Chemical Technology January 2015

Minerals Processing and Metallurgy

Purple Gold TM Patent Number 82596 Aspial Corporation www.purplegold.com. Jewellery available from Lee Hwa Jewellery and Goldheart, both in Singapore

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.

31

Chemical Technology • January 2015