An ab-initio study of the mechanism of dipole moment formation in KH₂PO₄

Abstract

Ab-initio molecular orbital cluster calculations with Gaussian-90 at Pittsburgh Supercomputing Center were carried out in order to understand the mechanism of dipole moment development and, hence, ferroelectricity in KH₂PO₄. Our specific goal was to evaluate the effect of the movement of H and P atoms on the magnitude and sign of the dipole moment. The equilibrium geometry was taken from earlier neutron diffraction studies. We designed two sets of clusters, H₂PO₄, and H₄PO₄, to perform the electronic structure calculations with bases set of 6–31G*, for paraelectric and ferroelectric phases. The calculations yield electric charge density distribution in the systems. It is seen that the dominant contribution to the development of dipole moment arises from the charge redistribution along the P-O-H bonds. The calculated results show clearly that the z-direction polarization develops as the H atoms move closer to the O atoms. Then, the polarization drives the P atom to move away from the center of the tetrahedron in order to reach the total energy minimum. At this stage of our study, while the calculated dipole moment and the displacement of the center P atom are in the right direction, the agreement with experimental results needs further improvement. Possible reasons for these discrepancies are outlined.