Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 161

a

b

Fig. 16 (a) The Ba subarray in hexagonal BaCl 2 (P62m) projected onto the ab plane. The red
spheres represent the Ba(1) atoms at (0, 0, 0), forming hcp layers (3 6 planar nets). The grey spheres
represent the Ba(2) atoms at (1/3, 2/3, 1/2) forming graphite-like layers (6 3 planar nets). The
structure is identical to that of o -Ti and similar to the AlB 2 structure drawn in Fig. 5d .(b) The
structure of TaN (P6/mmm) projected on the ab plane. Red and green spheres represent Ta atoms.
Blue circles are N atoms

AlB 2 structure, drawn in Fig. 5a, d . Looking at Fig. 16a , which is comparable to
Fig. 14e , one sees that the Ba (1) atoms (grey) form graphite-like 6 3 planar nets at
z 0. These 6 3 nets alternate with 3 6 layers formed by the Ba(2) atoms ( z 1/2),

represented by red spheres in Fig. 16a . Note that all Ba atoms were represented by
grey spheres in Fig. 14e . Compare also with the projection of the AlB 2 structure,
represented in Fig. 5d .
An inspection of the nitride TaN [ 66 ] and of the suboxide

¼ ¼

d -TiO 0.5 [ 67 ] reveals

that both the Ta and Ti atoms are forming an o -Ti structure type. In the case of

d -phases

of some binary alloys of titanium, but his observation becomes meaningful when
the

-TiO 0.5 , Andersson [ 67 ] had already noticed its structural relation to the o

-Ti phase was discovered some years later by Jamieson [ 65 ] . This phase of Ti
belongs to the AlB 2 -type structure and is formed by alternate 3 6 ( hcp ) and 6 3
(graphite-like) layers of Ti atoms, so that TiO 0.5 represents a new example of
how the metallic nets are preserved in their oxides [ 3 , 4 , 67 ].
The comparison of the structures of TaN and

o

d

-TiO 0.5 merits a further discus-
sion. As shown in Fig. 16b , the structure of TaN is similar to the Ba 3 Cl 3 subarray
(Fig. 14e ). More precisely, their cation arrays are coincident as it is shown in
Fig. 16a .
The explanation of such similarities could be seen as a challenge, in view of the
different chemical character of the atoms involved in these compounds (Ba, Ti, Ta,
In, etc.), specially when the most relevant feature is the general appearance of the 6 3
(graphite-like) planar nets. This structural motif, characteristic of carbon (Group 14
elements), is also formed by pseudo-atoms ( -C) (in the Klemm's approach) such

as the B atoms in AlB 2 or MgB 2 (Fig. 5 ) . In this context, we must recall that the
formation of skeletons of the Group 14 was suggested to be a general trend in
forming inorganic crystal structures [ 14 ] . Thus, the existence of such graphite-like
layers in both

C

-TiO 0.5 is in agreement with the fact that, in anatase
(TiO 2 ), the Ti atoms form a diamond-type network [ 68 ] , a behaviour that is also

o -Ti and d