FeLi[PO4]: Dissection of a Crystal Structure - Inorganic 3D Structures - page 73

This interpretation as a Fe-stuffed (LiP) array could be admitted if the charge
transfer from Fe to P atoms is considered as reasonable. In this case, the compound
should be formulated as pseudo-Fe 2+ [

-LiClO 4 ]. On the contrary, a charge transfer
from Fe to the Li atoms seems to be poorly founded. This transfer would give rise to
the pseudo-formula Fe 2+ [

C

-BPO 4 ]. Even if the two Fe electrons are distributed on
both Li and P atoms, the pseudo-formula Fe 2+ [

C

-BeSO 4 ] is obtained.
Our proposal is that the two last pseudo-formulae should be discarded because
neither Be[SO 4 ]norB[PO 4 ] are known to have cation arrays of the NiAs-type
structure which, on the other hand, exists with heavier elements like in Mg[SO 4 ]
[ 20 ] . However, the pseudo-formula [Li

C

C

-ClO 4 ] is physically meaningful because the
real compound Li[ClO 4 ], recently synthesized [ 21 , 22 ], is isostructural to Cu[SO 4 ]
(MgSO 4 )type(Pnma), with a Li[ C -Cl] subarray of the NiAs type, as represented in
Fig. 5b . It should be remarked that this interpretation seems more “plausible” because
the real halide LiCl crystallizes in the strongly related rocksalt structure. However,
the strong distortion of the hcp array of P atoms in triphylite (Fig. 5a ) in comparison
with the real Li[ClO 4 ] structure (Fig. 5b ) is a clear indication that other interpretations
of the structure might be more appropriate, as it will be shown later.

2.4 FeLiP, and FeLi[PO 4 ]: The Extended Zintl-Klemm Concept

In the above section, we have seen the difficulties of finding a coherent interpreta-
tion of the [FeLiP] subarray on the basis of the Fe-P pair of atoms. The unique
rational issue was the formation of the hugely distorted array of Fig. 5a , which
could be related to the distortion of the graphite-like Fe-P layers drawn in Fig. 3 .
However, if instead of drawing the Fe-P contacts running parallel to the a axis,
we connect the same atoms with the blue lines drawn in Fig. 6a , then a new pattern

a b

Fig. 6 (a) The FeLiP subarray of FeLi[PO 4 ] projected on the ac plane. The drawing outlines the
two views of the structure, i.e. the chains of trigonal prisms marked with black lines and also the
new description in terms of blocks of the PbO-type drawn with blue lines connecting the Fe and P
atoms ( green and red-violet spheres , respectively). (b) A perspective view of the FeLiP subarray
showing the four-connected skeleton of the Fe and P atoms