The displacements of the ions and the changes in cell parameters which occur on application of an electric field to a crystal of (\mathrm{AgGaS}{2}) have been determined using (\mathrm{X})-ray diffraction. The shifts in Bragg angle of 14 reflections, dte to a field of (2.6 \times 10^{6} \mathrm{~V} / \mathrm{m}) applied nearly parallel to ([22 \mathrm{t}]), were used to refine the change in cell parameters. The resulting piezoelectric coefficients are (d{14}=d_{25}=8.8) ((0.9) \times 10^{-12} \mathrm{C} / \mathrm{N}) and (d_{36}=7.6(1.8) \times 10^{-12} \mathrm{C} / \mathrm{N}). This leads to a value of (4.8(0.5) \times 10^{-12} \mathrm{pC} / \mathrm{N}) for the piezoelectric constant in the [221] direction, which compares well with a value of 5-6 (\times 10^{-12}) (\mathrm{C} / \mathrm{N}) measured directly. The ionic displacements were studied for two field directions. With (E=) (2.6 \times 10^{\mathrm{t}} \mathrm{V} / \mathrm{m}) in a direction parallel to [ (\mathrm{II} \mid), very small, but significant (\mathbf{a}) and (\mathbf{b}) direction displacements were observed fior all atoms, whereas the displacements in the (\mathbf{c}) direction were negligible as expected. Relative to the sulfur framework, the Ga ion displacement is considerably larger than that of the (\mathrm{Ag}) ions. The changes in scattering intensity for a field parallel to [221] were found to be much smaller, indicating smaller ionic displacements for that field direction. The ion displacements are analyzed with a model of nonpolarizable harmonic oscillators in an electric field. It is found that the observed displacements of the Ga ions are much larger than the calculated displacements, whereas the displacements of the Ag ion are in good agreement with theory. O1993 Academic Press, inc.