Size Factor compounds : (i) Laves phases

D(i) Laves Phases

qThese phases have a formula: AB2

qLaves phases can be regarded as Tetrahedrally Close Packed (TCP)* structures with an ideal ratio of the radii (rA/rB) = (3/2)1/2 ~1.225 [or usually rA/rB Î (1.1, 1.6)]

qIf rA/rB = 1.225 then a high packing density is achieved with the chemical formula AB2 with a average coordination number of 13.3

qCrystal structures:
Ø Hexagonal → MgZn2 (C15), MgNi2 (C36)
Ø FCC → MgCu2 (C14)

qThere are more than 1400 members belonging to the ‘Laves family’

qMany ternary and multinary representatives of the Laves phases have been reported with excess of A or B elements. Some ternary Laves phases are known in systems with no corresponding binary Laves phases.

qThe range of existence of the three phases (C15, C36, C14) in ternary Laves phases is influenced by the e/a ratio

qLaves phases containing transition metals as components have interesting Physical and mechanical properties. Engineering materials based on Laves phases are being developed for:

Ø High temperature applications

(for use in turbine blade fine precipitates of Laves phases is shown to improve fatigue strength)

Ø Hydrogen storage applications
(in nickel-metal hydride batteries)