FeLi[PO4]: Dissection of a Crystal Structure - Inorganic 3D Structures - page 79

3.2 The Exchange of Li and Fe in the Transition
Triphylite ! b -FeLi[PO 4 ]

The questions that arise now are: Does the exchange of Fe and Li atoms have any
physical meaning? Why the exchange takes place under pressure? To answer these
questions, we must return to the interpretation of the structures in terms of the EZKC.
In a first attempt, the exchange was thought to be a consequence of the inversion of the
charge transfer between Li and Fe. However, we must remember that the approach
which considered the triphylite FeLi[PO 4 ] structure (olivine-like) an Fe-stuffed C -Be

[SO 4 ](or -B[PO 4 ]), with the Fe atoms acting as donors, was later left as inadequate.

The detailed analysis of both structures reveals that the important feature is that the
structures of the respective [FeP] subarrays (

C

-FeS) should be coincident with struc-
tures of real phases of the FeS sulphide, that is, the PbO type in triphylite andNiAs type
in

C

-FeLiPO 4 . Thus, by assuming the expected electron transfer fromLi to Fe and/or P
atoms, both skeletons can be explained in a rational way. Even more, differences in the
structures of [FeP] subarrays would indicate that the PbO-type structure (Fig. 6a )
existing in the ambient pressure phase (triphylite) is converted into the NiAs-type
structure of

b

b C

-FeS]
subarray, must be correlated with the fact that real FeS, zinkblende at ambient condi-
tions undergoes, under compression, the transitions ZnS

-FeLi[PO 4 ] under pressure. This transition, undergone by the [

! PbO ! NaCl (NiAs).

The important result is not only the PbO ! NiAs transition but also that when the

Li atom transforms the FePO 4 subarray of -Fe[SO 4 ]; this trans-

formation is not only formal, but Fe[PO 4 ] reproduces the structure of real Fe[SO 4 ]
itself, whose [FeS] subarray is, in turn, identical to the NiAs-type phase of FeS. These
ideas are clearly expressed in Fig. 9 .
Now, we can give a satisfactory explanation of why the Fe and Li atoms exchange
their positions. At ambient pressure (triphylite), both Fe and P atoms must form the
PbO-type structure characteristic of FeS. At high pressure, the same atoms must form
a high pressure phase of FeS, that is, the NiAs-type structure. In both cases, however,
stable structures of

b -FeLi[PO 4 ]intoa C

-FeS are satisfied.
In support of our interpretation is the fact that when the Fe atoms are considered as
donors, the LiP subarray (NiAs type) of triphylite forms the irregular, extremely
flattened octahedra (Fig. 5a ), which contrast with the regularity of Li[ClO 4 ](Fig. 5b ) .
On the contrary, if Li acts as donor, forming the

C

-Fe[SO 4 ], represented in Fig. 9c ,
the structure drawn in Fig. 5a becomes useful and the two structures of FeLi[PO 4 ],
interpreted as in Figs. 8a and 9b , for triphylite and the

C

b -phase, respectively, become

physically meaningful.

4 FeLi[PO 4 ] and the High Temperature Phases of Na 2 [SO 4 ]

It has been mentioned above that the two phases of FeLi[PO 4 ]( mcm)

are isostructural with two high temperature phases of Na 2 [SO 4 ]. It must be recalled

P nma and C