Page 222 - Materials Chemistry, Second Edition

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3.2. Metallic Structures and Properties

intermetallic compounds with this structure include Ag 5 Zn 8 ,Cu 9 Al 4 ,Cu 31 Sn 8 ,
Na 31 Pb 8 ,Rh 5 Zn 21 , and Pt 5 Zn 21 . Additional zinc may continue to dissolve in this
phase until a concentration of ca. 75% Zn is reached, which results in the final hcp
phase referred to as e-brass, CuZn 3 (n ¼ 1.75). Other intermetallic compounds that
share this structure include AgZn 3 ,Ag 5 Al 3 ,Cu 3 Sn, and Cu 3 Si. Beyond this concen-
tration, additional zinc results in the hcp -brass phase which is no longer considered
an alloy, but pure Zn with n ¼ 2. Only the a and b phases are useful alloys; the
others are too hard and/or brittle. These various intermetallic structures are often
called electron compounds or Hume-Rothery phases since they are governed by the
ratio of # electrons: # atoms.
In addition to exhibiting simple ionic structures (e.g., CsCl, NaCl, CaF 2 , etc.) or
those of electron compounds, intermetallics may pack according to their atomic
sizes. For instance, Laves phases (also known as Frank-Kasper phases) are interme-
tallic compounds whose structures are governed primarily by atomic radii ratios.
These compounds are of form “AM 2 ”, where A is a larger metal than B and possess a
tetrahedral framework formed by Cu, Zn, or Ni. There are three primary motifs for
Laves phases (Figure 3.31): MgCu 2 (A:M radius ratio of 1.25; e.g., CaPt 2 , HfCo 2 ,
CeCo 2 , BaPt 2 , CsBi 2 , PbAu 2 , LaPt 2 ,VIr 2 , ZrFe 2 , etc.), MgZn 2 (A:M radius ratio of
1.17; e.g., BaMg 2 , b-FeBe 2 , TaFe 2 , MoFe 2 , WBe 2 , ZrRu 2 , TiCr 2 , TaCoCr, LiOs 2 ,
TiFe 2 , etc.), and MgNi 2 (A:M radius ratio of 1.28). There also exist mixed Laves
phases of type A(M’M ), where M’ and M are Cu, Ag, Zn, Al, or Si. For
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these structures, increasing electron concentration favors the Laves phases in the

order MgCu 2 (1.33–1.8 e /atom), MgNi 2 (1.7–1.9 e /atom), and MgZn 2 (ca. > 1.9

e /atom). AB 5 intermetallic structures such as AuBe 5 ,LuMn 5 , and MgSnCu 4 are
also Laves phases related to either MgCu 2 or MgZn 2 , with two arrays of Mg sites
occupied equally by the two metals.
A structural motif for intermetallic compounds that contain a Group I/II and late
transition metal are known as Zintl phases. Unlike other metallic alloys, these
compounds are typically diamagnetic insulators, with a high degree of brittleness.
Though late transition metals have similar electronegativities to those of the late
main group elements, it has only recently been accepted that transition metal atoms
present in alloys such as CsAu, K 34 In 96.19 Au 8.81 ,Yb 3 Ag 2 ,Ca 5 Au 4 , and Ca 3 Hg 2 exist

as Zintl anions of form M , [M-M] , etc. [13] However, the anions of late transition
4
metal elements behave differently than their late main group counterparts (e.g.,
halides), exhibiting covalent bonding within alloys when their p-shells are partially
filled.

The Zintl compound NaTl features a diamond lattice of Tl anions with Na +
cations in tetrahedral interstitial sites. However, unlike most other Zintl compounds
that are insulators, NaTl does not have a bandgap and exhibits metallic conductiv-
ity. [14] Insulating half-Heusler compounds of form AML (where A ¼ Grp. 3 such as
Sc, Y; M ¼ late transition metal such as Ni, Pd, Au; L ¼ heavy main group atom of
Grp. 14/15 such as As, Sn, Sb, Pb, Bi) are structurally related to NaTl, with M and L
forming a zincblende lattice and A atoms occupying 1/2 of the 10-coordinate sites
[15]
defined by a M 4 tetrahedron and L 6 octahedron (Figure 3.32).