what is the number of next nearest or second nearest neighbour in NaCl and CsCl crystal
and what is the method to find it
In NaCl structure, each Na+ ion has 12 Na+ ions as the next neighbour.
The NaCl structure can be regarded as two interpenetrating FCC lattices. Each has 6 nearest neighbours of opposite charges, i.e, the co-ordination number is 6 (which is the number of nearest neighbours of an atom in a crystal).
Each Ca+ ion has 6 Cs+ ions as the next nearest neighbour at a distance of r = dCl-Cl-.
The CsCl is an ionic structure showing two interpenetrating simple cubic lattices, one for each kind of ion. It has a coordination number 8.