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The physical properties of crystals such as dielectric polarization and piezoelectricity are well studied on macroscopic level. They are described by the tensors of dielectric susceptibilities and piezoelectric constants. Inmost cases their atomistic origin is not always clear. The evaluation of the structural origin of these phenomena requires in-situmeasurement of themicroscopic structural response under influence of an external perturbation. One possible experiment of this type is to monitor the change of X-ray diffraction intensities by a crystal under a permanent external electric field. Because the effect is small (ΔI/I<0.01) it has to be measured by means of the synchrotron radiation [2]. The shifts of diffraction peak positions are related to the change of crystal lattice parameters (converse piezoelectric effect). The variations of Bragg intensities are associated with the change of the fractional atomic coordinates (dielectric polarization). From one side electric field induced atomic displacements may be related to crystal dielectric properties. From other side they are directly connected to features of electron density and chemical bonding of studied crystal. Thus the microscopic information, derived from the electric field induced change of the electron density is related to crystal physical properties. Recently [1], we applied this strategy to investigate the site selective response in α-GaPO4 to a permanent external electric field. Here we have shown that the response of a crystal rather depends on the pseudoatomic charges of consistent atoms then to the strength of chemical bond. Therefore the PO4 with stronger bond strength deforms slightly larger compared with theGaO4 tetrahedra with smaller bond strength due to the higher pseudoatomic charge of P atom compared with Ga one. This outcome can be verified by inspection of other semi-covalent ternary compounds such as ZnSiP2 or similar chalcopyrite compounds. These are iso-structural to ZnS structure and contain two different bonds with different bond strength and pseudoatomic charges. The chalcopyrite ZnSiP2 crystals were grown by chemical transport reactions and prepared for synchrotron diffraction studies under external electric field. The electron density in this crystalline compound is investigated on the basis of density functional theory. The strength Zn-P and Si-P chemical bonds are studied and the pseudoatomic charges of Zn, P and Si atoms are determined. On the basis of the previous results the reaction of ZnSiP 2 crystals to an external electric field is predicted.

X-ray diffraction study of chemical bonds under external electric field