Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 74

The garnets are a large family of complex oxides, including both minerals and synthetic materials, some of which are important ferromagnetic materials; yttrium aluminium garnet (YAG) is used as a synthetic gemstone and when doped with neodymium is the key component in YAG lasers; more recently, garnets with high Li⁺ ion conductivity have been synthesised; garnets have hardness 6.5–7.5 on the Mho scale and are used industrially as an abrasive on sandpaper. They have the general formula A₃B₂X₃O₁₂: A = Ca, Mg, Fe, etc.; B = Al, Cr, Fe, etc.; X = Si, Ge, As, V, etc. A is a large ion with a radius of ~ 1 Å and has a coordination number of eight in a distorted cubic environment. B and X are smaller ions which occupy octahedral and tetrahedral sites, respectively. Garnets with A = Y or a rare earth: Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu and B, X = Fe³⁺ have interesting magnetic properties. One of the most important is yttrium iron garnet (YIG), Y₃Fe₅O₁₂, the structure of which is shown in Fig. 1.49. Many other A, B, X combinations are possible, such as those in Table 1.25.

Figure 1.49 The garnet crystal structure.

The unit cell of garnet is body centred cubic, a ≈ 12.4 Å, and contains eight formula units. The structure may be regarded as a framework built of corner‐sharing BO₆ octahedra and XO₄ tetrahedra. The larger A ions occupy eight‐coordinate cavities within this framework. In YIG and the rare earth garnets, the B and X ions are the same, Fe³⁺.