From WolfWikis

Spinel

Spinel is an important class of mixed-metal oxides, which has the general chemical composition of AB₂O₄. Normally A is a divalent atom of radius between 80 and 110pm, such as Mg, Fe, Mn, Zn, and Cu. And B is a trivalent atom of radius between 75 and 90pm, such as Ti, Fe, Al, and Co. The structure consist of a cubic closed-packed array of 32 oxide ions, which forms 64 tetrahedral holes and 32 octahedral holes in one unit cell (containing eight formula units (AB₂O₄)₈).[1] There are two types of sub-cells commonly described for the spinel structure, here shown a structure a and structure b in Figures 1-a and 1-b respectively. Structure a shows the filling of 2 tetrahedral sites within one-eighth of the unit cell, and structure b shows a filled octahedral site. The arrangement of these two cubic sub-cells in one unit cell is shown in Figure 2. There are 12 filled octahedral sites not centered in the sub-cells that are also filled to give a total of 16 filled octahedral sites. In a normal spinel structure, all the trivalent cations are located in half the octahedral sites, while all the divalent cations occupy 1/8 of the tetrahedral sites. The inverse spinel structure will be discussed with examples below.

Figure 1-a: Two kinds of occupied tetrahedral sites in spinel sub-cell a. A is in green and O is in red.

Figure 1-b: Occupied octahedral site in spinel sub-cell b. B is in gray, and O is in red.

Figure 2: Arrangement of structure a and b in one unit cell. shaded one reoresents structure a, while white one reprensents b.

Figure 3 shows an example spinel MgAl₂O₄. Notice the red oxygen atoms, the green magnesium atoms in the tetrahral holes, and the grey aluminum atoms in the octahedral holes. The pattern of structure a and structure b can also be seen in Figure 3.

The structure of spinel is composed of edge-sharing BO₆ octahedral chians. In one unit cell, along c axis, there are four layers of BO₆ octahedral chains. Neighboring layers are pependicular to each other, as shown in Figure 4. Between BO₆ octahedral chains in one layer are sited by tetrahedrons, the arrangment of octahedrons and tetrahedrons in two neignboring layers are shown in Figure 5.

Figure 3: Arrangement of atoms within the MgAl₂O₄ unit cell. Mg is in green, Al is in gray, and O is in red.

Figure 4: Arrangement of edge-sharing BO6 octahedral along c axis. a) the first and second layers of BO₆ octahedral chains, b) the second and third layers of BO₆ octahedral chians, c) the third and forth layers of BO₆ octahedral chains.

Figure 5: Arrangement of octahedrons and tetrahedrons along c axis.

CoFe₂O₄

Figure 6: Crystal Structure of CoFe₂O₄ where green atoms are Co, pink atoms are Fe, and blue atoms are O.

CoFe₂O₄ has an inverse spinel crystal structure. The normal crystal structure of an AB2O4 spinel consists of the A²⁺ atoms occupying all of the tetrahedral coordination sites and the B³⁺ atoms occupying all of the octahedral sites[2]. In the case of an inverse spinel such as CoFe₂O₄, the Co cation occupies one half of the octahedral coordination sites. Half the Fe³⁺ cations occupies the other half of the octahedral coordination sites as well as all of the tetrahedral coordination sites. The crystal structure of CoFe₂O₄ is shown in Figure 6.

CoFe₂O₄ has recently been studied as a component for ferrofluids. Ferrofluids are colloidal suspensions of ferromagnetic nanoparticles that exhibit different magnetic properties depending on their particle size such as superparamagnetism. Superparamagnetism occurs when ferromagnetic particles reach a critical size that is so small, that the particle diameter is less than a single magnetic domain[3]. The critical particle size is around 10 nm. In a ferromagnetic bulk material with particle sizes much greater than 10 nm, spontaneous magnetization domains occur. Each individual domain contains its own magnetization direction and is separated from other domains by domain walls[4]. In the case of nanoparticle systems, each particle behaves as a single domain, constraints are removed, and each single domain can spontaneously switch directions[4]. When an alternating magnetic field is applied to the superparamagnetic particles, the particles undergo thermal fluctuations as the spins overcome their blocking energy barrier and flip with the alternating magnetic field[4].

