Spinels half-metallicity

Tlie plienomenon of half-metallicity has gained much interest in order to understand the unusual band structures in various classes of materials and their potential applications in future electronic devices. For example, zinc blend pnictides and chalcogenides e.g. CrAs) are another class of non-oxide materials (apart from Heuslers) in addition to the many oxide classes that are potentially half-metallic materials. Alkali metal doped rare earth oxomanganates, (REi- A MnOs), rutile-Cr02, spinel-Fe304 and Sr2peMo06 double perovskite oxide are examples of important half-metallic oxides. 

A half-metallic character has been proposed in another spinel oxide, LiCr204, by spin-polarised first principles density functional calculations (Figure 5.8). Both full potential linearised augmented plane-wave (FP-LAPW) and the LMTO methods were used to calculate the band structures. The lattice and other internal parameters for the cubic spinel structure were optimised with FP-LAPW calculations and subsequently the optimised parameters were used in the band structure calculations. Flowever, the experimental realisation of Li(Cr " " " )204 is still a challenge to confirm its predicted electronic and magnetic properties. 

In the following, we start by assuming purely ionic structures. In spinel the oxide ions form a cubic closest-packing. Two-thirds of the metal ions occupy octahedral interstices, the rest tetrahedral ones. In a normal spinel the A ions are found in the tetrahedral interstices and the M ions in the octahedral interstices we express this by the subscripts T and O, for example Mgr[Al2](904. Since tetrahedral holes are smaller than octahedral holes, the A ions should be smaller than the M ions. Remarkably, this condition is not fulfilled in many spinels, and just as remarkable is the occurrence of inverse spinels which have half of the M ions occupying tetrahedral sites and the other half occupying octahedral sites while the A ions occupy the remaining octahedral sites. Table 17.3 summarizes these facts and also includes a classification according to the oxidation states of the metal ions. 

Spinels have a crystal structure in which there is a face-centered cubic arrangement of O2 ions. There are two types of structures in which cations have octahedral or tetrahedral arrangements of anions surrounding them. In the spinel structure, it is found that the +3 ions are located in octahedral holes and the tetrahedral holes are occupied by the +2 ions. A different structure is possible for these ions. That structure has half of the +3 metal ions located in the tetrahedral holes while the other half of these ions and the +2 ions are located in the octahedral holes. In order to indicate the population of the two types of lattice sites, the formula for the compound is grouped with the tetrahedral hole population indicated first (the position normally occupied by the +2 ion, A) followed by the groups populating the octahedral holes. Thus, the formula AB204 becomes B(AB)04 in order to correctly... 

Further adding to the complexity of the spinel structure are three possible arrangements of the metal ions in the cubic close-packed anions. The ordering of divalent metal ions (such as Mg2+) on the proper tetrahedral sites and all the trivalent ions (as Ai3+) in the correct octahedral sites, will give rise to the normal spinel structure. If the divalent ions occupy some of the octahedral sites and half of the trivalent ions move to the tetrahedral sites, the structure is then referred to as the inverse spinel structure. The last case exists when the tetrahedral sites and the octahedral sites are occupied by a mixture of di- and tri-valent ions. This type is known to generate the random spinel structure, and the exact composition and populations in the... 

The mixed-metal oxide spinel, MgA fTt, is one of the most important inorganic materials. The structure of spinel can be regarded as a ccp structure of O2-anions with Mg2+ ions orderly occupying /8 of the tetrahedral interstices, and Al3+ ions orderly occupying half of the octahedral interstices the remainder 7/x tetrahedral interstices and half octahedral interstices are unoccupied. The sites of the three kinds of ions in the face-centered cubic unit cell are displayed in Fig. 9.6.29. 

Nickel spinel, NiAl204, was used for making synthesis gas by C02 reforming of methane. The spinel structure is based on a cubic close-packed array of oxide ions. Typically, the crystallographic unit cell contains 32 oxide ions one-eighth of the tetrahedral holes (of which there are two per anion) are occupied by the divalent metal ion (N +), and one-half of the octahedral holes (of which there is one per anion) are occupied by the trivalent metal ion (AP+). These spinels are usually very stable and have been used for high temperature catalytic reactions [12,13]. 

The spinel structure is based on a face-centered cubic arrangement of O2- ions that results in cation sites that can be described as octahedral and tetrahedral holes. In the structure of spinel, the aluminum ions are surrounded by six oxide ions in an octahedral arrangement and magnesium ions are surrounded by four oxide ions in a tetrahedral arrangement. It should be noted that the +3 ions reside in octahedral holes, whereas the +2 metal ions reside in tetrahedral holes (see Chapter 3). In a variation of this structure, compounds of this type form an arrangement in which half of the M 3+ ions reside in tetrahedral holes and the M2+ and half of the M 3+ ions reside in octahedral holes. Such a structure is known as... 

Some mixed metal oxides AB2X4 in which at least one of the metals is a block element (e.g. CoFe204) possess an inverse spinel structure which is derived from the spinel lattice by exchanging the sites of the A ions with half of the B ions. 

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