonal prisms, i.e. the so-called TCTP [ 57 ], which form the
cotunnite structure drawn in Fig. 1 8c . The resulting structure has a reflection
twinning through the common plane (11 0) (Fig. 18a ).
An effective way of remarking this coincidence is to eliminate some of the atoms
forming Fig. 18a , in such a way that two such zigzag chains become isolated, as
shown in Fig. 18b . The chains are identical to those existing in the real cotunnite-
like BaCl 2 represented in Fig. 18c , and the reader can see that if the two isolated
chains of Fig. 18b approach each other, the cotunnite structure (Fig. 18c ) can be
reconstructed. This mechanism is only proposed as a didactic tool, just to show the
similarities between both phases, but the real transition should occur through a
reconstructive mechanism involving all atoms. A similar geometrical mechanism
was proposed by Hyde and Andersson [ 57 ] .
The two structure types must be very close in energy, as deduced from the
structure of the compound NbCoB [ 72 ] , which is an intergrowth of slabs of both
Co 2 Si- and Fe 2 P-type structures. From the experimental data reported for BaCl 2 [ 55 ] ,

a b c

Fig. 18 (a) The hexagonal (P62m) phase of BaCl 2 , showing the TCTP centred by the Ba atoms.
(b) Zigzag chains of trigonal BaCl 6 prisms existing in hexagonal BaCl 2 . The chains have been
isolated by eliminating both one Ba and one Cl atom from the array shown in (a). If the two
chains approached each other, the cotunnite (Co 2 Si-type) structure represented in (c) would be
reconstructed