Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 62

Anion‐deficient perovskites occur commonly and, in cases where the anions are ordered, a variety of more complex structures are generated. As a consequence of anion deficiency, the B site coordination number must reduce from six on average. In the brownmillerite structure, Ca(Fe,Al)O₂.₅, Fig. 1.42(b), the average B site coordination number is expected to be five and this is achieved in a layered structure with alternating sheets of tetrahedra and octahedra. In LaNiO₂.₅, Fig. 1.42(c), the Ni coordination is a mixture of NiO₆ octahedra and NiO₄ square planes which are ordered to give a 2 × 2 × 2 supercell relative to the perovskite subcell. In the a and b directions, octahedra and square planes alternate but in the c direction chains of corner‐sharing octahedra form which are connected to adjacent chains by the square planar units. The Ni polyhedra exhibit antiphase tilting along the three unit cell axes and therefore, may be described in Glazer notation as a ^– a ^– c ^– . This material was produced from LaNiO₃ by hydrogen reduction, but a residual amount of oxygen remained in the O(4) sites, giving a composition, for the particular material that was studied, of LaNiO₂.₅₆₍₁₎; possibly, using different preparative conditions, materials with different oxygen contents could be prepared.

The above are two examples of anion‐ordered crystal structures which occur at specific compositions. Other anion ordering arrangements are known both at these and other compositions. In addition, solid solutions form in many systems in which the anion content is variable and can be represented by the general formula ABO_3–δ . In these, the oxygen vacancies may be distributed at random through the perovskite structure or locally ordered structures may form in which small domains of a particular structure type are distributed at random through a disordered perovskite network.