Question

If Fe and Cu substituted for one another randomly, as they do above 540•c what would happen to the unit cell?

Answer 1

Iron appears in body centred cubic lattice whereas copper appear in face centred cubic lattice.

There are 2 atom of iron in BCC as

There are 4 atom of copper in FCC as

Iron gets substituted by copper may result in increased magnetisation of the unit cell due to increase in number of neighbour copper atom in BCC lattice of iron which can accommodate only 2 atom of copper in it. Copper having positive enthalpy in comparison to iron and with 2 free atom of copper impurity in BCC lattice may result in bonding of Cu-Cu pair this leads to increase in number of copper in BCC lattice. According to Heisenberg the magnetisation depends of number of free atoms in the lattice which lead to increased magnetization.

When all the copper atom get removed from lattice to leave the space for 2 iron atom. This results in clustering of iron atom and remaining space in the FCC lattice of copper again occupy by copper atom. This clustering causes strong Fe-Fe interaction. The impurities of iron may result in disruption of orthorhombic structure of FCC by creating cross link between iron and copper and the lattice tends to appear as tetragonal symmetry.