FeLi[PO4]: Dissection of a Crystal Structure - Inorganic 3D Structures - page 70

Fig. 1 The structure of the
olivine-like FeLi[PO 4 ]
projected on the ab plane. The
colour codes are: green (Fe),
grey (Li), violet (P) and
yellow (O). This projection
shows the hcp array of
O atoms connected by blue
lines

not distributed at random but separated in two different sets. The Li atoms are at
4 a (0, 0, 0) and Fe atoms at 4 c ( x ,¼, z ). This separation is an important aspect which
is normally ignored in the crystal-chemical discussions. Because it is generally
admitted that size effects play an essential role in the type of voids occupied by each
cation, here the question is of special interest because both Li and Fe atoms are at
octahedral voids, and there would be no reason for this precise observed splitting.
The complete structure is shown in Fig. 1 , which shows the great irregularity of the
hcp array of O atoms. It has been suggested [ 5 ] that the irregularity implies that the
classical description of structures based on closed packed array of anions should be
considered as inadequate.
As mentioned above, our description will be based on the cation array which, in
the case of olivine-like compounds, is of Ni 2 In type. It should be remarked that even
a simple structure, like the Ni 2 In type, admits multiple descriptions which will be
considered in the following sections.

2.1 Olivine FeLi[PO 4 ] and the Ni 2 In Structure

The most frequent description of the Ni 2 In structure consists of chains of trigonal
prisms, formed by the (Li, Fe) atoms, which are centred by the P atoms. Thus, FeLi
[PO 4 ] can be described as an O-stuffed [FeLiP] alloy whose structure is of the
distorted Ni 2 In type [ 5 ] . The cation array is represented in Fig. 2 , after omitting the
O atoms, which were drawn in Fig. 1 .

2.2 FeLi[PO 4 ] and the AlB 2 Structure

A second description relates olivine to the AlB 2 -type structure. More precisely,
Ni 2 In has been described as a superstructure of the AlB 2 type [ 16 ]. This relationship
has also applied to hexagonal structures of related compounds, such as KZnP,
which have been the object of theoretical calculations [ 17 ].
The AlB 2 structure consists of the 3 6 planar nets of Al atoms alternating with 6 3
graphite-like layers of B atoms. In the hexagonal Ni 2 In structure (

6 3 /mmc), the 3 6

P