Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 72

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.