Spinel lattice parameters

Figure 2.3. High-resolution scans over characteristic spinel reflections of some catalyst precursors using the Guinier transmission geometry and monochromated Co radiation. The graph indicates the dependence of the spinel lattice parameter determined from the dependence of the spinel lattice parameter determined from the (440) reflection on the aluminum content built into the lattice. Note that all catalyst samples contain nominally the same amount of aluminum. Lattice parameters above the line indicate the presence of an excess of calcium besides aluminum. The data Ml-3 are pure alumina spinel samples.

The unit cell of (3-alumina is hexagonal, with lattice parameters a = 0.595 nm, c = 2.249 nm. The dominant features of the idealized structure of (3-alumina, with composition NaAlnOi7, are layers called spinel blocks stacked perpendicular to the c axis (Fig. 6.10a). These blocks are composed of four oxygen layers in a... 

The spinel blocks in (3-alumina are related by mirror planes that mn through the conduction planes that is, the orientation of one block relative to another is derived by a rotation of 180°. A second form of this compound, called (3"-alumina, has similar spinel blocks. However, these are related to each other by a rotation of 120°, so that three spinel block layers are found in the unit cell, not two. The ideal composition of this phase is identical to that of (3-alumina, but the unit cell is now rhombohedral. Referred to a hexagonal unit cell, the lattice parameters are a = 0.614 nm, c = 3.385 nm. The thickness of the spinel blocks and the conduction planes is similar in both structures.3... 

The spinel ferrites were fabricated by solid state reaction technique. Cobalt and Zinc ferrites CoxZnyFe204,(x=0.7,0.3,0.4,0.2 and y=0.3,0.7,0.6,0.8) were prepared by solid state reaction technique. The crystalline structure of the sample was investigated by X-ray diffraction(XRD). All samples show cubic spinel structure. The lattice parameter decreases with increasing cobalt content. Magnetic properties shows that the prepared sample exhibit ferromagnetic behaviour at room temperature. The saturation magnetization increases with increasing cobalt content. Curie temperature... 

Fig. 7.16 (a) Lattice parameters and (b) voltage vs x for two forms of Li TiSj. The c parameter for the layered (IT) form is the distance between the layers the a parameter in the cubic (spinel) form is the edge of the cubic unit cell. Data from Sinha and Murphy (1986), Dahn and McKinnon (1984), and Dahn and Haering (1981b). 

Figure 12. Correlation of the lattice parameter of the spinel Lii+JV[n2-/)4 with (a) the lithium content, (b) manganese oxidation state, and (c) capacity loss of the cell over the first 120 cycles, after ref 157.

Both powders collected from the reactor wall and from the reactor bottom showed similar diffraction patterns (Fig. 5.) indicating the presence of spinel as the dominant phase (amount of sample C was negligible in these experiments). Composition of the spinel phase was estimated from the lattice parameters (a), assuming, that the increase of lattice parameter due to Zn incorporation is proportional to the Zn concentration. From these calculations the compositions of sample R and sample RB were Zno.7Fe2.3O4 and Zno.4Fe2.6O4, respectively. Consequently, some of the Zn content of the starting mixture could not build into the spinel structure and left the system through the exhaust in the form of very fine ZnO powder. 

The variance of the lattice parameters in the 111 direction for this Ni/Al203 catalyst, and the paracrystal size in this direction were found to be very similar to those that had been found earlier for an NH3 synthesis catalyst. It was suggested that there could be a common mechanism for the structural perturbation in each catalyst involving solution of a spinel phase in the host metal. Since the ratio of the volume of a Ni atom in bulk Ni to that of an oxygen anion in nickel aluminate is 1.5 1, it was suggested that the substitution could involve one (A102) group for three nickel atoms. 

The crystal structure of BAM is shown in fig. 34. It has the ft -alumina structure with lattice parameters a = 5.6275 k,b = 22.658 A (fyi et al., 1986). There are layers consisting of Ba2+ and O2- ions between the spinel blocks and Eu2+ is generally believed to stay within these Ba-0 layers. The positions of BR, aBR, and mO sites within this Ba-0 layer are shown in fig. 34. The BR site is the substitutional site of Ba and the other two sites are interstitial sites. 

