Deformation Potentials of the Alkali Halides

The ratios of the shear deformation potentials $b_{\mathrm{ex}}$ and $d_{\mathrm{ex}}$ to the hydrostatic deformation potential $a_{\mathrm{ex}}$ of the first excitonic transition of KI, RbI, and CsI have been determined from piezoreflectance measurements. The obtained values do not agree with obtained values from piezobirefringence measurements of Gavini and Cardona. The difference is qualitatively explained in terms of the difference between the band-edge deformation potentials and the deformation potentials of the exciton.