Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 140

2.3 A Possible Mechanism for the Ni 2 In ! TiSi 2 ! MgCu 2
Transitions

Finding a topological mechanism for the Ni 2 In MgCu 2 transitions is a

difficult task. However, we have discovered new structural relationships between
both the Ni 2 In and the TiSi 2 structures, which deserve a special discussion that is
presented next.
It has been reported [ 14 , 24 ] that the hexagonal Ni 2 In-type structure can be
regarded as a superstructure of the AlB 2 type (MgB 2 ). In this structure, both
graphite-like (6 3 planar nets) and hcp (3 6 ) layers alternate along the c axis, as
shown in Fig. 5a . Hereafter, these layers will be denoted as G (graphite-like) and
H (hexagonal).
In AlB 2 , the G layers are formed by only B atoms, whereas in Ni 2 In they are
formed by both Ni(2) and In atoms, producing in this way a doubling of the c axis
of the AlB 2 unit cell (Fig. 5a ). The H layers are then formed by the Ni(1) atoms.
It is worthy of mention that the G and H layers are complementary in such a way
that the atoms of the H layer are capping the centre of the graphene hexagons.
Thus, compression of either the AlB 2 or the Ni 2 In structures, along the c axis,
would result in the formation of a denser, hexagonal close packed ( hcp ) layer
(G + H) stacked in the

! TiSi 2 !

sequence. These layers are identical to those
of TiSi 2 , although in this last case they are stacked in the

... AAAA ...

... ABCDABCD ...

sequence. Such a filled hexagon (G þ H) is drawn in Fig. 3a and one complete

layer is drawn in Fig. 5c .

a

b

c d

Fig. 5 (a) The structure of AlB 2 showing the G (boron, yellow atoms) and H (aluminium, grey
atoms) layers. (b) The Ni 2 In-type structure to illustrate the intergrowth of G (NiIn) and H (Ni)
layers. Recall that this structure is the same than that of Fig. 6b . (c) An isolated (G H) layer

which forms the TiSi 2 ( C 54) structure. Both the Si ( grey spheres ) and Ti atoms ( green spheres ) are
coplanar and are projected onto the ab plane. Compare with Fig. 2b . (d) The structure of AlB 2
projected on the ab plane to show the similarity with the (G

þ

þ H) layers