The arrangement of ions in ceramics

When atoms combine to form solids their outer electrons enter new states whilst the inner shells remain in low-energy configurations round the positively charged nuclei. 

The relative positions of the atoms are determined by the forces between them. In ionic materials, with which we are mainly concerned, the strongest influence is 

Ionic size is determined from the distances between the centres of ions in different compounds and is found to be approximately constant for a given element in a wide range of compounds provided that account is taken of the charge on the ion and the number of oppositely charged nearest neighbours (the coordination number). Widely accepted values, mostly as assessed by R.D. Shannon and C.T. Prewitt [2], are given in Table 2.2. 

In perovskite (CaTiOs) and its isomorphs such as BaTiC 3, the large alkaline earth ions replace O2- ions in the anion lattice and the Ti4+ ions occupy all the octahedral interstices that are surrounded only by O2- ions, i.e. no Ti4+ ions are immediately adjacent to divalent cations. 

The ionic radius concept is useful in deciding which ions are likely to be accommodated in a given lattice. It is usually safe to assume that ions of similar size and the same charge will replace one another without any change other than in the size of the unit cell of the parent compound. Limitations arise because there is always some exchange interaction between the electrons of neighbouring ions. 

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