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Chapter 25: Problem 14

Discuss the crystal-field effects on spinel structures.

Short Answer

Expert verified
Crystal-field theory describes how the asymmetrical electric field in a crystalline structure affects the energy levels of the central cation's d or f orbitals. In a spinel structure, the field created by the surrounding oxygen anions creates an environment that causes the d-orbitals of the transition metal ions to split into distinct energy levels. This can cause changes in magnetic and optical properties, including shifts from a normal to an inverse spinel structure.

Step by step solution

01

Understanding crystal-field theory

Crystal field theory (CFT) is a bonding model that explains many important properties of transition-metal complexes, especially colors, magnetism, and spectral behavior. In essence, CFT focuses on the non-covalent interaction of a central transition metal ion with a set of surrounding anions or neutral molecules. The electrical field from the surrounding ions or molecules may cause the d or f energy levels of the central cation to split into two or more new energy levels.
02

The basic spinel structure

Spinel is a class of minerals which crystallize in the isometric (cubic) crystal system with the general formula AB2O4. In a normal spinel, the divalent (2+) cation (A) occupies the octahedral site and the trivalent (3+) cation (B) occupies the tetrahedral site of the crystal lattice. The O4 forms a close packed array with A and B in the other voids.
03

The crystal-field effect in a spinel structure

In the context of a spinel structure, a crystal field created by the surrounding oxygen anions will lead to an interruption of the degeneracy of the 5 d-orbitals of the transition metal ions. This degeneracy would be apparent in a free ion. However, in the crystalline spinel, the energy levels of the d-orbitals of the cations split, affecting its magnetic and optical properties. As a result, the structure might change from a normal to an inverse spinel under certain conditions, where the divalent (2+) cation (A) occupies the tetrahedral site and the trivalent (3+) cation (B) occupies the octahedral site in the lattice.

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