Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 159

a b

c

Fig. 15 (a) The hexagonal structure ( 6 3 /m) of LaCl 3 projected on the ab plane. La- blue ,

Cl- green . The unit cell, marked with black lines , contains two columns of TCTP. Each La atom
is surrounded by six Cl atoms of the trigonal prisms, plus three Cl atoms coplanar with the La
atoms, completing so the CN ¼ 9. An alternative unit cell ( red lines ) has been drawn by displacing
the origin along [1 1 0] in which the columns of TCTP coincide with those of Fe 2 P that are centred
at the cell origin in (b). (c) The LaCl 3 structure projected as in (a) but where fictitious La atoms
( red spheres ) have been inserted at the origin to show the ideal formation of a structure similar to
that of Fe 2 P. In this notional structure, the hexagonal tunnels are formed by alternating small and
great trigonal prisms centred by La and red-La atoms, respectively

P

of both structures, the LaCl 3 structure (Fig. 15a ) has been modified in the following
way: on the one hand, an alternative unit cell has been drawn with red lines so that
the columns of trigonal prisms are centred at the cell origin, and match those
existing in the Fe 2 P structure of Figs. 14b and 15b . On the other hand, we have
added one fictitious atom (red sphere) located at the origin of the real unit cell as
depicted in Fig. 15c . It is also noteworthy that a structure similar to that of Fe 2 P
(Fig. 15b ) can be built with the aid of that “red” atom. The hexagonal tunnels
formed in Fe 2 P can also be achieved in Fig. 15c by joining the smaller La-centred
prisms with the bigger ones that are centred by the fictitious “red” atoms.
Taking as reference the “red” unit cell of LaCl 3 (Fig. 15a, c ) , we see that the left
half of the cell contains one TCTP that is formed by six Cl atoms (green spheres)
and is centred by the La atom (blue sphere). The nine La-Cl bonds have been drawn
with red lines to emphasize that each trigonal prism is surrounded by other three,
displaced c/2 with respect to the central one.
If we look now at Fig. 15b (Fe 2 P), we see that the corresponding trigonal prism,
also located at the left half of the unit cell, is formed by six Fe atoms (brown
spheres) and is centred by the P atom (violet sphere). Also here, the P atom is