The bond-orbital model is generalized to the treatment of elastic shear constants by assuming a reduction in covalent energy $V_2$ proportional to the square of the angular misalignment of the two hybrids forming a bond. The result, proportional to $[{(1-{{\alpha }_p}^2)}^{\frac{1}{2}}-S]\times V_2$, gives a good account of the shear constants in all zinc-blende and wurtzite crystals for which they are known, except for the noble-metal halides.

• Received 3 June 1974

DOI:https://doi.org/10.1103/PhysRevLett.33.410