Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 55

There are five main AX structure types, rock salt, CsCl, NiAs, sphalerite and wurtzite, each of which is found in a large number of compounds. There are also several less common AX structures. Some are distorted variants of one of the main structure types, e.g.:

1 FeO at low temperatures, <90 K, has a rock salt structure with a slight rhombohedral distortion (the α angle is increased from 90 to 90.07° by a slight compression along one threefold axis). This rhombohedral distortion is associated with magnetic ordering in FeO at low temperatures (see Chapter 9).

2 TlF has a rock salt‐related structure in which the fcc cell is distorted into a face centred orthorhombic cell by changing the lengths of all three cell axes by different amounts. The distortion arises because, in this structure, Tl+ (Xe core) 4f145d106s2 is a non-spherical cation exhibiting a stereochemically-active inert (or lone) pair effect.

3 NH4CN has a distorted CsCl structure (as in NH4Cl) in which the CN– ions do not assume spherical symmetry but are oriented parallel to face diagonals. This distorts the symmetry to tetragonal and effectively increases a relative to c.

Other AX compounds have completely different structures, e.g.:

1 Compounds of the d 8 ions, Pd and Pt (in PdO, PtS, etc.), often have a square planar coordination for the cation, associated with the Jahn-Teller effect; d 9 ions also show this effect, e.g. Cu in CuO.

2 Compounds of heavy p‐block atoms in their lower oxidation states (e.g. Tl+, Pb2+, Bi3+) often have distorted polyhedra in which the cation exhibits the inert pair effect, as with TlF, above. Thus, in PbO and SnO the M2+ ion has four O2– neighbours to one side giving a square pyramidal arrangement, Fig. 3.14. InBi is similar with Bi3+ showing the inert pair effect and irregular coordination.