Single-phase alloys crystal structures

From an X-ray powder analysis of a nearly single-phase alloy Rossi et al. (1979) observed the compound YbAu2Si2 crystallizing with the ThCrjSij-type of structure (I4/mmm, a = 4.287, c = 10.050, = 11.13 kg/dm ). For sample preparation, see... 

Another characteristic point is the special attention that in intermetallic science, as in several fields of chemistry, needs to be dedicated to the structural aspects and to the description of the phases. The structure of intermetallic alloys in their different states, liquid, amorphous (glassy), quasi-crystalline and fully, three-dimensionally (3D) periodic crystalline are closely related to the different properties shown by these substances. Two chapters are therefore dedicated to selected aspects of intermetallic structural chemistry. Particular attention is dedicated to the solid state, in which a very large variety of properties and structures can be found. Solid intermetallic phases, generally non-molecular by nature, are characterized by their 3D crystal (or quasicrystal) structure. A great many crystal structures (often complex or very complex) have been elucidated, and intermetallic crystallochemistry is a fundamental topic of reference. A great number of papers have been published containing results obtained by powder and single crystal X-ray diffractometry and by neutron and electron diffraction methods. A characteristic nomenclature and several symbols and representations have been developed for the description, classification and identification of these phases. 

Identification of unknown crystal structures and determination of phase fields by X-rays can be problematical if the characteristic patterns of the various phases are quite similar, for example in some b.c.c. A2-based ordered phases in noble-metal-based alloys. However, in many cases the characteristic patterns of the phases can be quite different and, even if the exact structure is not known, phase fields can still be well established. Exact determination of phase boundaries is possible using lattice-parameter determination and this is a well-established method for identifying solvus lines for terminal solid solutions. The technique simply requires that the lattice parameter of the phase is measured as a function of composition across the phase boimdary. The lattice parameter varies across the single-phase field but in the two-phase field becomes constant. Figure 4.12 shows such a phase-boundary determination for the HfC(i i) phase where results at various temperatures were used to define the phase boundary as a fimction of temperature (Rudy 1969). As can be seen, the position of is defined exactly and the method can be used to identify phase fields across the whole composition range. 

Assume, for example, that two metals A and B are completely soluble in the solid state, as illustrated by the phase diagram of Fig. 12-1. The solid phase a, called a continuous solid solution, is of the substitutional type it varies in composition, but not in crystal structure, from pure A to pure B, which must necessarily have the same structure. The lattice parameter of a also varies continuously from that of pure A to that of pure B. Since all alloys in a system of this kind consist of the same single phase, their powder patterns appear quite similar, the only effect of a change in composition being to shift the diffraction-line positions in accordance with the change in lattice parameter. 

The major alloy of tin recovered from archaeological sites is pewter. This can be divided into those containing lead and lead-free alloys. The former could have a lead content ranging from 67 % (equivalent to plumbers solder) down to 15%. The French in Elizabethan times kept the lead of their wine goblets to below 18% as above this, the wine would become tainted As the lead and tin are insoluble in one another, they are classed as a two-phase alloy and articles could only be manufactured by casting. The lead-tree pewter was invariably an alloy of tin with a small amount of copper (0.5-7% for pewter recovered from the Maty Rose). The copper dissolved in the tin crystal structure resulted in a single-phase structure, which was considerably harder than pure tin. Hence this class of pewter could be subjected to a limited amount of mechanical working to achieve the final shape. 

From arc-melted alloys annealed at 900°C for 2 weeks Klepp and Parthe (1982) were able to isolate a single crystal of YjNiSij with the Ta3B4 type. The crystal structure was refined from single crystal X-ray counter data, R = 0.088. Atomic parameters derived were as follows (Immm) Y in 4j) 1/2, 0, 0.18307(9) Si in 4j) 1/2, 0, 0.3599(3) Y in 2a) 0, 0, 0 and a statistical distribution of 0.5 Ni -i- 0.5 Si in 4i) 0, 0, 0.4349(2). The location of this compound in the phase diagram of fig. 59 has been marked by an asterisk. Possibly this phase is a high-temperature phase T> 800°C) or more likely corresponds to the earlier observed Y5NiSi4 (Bodak et al., 1974a,b). 

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