1.

Find the breaking stress of an ionic crystal neglecting the effect of all the ions except the nearest neighbours. Do numerical calculations for a sodium chloride crystal in which the distance between the centres of neighbouring ions (the lattice constant a) is, according to data obtained with the aid of X-ray structural analysis, equal to 2.81 Å. Do the same for a lithium fluoride crystal, in which a = 2.01 Å.

Answer»


Solution :For the purpose of calculation consider a plane with ions arranged in staggered rows (chessboard order) (FIG.). In this case the breaking STRESS is `sigma_m = F_0 n`, where `F_0 = e^2//(4 pi epsilon_0 a^2)` is the force of interaction between NEIGHBOURING ions, `n = a^(-2)` is the ion concentration per UNIT area.
Hence
`sigma_m = (e^2)/(4PI epsilon_0 alpha^4)`


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