A-face-centred

In Figure 8.19 is shown the X-ray photoelectron spectrum of Cu, Pd and a 60 per cent Cu and 40 per cent Pd alloy (having a face-centred cubic lattice). In the Cu spectrum one of the peaks due to the removal of a 2p core electron, the one resulting from the creation of a /2 core state, is shown (the one resulting from the 1/2 state is outside the range of the figure). 

EXAFS spectra of platinum metal, having a face-centred cubic crystal stmcture, have been obtained at 300 K and 673 K. Explain what qualitative differences you might expect. How many nearest-neighbour atoms are there in this stmcture Illustrate your answer with a diagram. 

The process requires the interchange of atoms on the atomic lattice from a state where all sites of one type, e.g. the face centres, are occupied by one species, and the cube corner sites by the other species in a face-centred lattice. Since atomic re-aiTangement cannot occur by dhect place-exchange, vacant sites must play a role in the re-distribution, and die rate of the process is controlled by the self-diffusion coefficients. Experimental measurements of the... 

Fig. 8.12. The structure of 0.8% carbon martensite. During the transformation, the carbon atoms put themselves into the interstitial sites shown. To moke room for them the lattice stretches along one cube direction (and contracts slightly along the other two). This produces what is called a face-centred tetragonal unit cell. Note that only a small proportion of the labelled sites actually contain a carbon atom.

Figure 3.16. WiditinnstiiUcn precipitation of a hexagonal close-packed phase from a face-centred cubic phase in i Cu Si alloy. Precipitation occurs on [ I I Ij planes of the matrix, and a simple epitaxial erystallographie correspondence is maintained. (0 0 0 Di, , (I I (after Barrett...

Except for Ceo, lack of sufficient quantities of pure material has prevented more detailed structural characterization of the fullerenes by X-ray diffraction analysis, and even for Ceo problems of orientational disorder of the quasi-spherical molecules in the lattice have exacerbated the situation. At room temperature Cgo crystallizes in a face-centred cubic lattice (Fm3) but below 249 K the molecules become orientationally ordered and a simple cubic lattice (Po3) results. A neutron diffraction analysis of the ordered phase at 5K led to the structure shown in Fig. 8.7a this reveals that the ordering results from the fact that... 

The selection of materials for high-temperature applications is discussed by Day (1979). At low temperatures, less than 10°C, metals that are normally ductile can fail in a brittle manner. Serious disasters have occurred through the failure of welded carbon steel vessels at low temperatures. The phenomenon of brittle failure is associated with the crystalline structure of metals. Metals with a body-centred-cubic (bcc) lattice are more liable to brittle failure than those with a face-centred-cubic (fee) or hexagonal lattice. For low-temperature equipment, such as cryogenic plant and liquefied-gas storages, austenitic stainless steel (fee) or aluminium alloys (hex) should be specified see Wigley (1978). 

Horse-spleen apoferritin crystallizes in a face-centred, close-packed, cubic arrangement, in the space group F432, with molecules at the 432 symmetry points of the crystal lattice (Harrison, 1959). This publication was the logical extension of the DPhil thesis of the Oxford chemist Pauline M. Cowan (as she was before her marriage to Roy Harrison), and represented the first publication in what was to be a long and distinguished series of contributions on ferritin from the undisputed Iron Lady of iron metabolism. ... 

Mono- or single-crystal materials are undoubtedly the most straightforward to handle conceptually, however, and we start our consideration of electrochemistry by examining some simple substances to show how the surface structure follows immediately from the bulk structure we will need this information in chapter 2, since modern single-crystal studies have shed considerable light on the mechanism of many prototypical electrochemical reactions. The great majority of electrode materials are either elemental metals or metal alloys, most of which have a face-centred or body-centred cubic structure, or one based on a hexagonal close-packed array of atoms. 

The summation in the last expression is over all different configurations, 123, of three defects, each one being within nearest-neighbour distance of at least one other, and yt is a geometric factor. (For example, four configurations of a trivacancy can be drawn for a face-centred cubic lattice. In only one of these... 

Transformation (deformation) of a face-centred cubic unit cell into a body-centred cubic cell. 

A face-centred cubic cell (having a unit edge a0), compressed along an axis, when the reduction corresponds to a face-centred tetragonal cell with a c a ratio... 

Figure 3.12. A face-centred cubic cell (edge ac) can be described in terms of a body-centred tetragonal cell with a da ratio having the value - 2 (a, = ac yfl /2, ct = ac). By a convenient compression along the c axis, the body-centred tetragonal cell can be transformed into a body-centred cubic cell.

Nevertheless it may be observed that, in some specific cases, reference to a pseudo-face-centred tetragonal cell may be useful, for instance to compare the structure under examination with a face-centred cubic structure. 

The otherl4th group elements, Si, Ge and oSn have the diamond-type structure. The tI4- 3Sn structure (observed for Si and Ge under high pressure) can be considered a very much distorted diamond-type structure. Each Sn has four close neighbours, two more at a slightly larger and another four at a considerable larger distance. Fig. 7.13 shows the (3Sn unit cell. Lead, at ambient pressure, has a face-centred cubic cF4-Cu type structure. 

It is a close-packed superstructure based on a face-centred cubic pseudo-cell. Distorted close-packed triangular layers are stacked in close-packed ABC sequence. 

In the FeS2 (pyrite), cP12, structure, the Fe atoms form a face-centred cubic array, the... 

A) Face-centred cubic structure of NaCI (B) Body-centred cubic structure... 

A similar situation is illustrated by Bi203. The high-temperature 5-Bi203 phase has a face-centred-cubic Bi-atom array with O atoms occupying in a disordered manner three-quarters of the tetrahedral interstices. At lower temperatures, the anion vacancies become ordered on these sites, and the energy splitting AH of the vacant and occupied sites varies with the degree of order as described by equation (3.23). 

The F -ion conductor first discovered by Faraday represents a more complex order-disorder transition to fast ionic conduction. At all temperatures, PbF2 is reported to have the fluorite structure in which the F ions occupy all the tetrahedral sites of a face-centred-cubic Pb -ion array however, the site potential of the Pb ions is asymmetric, and a measurement of the charge density with increasing temperature indicates that the F ions spend an increasing percentage of the time at the... 

In support of this model, it is noted that LnFj - which has F ions on both octahedral and tetrahedral sites of a face-centred-cubic Ln -ion array - becomes a fast F -ion conductor below its melting point without any change in the cation array (O Keeffe and Hyde, 1975). This observation shows that some low-energy excitation other than the displacement of F ions into octahedral sites is operative, as is postulated with the cluster-rotation model for PbF2. 

The amphoteric nature of hydrated thorium oxide has been established. Microgram scale preparation of Cm02, which has a face-centred cubic... 

At -140 °C solid phosphine crystallises in a face-centred cubic form with four molecules in the unit cell lattice constant is a = 6.31 0.01... 

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