Unit cell: orthorhombic; for LiFePO4, a = 10.33, b = 6.01, c = 4.70 Å; Z = 4.
There is much current interest in LiFePO4 as the cathode material in rechargeable lithium batteries. On charging, Li deintercalates from the structure and a corresponding amount of Fe is oxidised from Fe2+ to Fe3+, as follows:
Lithium ions occupy channels parallel to the y axis, Fig. 1.45, which allows them to leave and enter the structure readily during cell charge and discharge. This is an example of a solid state redox reaction with a cell potential of about 3.08 V. LiFePO4, and associated LiMnPO4, are of interest because the redox reaction and the process of lithium removal and insertion are reversible over many cycles, giving a high cell capacity, and the materials are cheap, non‐toxic and environmentally friendly.
1.17.11 Corundum, ilmenite and LiNbO3
These three closely related structure types can be regarded, ideally, as hcp oxide ions with cations occupying two‐thirds of the octahedral sites. Conceptually, they are related to the NiAs structure in which all the octahedral sites are occupied, and to the CdI2 structure in which only half the octahedral sites are occupied, Table 1.4. The crystal structures are shown in Fig. 1.46 and some compounds adopting these structures are listed in Table 1.24. Corundum contains only one cation, Al3+, whereas ilmenite contains two cations that are ordered over the octahedral sites that are occupied by Al in corundum. In LiNbO3, the same octahedral sites are occupied but the cation ordering arrangement is different.
The unit cell of all three structures is hexagonal and has six cp oxygen layers parallel to the basal plane, shown in Fig. 1.46(a) at c heights 1/12, 3/12, 5/12, 7/12, 9/12 and 11/12. Cations are in octahedral sites mid‐way between the oxygen layers; alternate layers of cation sites are occupied by Fe and Ti in ilmenite, Fig. 1.46(b). Pairs of octahedra share a common face in the c direction and cation repulsion between the cation pairs causes distortion from an ideal hcp structure. In all cases, the cation octahedra are distorted with three long and three short bonds. Repulsion between Nb5+ and Li+ in LiNbO3 causes displacement of Li to a position near the triangular face at the opposite side of the octahedron. LiNbO3 and LiTaO3 are ferroelectric materials and cation displacements within the face‐sharing octahedra are responsible for the polar crystal structures and dipole reorientation in an applied electric field, which is a characteristic feature of ferroelectrics.
The cation ordering sequence in LiNbO3 is different to that in ilmenite, Fig. 1.46(c) and (d). Li and Nb are both present, ordered, between any pair of close packed oxide ion layers whereas Fe and Ti occupy alternate sets of layers in ilmenite. An alternative view of the LiNbO3 structure is given in (d), which illustrates that LiNbO3 can also be regarded as a grossly distorted perovskite structure. Tilting and rotation of the NbO6 octahedra (B sites) reduce the coordination of the A sites from 12 to distorted octahedral, and these are occupied by Li. If we regard LiNbO3 as a distorted perovskite, its tolerance factor is 0.78, which, in practice, represents the lower limit for materials that can be regarded as distorted perovskites.
1.17.12 Fluorite‐related structures, pyrochlore, weberite and rare earth sesquioxides
The fluorite structure of CaF2 can be described as eutactic ccp Ca2+ ions with F– ions occupying all tetrahedral sites, Fig. 1.29. A number of more complex fluorite‐related structures occur with an excess or deficiency of anions or with cation ordering.
A remarkable anion‐excess fluorite is LaH3; LaH2 forms the basic fluorite structure and extra hydrogens occupy fully the octahedral sites. The structure therefore has all tetrahedral and octahedral sites occupied by H within a ccp La array, Fig. 1.24. A similar structure is observed in intermetallics such as Li3Bi and Fe3Al. These structures represent an extreme with full occupancy of all tetrahedral and octahedral sites. Partial tetrahedral site occupancies occur in the lanthanum hydrides which form a continuous solid solution between LaH1.9 and LaH3.
Figure 1.46 Crystal structures of (a) corundum, (b) ilmenite and (c, d) LiNbO3.
Table 1.24 Some compounds with corundum‐related structures
| Corundum | M2O3: M = Al, Cr, Fe (hematite), Ti, V, Ga, Rh |
| (α‐alumina) | Al2O3: with Cr dopant (ruby) Al2O3: with Ti dopant (sapphire) |
| Ilmenite | MTiO3: M = Mg, Mn, Fe, Co, Ni, Zn, Cd MgSnO3, CdSnO3 NiMnO3 NaSbO3 |
| LiNbO3, LiTaO3 |
Oxygen‐excess fluorites occur in UO2+x (see Fig. 2.10); the structure is distorted locally and the extra oxide ions are displaced off cube body centres. This has a knock‐on effect in which some of the corner oxide ions are displaced onto interstitial sites. The UO2+x system has been studied in considerable detail because of its importance in the nuclear industry as a fuel in nuclear reactors.
Mixed anion oxyfluorides such as LaOF and SmOF form the fluorite structure in which similarly sized O2– and F– ions are disordered over the tetrahedral sites. Various examples of mixed‐cation fluorites are known in which two different cations are ordered, as shown for several examples in Fig. 1.47. These structures are rather idealised, however, since the anions are displaced off the centres of the tetrahedral sites in various ways to give, for instance, a distorted tetrahedral environment for Cr2+ in SrCrF4 and distorted octahedral coordination for both Ti and Te in TiTe3O8.
Li2O has the antifluorite structure and various Li‐deficient antifluorites are good Li+ ion conductors; for instance, Li9N2Cl3 has 10% of the Li+ sites vacant, giving rise to high Li+ ion mobility.
The pyrochlore structure may be regarded as a distorted, anion‐deficient fluorite with two different‐sized cations A and B. Its formula is written as either A2B2X7 or A2B2X6X′. The unit cell is cubic with a ~11 Å and contains eight formula units. In principle,