some olivines are listed in Table 1.23. Olivines occur mainly with oxides but also with sulphides, selenides and some fluorides. Various cation charge combinations occur, such that in oxides the three cations have a net charge 8+.
Olivines (mainly forsterite and fayalite) are believed to be the main mineralogical constituent of the Earth's upper mantle. At high pressures, many olivines transform to the spinel structure and spinels are probably the main constituent of the Earth's lower mantle. Volume changes associated with the olivine to spinel phase transformation may have had fundamental geological consequences during the evolution of the Earth, involving the formation of mountain ranges and under‐sea ridges. When spinel material from the lower mantle was pushed upwards to the Earth's surface, it transformed to olivine due to the reduction in pressure. The reverse transformation, olivine to spinel, with a volume contraction may be a contributing factor to earthquakes.
Figure 1.45 Olivine structure of LiFePO4.
Modified from J. J. Biendicho and A. R. West, Solid State Ionics 203, 33 (2011).
Table 1.23 Some compounds with the olivine structure
| General formula | |||
|---|---|---|---|
| Octahedral site | Tetrahedral site | hcp anion | Examples |
| II2 | IV | O4 | Mg2SiO4 (forsterite) Fe2SiO4 (fayalite) CaMgSiO4 (monticellite) γ‐Ca2SiO4 A2GeO4: A = Mg, Ca, Sr, Ba, Mn |
| III2 | II | O4 | Al2BeO4 (chrysoberyl) Cr2BeO4 |
| II, III | III′ | O4 | MgAlBeO4 |
| I, II | V | O4 | LiFePO4 (triphylite) LiMnPO4 (lithiophylite) |
| I, III | IV | O4 | LiRESiO4: RE = Ho, …, Lu NaREGeO4: RE = Sm, …, Lu LiREGeO4: RE = Dy, …, Lu |
| II2 | IV | S4 | Mn2SiS4 Mg2SnS4 Ca2GeS4 |
| I2 | II | F4 | γ‐Na2BeF4 |
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