The ability to produce heat from the thermal fluctuations makes CoFe₂O₄ collodial suspensions a good candidate for hyperthermia based cancer treatments. This treatment works by directing the CoFe₂O₄ nanoparticles to the cancerous area and raising the temperature of the particles when a magnetic field is applied which will in turn cook the cancerous tissue enough to lower its resistance to chemotherapy[5].

LiMn₂O₄

Lithium-ion batteries are a type of rechargeable battery developed for use in consumer electronics. The popularity of these types of batteries has increased because of their non-toxicity, slow loss of charge, and no memory effects. In a battery, the cathode site is composed of a Lithium-metal oxide, and the anode is usually Li impregnated graphite. These types of batteries have recharging capabilities and are also known as secondary batteries. Higher potential is usually the most desired behavior of a battery, but in order to be rechargeable, lithium must be able to diffuse in and out of the compound structure. The ability of Li¹⁺ to be re-inserted into its parent structure by a reverse electrochemical potential gives rise to the battery’s rechargeable capability [6].

Figure 7: LiMn₂O₄. The green atoms are Lithium, the pink atoms are Manganese, and the red atoms are oxygen.

LiMn₂O₄ is commonly used as the cathode for Li-ion batteries. It adopts a spinel-type structure. This spinel compound exhibits a high conductive potential (4V) which can be used to operate a wide variety of electronics (for comparison, a typical AA battery = 1.5V) [7]. This is attributed to the multiple oxidation states of Mn, which because of minor impurities within the MnO₂ structure can become a +3 cation, allowing charge transfer to occur [8].

LiMn₂O₄ is synthesized by conventional solid state methods. A lithium salt is combined with MnO₂, then ground, pressed, and heated to high temperatures. In addition, other methods that combine lithium metal with electrolytic MnO₂ have shown to reduce minor impurities during synthesis [9]. The structure consists of Manganeses atoms octahedrally coordinated to six Oxygens and Lithium atoms tetrahedrally coordinated to four Oxygens (See Figure 7.) Figure 8 shows a polyhedral structural representation of LiMn₂O₄. The MnO₆ octahedra are colored maroon, while the green Lithium ions fill the interstitial sites.

Figure 8: Extended structure of LiMn₂O₄ shown with MnO₆ polyhedra. With larger lattice parameters, the Li ions can diffuse more freely through the structure [7].

Once prepared, the solid is compressed into a pellet to study its conductive and resistive effects. Characterization shows that the spinel exhibits capacitance, which means it has the ability to store charge [8]. This conductive performance can be altered by varying the sythetic parameters. For example, during synthesis, the size of the lattice increases gradually along with the cooling rate. A larger lattice allows better ion diffusion of the lithium atoms into and out of the structure, but at the same time decreases charge transfer potential as the Mn-O bond elongates. Conversely, rapid quenching of the sample results in a higher observed potential, but a smaller lattice size, and therefore slower ion diffusion [7]. Diffusion of the Li ion is necessary for the battery to work. It carries the metal oxide to and from the cathode and anode. The transition metal is oxidized or reduced, depending on which terminal it is located at. This process creates the charge of the battery (see equation 1.)

Eq. 1     LiMn2O4 + C6  <--> Li1-xMn2O4 + LixC6

Fe₃O₄

Magnetite - Fe₃O₄, also known as lodestone,adopts a spinel type structure with the lattice parameter of 8.3941Å. Magnetite is an inverse-spinel. Cations are arranged with one Fe³⁺ per filled tetrahedral hole, and Fe²⁺ and the remaining Fe³⁺ randomly distributed in the octahedral holes. This places half of the smaller cations, Fe³⁺, in the smaller tetrahedral sites (as compared to the larger Fe²⁺ cations and the larger octahedral sites) [10]. A normal spinel has the formula AB₂O₄ with A atoms in the tetrahedral sites and both B atoms in the octahedral sites. Inverse spinels have B atoms in the tetrahedral sites and both A and B atoms in the octahedral sites. This inverse structure for magnetite was first suggested to explain the fast electron hopping - continuous exchange of electrons between Fe²⁺ and Fe³⁺ in the octahedral positions [10]. It has more recently been proposed that the magnetite structure should instead be described with Fe^2.5+ in the octahedral sites at ambient conditions to make the electron delocalization more obvious.[11].

