Cu type

Nickel is usually alloyed with elements including copper, chromium, molybdenum and then for strengthening and to improve corrosion resistance for specific applications. Nickel-copper alloys (and copper-nickel alloys see Section 53.5.4) are widely used for handling water. Pumps and valve bodies for fresh water, seawater and mildly acidic alkaline conditions are made from cast Ni-30% Cu type alloys. The wrought material is used for shafts and stems. In seawater contaminated with sulfide, these alloys are subject to pitting and corrosion fatigue. Ammonia contamination creates corrosion problems as for commercially pure nickel. 

Base-metal vein-type deposits may be divided into Pb-Zn-type and Cu-type (Otsu and Harada, 1963). However, this sub-classification is not considered here for simplicity of discussion. 

Cr3Si-type structure W + / AgMgAs-type structure F + F + F" CaTi03-type structure P + P + J Cu2Mg-type structure T + D. For a few element structures Cu type structure F W-type structure 7 a-Po-type structure P Mg-type structure E C-diamond-type structure D. 

The sequence ABCABC... having a cubic symmetry is shown in Fig. 3.21. It is the cubic (face-centred cubic) close-packed structure, also described as cF4-Cu type structure. 

Figure 3.21. The face-centred cubic close-packed structure (Cu type). On the left a block of eight cells is shown (one cell darkened). On the right a section of the structure is presented it corresponds to a plane perpendicular to the cube diagonal. Notice that the plane is the same presented on the left in Fig. 3.19. The sequence of the layers in this structure is shown in Fig. 3.20 in comparison with other close-packed elemental structures.

This filled-up superstructure may therefore be described in terms of the occupation by N of an interstice (centred in Vi, A, Vi) of a Cu-type (or AuCu3-type) structure. The N atom is octahedrally surrounded by 6 Fe atoms. This structure could also be described as a deficient NaCl-type derivative structure the Fe atoms are in the same positions as the Na atoms in NaCl and one out of the four Cl positions is occupied by the N atoms. 

Another example of a superstructure based on a close-packed structure but related to the hexagonal close-packed one is that corresponding to the hP8-Ni3Sn prototype. Just as the AuCu3 type can be derived by ordering the Cu-type structure, so the Ni3Sn type can be obtained from the hP2-Mg type. Details of this structure and of some stacking variants are described in Chapter 7. 

In the preceding paragraphs examples of a number of so-called superstructures have been considered. Generally, it has been observed that a derivative structure has fewer symmetry operations than the reference structure it has either a larger cell or a lower symmetry (or both) than the reference structure. Typically the passage from the reference structure to the derivative structure (superstructure) may be related to the fact that a set of equipoints of a certain structure (the reference one) has to be subdivided into two (or more) subsets in order to obtain the description of the other structure. The structure of the Cu type (cF4 type), for instance, corresponds to 4 Cu atoms in the unit cell, placed in 0, 0, 0 14, 14, 0 14, 0, 14 0, 14, 14, whereas in the cP4-AuCu3 type structure the same atomic sites are subdivided, in another space group, into two sets with an ordered distribution of the two atomic species (1 Au atom in 0, 0, 0 and 3 Cu atoms in 14, 14, 0 14, 0,14 0,14,14). 

A special case of long-period structure to be considered is the oI40-AuCu(II) type structure which has ID substitutional and displacive modulations (Fig. 3.41). We must first mention that ordering of the Au-Cu face-centred cubic (cF4-Cu type) solid solution, having a 50-50 atomic composition, re-distributes Cu and Au atoms... 

In the cP2-W type (CN 8) structure Vsph is 0.68 Vat (only a portion of the available space is occupied by the atomic sphere ). In the cF4-Cu type and in the ideal hP2-Mg type (CN 12) structures, Vsph is 0.74 Vat. Considering now the previously reported relationship between RCs n and i CN8, we may compute for a given element very little volume (Vat) change in the allotropic transformation from a form with CN 12 to the form with CN 8, because the radius variation is nearly... 

Typical space-filling parameters of elemental structures are the following cF4-Cu type p = 0.740... 

Their normal crystal structure, at ambient conditions, corresponds to the body-centred cubic cI2-W-type structure. At very low temperatures, the close-packed hexagonal hP2-Mg-type structure has been observed for Li and Na, while for Rb and Cs the face-centred cubic close-packed cF4-Cu-type structure is known at high pressure. No polymorphic transformation has been reported for potassium. 

