Question

1) Consider a material consisting of two types of ions with charges +e and −e, respectively. In addition to the Coulomb interaction, they have a short-range repulsive potential of the type A/r12.
a) Assume the substance crystalizes in the NaCl structure. Find the lattice constant by minimizing the cohesive energy. The Madelung constant for the NaCl structure is α = 1.7476. (You can restrict the repulsive interaction to the nearest neighbor sites on the lattice.
b) Do the same for the CsCl structure with a Madelung constant of α = 1.7627.
c) Which structure will the material choose?

Answer 1