O Neill, H. St C. Navrotsky, A. (1983) Simple spinels crystallographic parameters, cation radii, lattice energies, and cation distribution. Amer. Mineral., 68,181-94. 

In general, the sequence of oxide-formation from a MgAl(C03) LDH proceeds as follows. Firstly, the LDH is converted to a mixed MgAl oxide with the MgO rock-salt type structure at approximately 400°C. The lattice parameters of the mixed oxide are generally lower, however, than those measured for pure MgO, indicating that the Al3+ ions are inserted into the structure, which also introduces lattice defects. At higher temperatures the mixed oxide decomposes into MgO and spinel, MgAl204 [160-164], A similar sequence has been observed for the thermal decomposition of a NiAl-LDH by Sato el al. [28],... 

Room-Temperature Lattice Parameters for Several Spinels Containing Mn3+ and/or Cu2+ Cations... 

According to Fig. 6.26, spinels, prepared by mechanical activation followed by heat treatment, are characterized by very low lattice parameter a, which increases as temperature rises. It is well known that the radius of Mu ions is lower than that of Mn . Mn ions are more stable at lower temperatures. Thus, low values of a parameter for the samples with molar ratio Li/Mn=l/2 point to the increased amount of Mn ions comparing with that of stoichiometric spinel [93]. One can see that in the samples prepared from Li2C0 by mechanical activation and heating at 450°C, the value of a and the amount of Mn ions are higher than in the case of LiOH samples because of reducing character of CO2, eliminating in the course of interaction [98]. 

For y=0.005, a=8.24lA y=0.01, a=8.233A y=0.015, a=8.227A. The lattice parameters show a decrease with Co and Y doping. This is perhaps due to the effects of the Co-0 bond and Y-0 bond in the octahedron when the cations are incorporated into the spinel structure. 

In high radiation fields, the spinel crystal structure has been shown to change. The structure, while still cubic, becomes disordered with a reduction in lattice parameter. The disordered rock-salt structure has a smaller unit cell reflecting the more random occupation of the octahedral sites by both trivalent and divalent ions. Increased radiation damage results in the formation of completely amorphous spinels. Radial distribution functions (g(r)) of these amorphous phases have Al-0 and Mg-O radial distances that are different from equivalent crystalline phases. The Al-0 distance in the amorphous form is reduced from Al-O of 0.194nm in the crystalline phase to 0.18nm in the amorphous phase, while the Mg-O distance is increased (0.19nm in the crystal to 0.21 nm in the amorphous phase). Differences between the Al-O distances of crystalline and amorphous phases are a characteristic of both calcium and rare earth aluminates. 

The oxide NiAl204 adopts the spinel structure, with a cubic lattice parameter of 0.8048 nm. The structure is derived from a face-centred cubic lattice. Making use of Table 6.4, calculate the angles of diffraction of the first six lines expected on a powder diffraction pattern. 

If without additives, a carrier such as A3 consists of a mixture of 8-, 0-, and a-aluminas between 900° and 1000°C. The presence of the metal oxides, introduced by impregnation, effectively maintains a cubic type structure at a calcining temperature of 900°-1000°C. With alumina, these oxides form spinel-structured compounds which are more or less well crystallized. Because of the insertion of alumina, the lattice parameter of these compounds is expanded with respect to that of the stoichiometric spinel. The properties of the carrier are thus maintained up to a temperature which depends on the considered mixed oxide. At this temperature—about 1000 °C with magnesium aluminate and 900°C with zinc and copper aluminates—the stoichiometric spinel recrystallizes while a-alumina is rejected (6). The carrier then suddenly loses its mechanical and structural properties. None of the mentioned additives could improve the stability of the Cl carrier above 1000°C. 

This expression accounts for 96.7% of the variations in the lattice parameter of 149 spinel oxides (Hill, Craig Gibbs, 1979). 

Table 2.2. Cation distribution, lattice parameter and u value for several spinels.

Estimate (a) the saturation magnetisation and (b) the magnetic inductance for the cubic ferrite CoFe204 with the inverse spinel stmcture. Co " " is a d ion. The cubic unit cell has a lattice parameter of 0.8443 nm and contains eight formula units. Assume that the orbital angular momentum is completely quenched in this material. 

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