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.

• Received 4 February 1974

DOI:https://doi.org/10.1103/PhysRevLett.32.1196