Polymorf - Knowhwere- Cu2AlMn

MINERALS AND SYNTHETIC COMPOUNDS, AB_mX_n, W/ CLOSE PACKED X ATOMS,

AND A ATOMS OCCUPY TETRAHEDRAL AND B ATOMS OCTAHEDRAL INTERSTICES

Cu₂AlMn

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tetra/octa	100%	fcc	3-d	all faces	Al- 6c/fcc; Cu- 4c, Mn - 6c

structure

In the crystal structure of Cu₂AlMn the aluminum atoms are arranged FCC, the

copper atoms occupy 100% of the tetrahedral voids, and the manganese atoms

occupy 100% of the octahedral voids. The constituent atoms have relatively small

differences in their volumes and electro negativities with non-directional metallic

bonding. Since their radius ratios are nearly equal to one, the crystal structure is

topologically equivalent to an expanded FCC lattice similar to diamond previously.

That is, the large "interstitial atoms" expand the FCC arrayed aluminum atoms so

that they do not contact each other, only the copper or manganese atoms they are

coordinated around.

Ex. Cu₂AlMn ( Cu - A , Mn - B ; Al - X )

Crystal lattice

Since all the atoms are approximately the same size the lattice model for the FCC

elements shown previously can represent the crystal structure. It is understood that

the tetrahedra and octahedra represent the AX₄ and BX₆ coordinated groups of atoms

respectively.

Polyhedral framework

The polyhedral framework model pictured here emphasizes that the crystal is

built up from the packing together of the AX₄ and BX₆ coordination groups of atoms.

It is simply a space filling of tetrahedra and octahedra. The formula reflects the 2:1

ratio of tetrahedra to octahedra in the packing and hence there is twice as many

copper atoms as manganese atoms.

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Page 69 - Structure matters - Cu₂AlMn