Figure 5.26. Iron binary alloys. Examples of the effects produced by the addition of different metals on the stability of the yFe (cF4-Cu type) field are shown. In the Fe-Ge and Fe-Cr systems the 7 field forms a closed loop surrounded by the a-j two-phase field and, around it, by the a field. Notice in the Fe-Cr diagram a minimum in the a-7 transformation temperature. The iron-rich region of the Fe-Ru diagram shows a different behaviour the 7 field is bounded by several, mutually intersecting, two (and three) phase equilibria. The Fe-Ir alloys are characterized, in certain temperature ranges, by the formation of a continuous fee solid solution between Ir and yFe. Compare with Fig. 5.27 where an indication is given of the effects produced by the different elements of the Periodic Table on the stability and extension of the yFe field.

Crystal data summarized first are those characteristic of structures of metallic elements, typically having highly symmetric and dense atomic arrangements. Only a few notes are reported for the close-packed structures (Mg, Cu types), since for these structures several details are presented in 3.7.6 and 3.9.2.I. Subsequently, particular structures observed for a few selected specific metals and, finally, a few typical structures of non-metallic elements are described. 

Notice that the structures presented in this paragraph are unary structures, that is one species only is present in all its atomic positions. In the prototypes listed (and in the chemically unary isostructural substances) this species is represented by a pure element. In a number of cases, however, more than one atomic species may be equally distributed in the various atomic positions. If each atomic site has the same probability of being occupied in a certain percentage by atoms X and Y and all the sites are compositionally equivalent, the unary prototype is still a valid structural reference. In this case, from a chemical point of view, the structure will correspond to a two-component phase. Notice that there can be many binary (or more complex) solid solution phases having for instance the Cu-type or the W-type structures. Such phases are formed in several metallic alloy systems either as terminal or intermediate phases. 

Figure 7.11. Section sequence parallel to the base plane of the cF4-Cu type structure.

Cu-derivative, substitutional and interstitial superstructures. As discussed in 3.8.1 ff, the Cu-type structure is also an important reference structure because it may be considered the ancestor of several derivative structures. 

The In cell may be considered a distortion of the Cu type, face-centred cubic, cell. The unconventional face-centred tetragonal cell (equivalent to the tI2 cell), corresponds to a = aJY = 459.8, c = c = 494.7 and c /a = 1.076. Protactinium has a similar structure, which however with a c/a value lower than one, can be considered a distortion of the body-centred cubic structure. 

The 6Pu (Cu type), 6 Pu (In type), and ePu (W type) allotropes stable at higher temperatures correspond to typical common metal structures. 

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. 

A large number of compounds belong to the NaCl-type structure for instance those given by the alkaline earths with O, S, Se, Te, Po, etc. and nearly all the (partially ionic-covalent and metallic) 1 1 compounds formed by the rare earths and the actinides with N, P, As, Sb, Bi, S, Se, Te, Po. Notice that this structure may also be described as a derivative of the cubic close-packed structure (cF4-Cu type) in... 

Sphalerite and wurtzite structures general remarks. Compounds isostructural with the cubic cF8-ZnS sphalerite include AgSe, A1P, AlAs, AlSb, BAs, GaAs, InAs, BeS, BeSe, BeTe, BePo, CdS, CdSe, CdTe, CdPo, HgS, HgSe, HgTe, etc. The sphalerite structure can be described as a derivative structure of the diamond-type structure. Alternatively, we may describe the same structure as a derivative of the cubic close-packed structure (cF4-Cu type) in which a set of tetrahedral holes has been filled-in. This alternative description would be especially convenient when the atomic diameter ratio of the two species is close to 0.225 see the comments reported in 3.7.3.1. In a similar way the closely related hP4-ZnO... 

This is apparent from Fig. 3.12 where the relation between a small tetragonal cell (tP2 or tI2) and a larger (nearly cubic) cell is shown. The large cell has ac = 280.4 2= 396.5pm and c = c = 367.3. It is similar to a Cu-type cell, slightly compressed (c/a = 0.926) and in which the atoms placed in the centre of the side... 

As pointed out in the description of the cubic close-packed structure (cF4-Cu type), this structure may be described (especially for certain values of the atomic diameter ratio) as a derivative of the Cu-type structure in which two sets of tetrahedral holes have been filled-in. 

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