FeLi[PO4]: Dissection of a crystal structure: The parts and the whole

Abstract

The structures of the dimorphic FeLi[PO4] phosphate are analysed on the basis of their cation arrays. At ambient conditions, the compound, known as the mineral triphylite, is olivine type (Pnma) (CaMg[SiO4]) and transforms into the olivine-related NaCaVO4-type (Cmcm) 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 → β-LiFe[PO4] 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 FePO4. The EZKC allows that LiFe[PO4] can be written with the pseudo-formula Li +[Ψ-FeSO4], so that the [FeP] subarray is converted into Ψ-[FeS], whose structure coincides with that of real FeS. The significant result is that the structure of FeS persists in the pseudoarrays Ψ-[FeS] of both Fe[PO4] and LiFe[PO4]. 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 β-LiFe[PO4], 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 substructures act as if they were resonance structures which cooperate to the stabilization of the whole network. © 2011 Springer-Verlag Berlin Heidelberg.