Chemical Technology January 2015
Minerals Processing and Metallurgy
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bcc-based B2 (Pearson symbol c P2) or CsCl-structure for composition AB, and fcc-based C1 (Pearson symbol c F12) or CaF 2 -structure for composition AB 2 . This again confirms the three requirements stipulated above that coloured in- termetallic compounds need to have a crystal structure of highest symmetry ensuring a sufficiently simple electronic structure for distinct absorption bands of high intensity. According to this approach, only the following potential coloured binary gold, palladium and platinum intermetallic compounds can result:
CsCl-structure: PdIn, PdBe, PdMg
(a) (b) Figure 1: Regions of high-symmetry structure in the AB (a) and AB 2 (b) Pettifor maps. Mendeleev numbers and prototypes (c is cubic, t is tetragonal, o is orthorhombic, h is hexagonal) are shown. Limits for expected coloured intermetallic compounds are shown by the dashed line [33].
CaF AuAl
2 -structure: PdAl 2
, PtSn
, PtGa
, PtAl
, PtIn
AuGa
,
2
2
2
2,
2
, AuIn
.
2
2
Element B is of Groups 13 and 14 in the Periodic Table with the exception of PdMg. Further, PtAl (tetragonal structure) also exhibits colour according to Figure 1 [33]. Hume-Rothery electron concentration An electron-to-atom ratio is stipulated for the Hume- Rothery phases, where the electron concentration (e/a) is defined as the sum of the valence electrons per atom of the compound: e/a = 1/100 - Σ a i v i where a i is the concentration in at.%, and v i is the number of valence electrons of the element i .
(a)
(b)
Figure 2: Crystal structures: (a) CsCl-structure, and (b) CaF 2
-structure.
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Chemical Technology • January 2015
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