Photoelastic Tensor of Silicon and the Volume Dependence of the Average Gap

The first accurate measurements of the algebraic values of the photoelastic tensor of silicon are reported. The values, determined acousto-optically, are $p_{1111}=-0.094\mathrm{}$, $p_{1122}=+0.017\mathrm{}$, and $p_{1212}=-0.051\mathrm{}$. The results are in strong disagreement with calculations based on the Phillips-Van Vechten theory of ionicity. It is shown that the widely used assumption of a universal power law describing both the intermaterial and intramaterial variations of the homopolar average energy gap with volume must be reinterpreted or revised.