Chemistry Reference
In-Depth Information
FeLi[PO
4
]: Dissection of a Crystal Structure
The Parts and the Whole
Angel Vegas
Abstract The structures of the dimorphic FeLi[PO
4
] phosphate are analysed on the
basis of their cation arrays. At ambient conditions, the compound, known as
the mineral triphylite, is olivine type (
P
nma) (CaMg[SiO
4
]) and transforms into
the olivine-related NaCaVO
4
-type (
mcm) structure, at high temperature and high
pressure. This phase transition takes place with an exchange in the positions of the
Li and Fe atoms, both filling octahedral voids. This feature, impossible to explain
with the traditional model of cation-centred anion polyhedra, can be understood in
the light of the extended Zintl-Klemm concept (EZKC) which contemplates as
possible the electron transfer even between “cations”. A detailed dissection of both
structures leads to the conclusion that the observed phase transition triphylite
C
!
b
-LiFe[PO
4
] is controlled by the [FeP] cation subarrays which undergo a PbO-
type
NiAs-type transition at high pressure. In addition to this relevant feature, it
is shown that the [FeP] subarray, PbO type in triphylite, exits in the delithiated
phase FePO
4
. The EZKC allows that LiFe[PO
4
] can be written with the pseudo-
formula Li
+
[
!
-[FeS], whose
structure coincides with that of real FeS. The significant result is that the structure
of FeS persists in the pseudoarrays
C
-FeSO
4
], so that the [FeP] subarray is converted into
C
-[FeS] of both Fe[PO
4
] and LiFe[PO
4
]. The
“autopsy” also reveals that several substructures, formed by different pairs of
atoms, co-exist with those of the PbO type and NiAs type, in triphylite and
b
C
-LiFe[PO
4
], respectively. Fragments of metallic lithium, layers of metallic iron
and the rocksalt LiCl are some of the structures that can be revealed through the
dissection process. The conclusion is that the formation of any structure implies that
several substructures need to be satisfied simultaneously. These multiple substruc-
tures act as if they were resonance structures which cooperate to the stabilization of
the whole network.
Keywords Alloys
Cation arrays
Crystal chemistry
Phase transitions
Resonance structures
Zintl phases
