New insights in the atomic origin of the piezoelectric effect in α-quartz and α-GaPO4

In order to understand the atomistic origin of the inverse piezoelectric effect in alpha-SiO₂ and alpha-GaPO₄ we have measured the changes of integrated X-ray intensities of selected Bragg reflection under influence of an external high electric field up to 8 kV/mm. To do this 300 mum thick samples were sandwiched between metallic contacts and the X-ray intensities were recorded with and without the applied field. The difference intensities were evaluated in terms of difference electron density of atoms within the respective crystallographic unit cell. Because the up to now accepted model of the field induced displacement of ionic sublattices against each other fails for the interpretation of experimental data we proposed a new model of the inverse piezoelectric effect, which considers the strong covalent bond between silicon and oxygen and gallium/phosphorous and oxygen, respectively. Here the main effect of screening the external electric field is a change in the Si-O-Si and Ga-O-P bonding angles, i.e. the rotations of nearly rigid MO4 tetrahedra.