Atomistic origin of the inverse piezoelectric effect in α-SiO2 and α-GaPO4

In order to understand the atomistic origin of the inverse piezoelectric effect, the changes of integrated intensities of selected Bragg reflection of alpha-SiO2 and alpha-GaPO4 were studied, which were induced by an external high electric field of up to E = +/- 8 kV/mm. Because the model of the field-induced displacement of ionic sublattices against each other fails for the interpretation of experimental data, we propose a model of the inverse piezoelectric effect, which considers the strong covalent bond between Si and O atoms in alpha-SiO2. Here the main effect of screening the external electric field is a change in the Si-O-Si bonding angles, i.e. the rotations of rigid SiO4 tetrahedra. The same model holds for alpha-GaPO4, which is an isostructural compound to alpha-SiO2. For the first time a similar experiment was performed at low temperatures. Between 50K less than or equal to T less than or equal to 300K the piezoelectric coefficient d(111) of both substances behaves nearly temperature independent. On the other hand, the field-induced change of the intensities increases for decreasing temperature. This can be interpreted by the rotation of tetrahedra, which is partially originated by the temperature decrease and by the external electric field, respectively, accompanied by a field-induced deformation of tetrahedra.