Ionic Displacements and Piezoelectric Constants of AgGaS2 from X-Ray Diffraction of a Crystal in an External Electric Field

Abstract

The displacements of the ions and the changes in cell parameters which occur on application of an electric field to a crystal of AgGaS2 have been determined using X-ray diffraction. The shifts in Bragg angle of 14 reflections, due to a field of 2.6 × 106 V/m applied nearly parallel to [221], were used to refine the change in cell parameters. The resulting piezoelectric coefficients are d14 = d25 = 8.8 (0.9) × 10-12 C/N and d36 = 7.6 (1.8) × 10-12 C/N. This leads to a value of 4.8 (0.5) × 10-12 pC/N for the piezoelectric constant in the [221] direction, which compares well with a value of 5-6 × 10-12 C/N measured directly. The ionic displacements were studied for two field directions. With E = 2.6 × 106 V/m in a direction parallel to [110], very small, but significant a and b direction displacements were observed for all atoms, whereas the displacements in the e direction were negligible as expected. Relative to the sulfur framework, the Ga ion displacement is considerably larger than that of the 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.