When magnetite is heated to just above 122K, a threshold called the Verwey temperature, conductivity increases more than two orders of magnitude [12]. The Verwey temperature was originally believed to be the temperature above which the fast electron hopping could occur. More recently it has been discovered that at very similar temperatures ~120K a coordination crossover (CC) transition occurs. Below 120K magnetite is a normal spinel, with Fe²⁺ in the A or tetrahedral sites and Fe³⁺ in the B or octahedral sites. T_CC and T_v both show pressure dependence supporting the theory that the locations of the smaller Fe³⁺ ions and larger Fe²⁺ affect these properties. The distribution of Fe²⁺ and Fe³⁺ between the tetrahedral and octahedral holes also explains why magnetite is has a larger magnetic moment below 120K - the net magnetic moment is a sum of the alligned moments of Fe³⁺ within one local environment and the smaller magnetic moments of Fe²⁺ in the opposite direction decrease the magnitude only slightly. Above 120K half the Fe³⁺ moments are aligned with the Fe²⁺ magnetic moments, but the remaining Fe³⁺ magnetic moments in the opposite direction decrease the net magnetic moment. [11]

Figure 9: Magnetite unit cell. Red atoms are oxygen, blue and green represent iron - different colors for Td/Oh sites respectively.

Figure 10: Iron polyhedra nextwork (blue tetrahedra and green octahedra) in magnetite unit cell.

Figure 11: Edge sharing network of octahedra in magnetite.

Magnetite is the oldest known magnetic mineral (hence the name). It was used by early navigators to locate the magnetic North Pole and is used by many species including fish and birds for their sense of navigation [11]. Many types of bacteria thrive only at specific depths in water or sediment; they use magnetite and other magnetic minerals to sense whether they should swim up or down following the magnetic field of the earth. [13] The synthesis of these magnetite crystals, sometimes along with other magnetic minerals such as greigite (Fe₃S₄) is known as magnetosome synthesis and has been studied in the bacteria Magnetospirillum magnetotacticum. [14]

Sulfospinels

The spinel structure is not limited to oxides but is quite common amongst mixed sulfides (and selenides) of transition metals [15]. They can display a wide range of magnetic and electrical properties and some them are being investigated for use in solid state lithium based batteries. For example FeCr₂S₄ is an antiferromagnetic semiconductor. HgCr₂S₄ by contrast is ferromagnetic with T_c= 36K. There are also magnetic helix structures and giant magnetoresistance effects in these materials.

Greigite, Fe₃S₄, mentioned in the magnetite section above has an inverse spinel structure.

A special case are the sulfospinels involving the element copper. Although spinels typically involve divalent and trivalent ions this element remains monovalent in the presence of sulfur or selenium. If one half of the (divalent) iron ions in FeCr₂S₄ is replaced by copper, the material is once again semiconducting with all trivalent Fe ions. The intermediate members of the series Cu_xFe_1-xCr₂S₄ 0<x<0.5 however are metallic conductors. The metallic conduction is associated with a band of iron 3d character mixed with anion p-character.

Without iron CuCr₂S₄ is a metal with a full hole per unit in the sulfur-p valence band. This ocean of holes acts as a ferromagnetic coupling medium and the material is ferromagnetic even at room temperature.

References

[1] Gary Wulfsberg, Inorganic Chemistry, 2000, University Science Books, 691.

[2] Modern Ferrite Technology: Crystal structures of Ferrites, 52-69.

[3] Rao, C. N. R.; Gopalakrishnan, J. New Directions in Solid State Chemistry, 1997, Cambridge University Press, 296-301.

[4] Liu, C., et. al. J. Am. Chem. Soc.,2000, 122, 6263.

[5] Fortin, J. P., J. AM. CHEM. SOC., 2007, 129, 2628.

[6] Tarascon, J.M. et al., J. Electrochem. Soc. 1991, 138, 2859.

[7] Tao, L. et al., Mater. Lett., 2006, 60, 1251.

[8] Ferracin, L.C. et al., Solid State Ionics, 2002, 130, 215.

[9] Wan, C. et al., Mater. Lett., 2002, 56, 357.

[10] Fleet, M. E. Acta Cryst. 1981, B37, 917.

[11] Rozenberg, G. K. et al. Phys. Rev B 2007, 75, 020102.

[12] Garcia, J. et al. Phys. Review B 2001, 63, 054110.

[13] Alivasatos, A. P. Sci. Am. 2001, 66.

[14] Frankel, R. B; Bazylinski, D. A. Trends in Microbiology, 2006, 14, 329.

[15] Palmer, H.M., Greaves, C.: J. Mater. Chem. 9, 637 (1999)