Simon Phillpot, professor of Materials Science at the University of Florida explains his recently published work on the spontaneous polarization of LaBGeO5, a well-known borosilicate stillwellite.
In the presence of an electric field, all materials exhibit a polarization that is dependent on the strength of the applied field. For most materials, when the field is removed, the polarization vanishes. But, for a certain class of materials termed ferroelectrics, the polarization retains a nonzero value known as the spontaneous polarization. By applying a sufficiently strong electric field to a ferroelectric material and then reversing its direction, it is possible to switch the direction of the spontaneous polarization. This switching ability makes these materials ideal candidates for memory applications, such as ferroelectric RAM. In real materials, the process of polarization reversal is complicated, involving domain wall motion and domain nucleation.
Recent advances in the quantum theory of electronic polarization have made calculations of the polarization from first principles a routine task for virtually any material. However, the result of such calculations is not a single vector quantity for the polarization, but a lattice of values that is defined modulo a vector known as the polarization quantum, which is related to the crystal lattice. Therefore, calculation of the spontaneous polarization not only requires knowledge of the ferroelectric state, but also of a continuous path that brings it to another state with the same polarization but in the opposite direction. Luckily, such a path need not contain the complicated behavior observed in experiments, but may be any continuous transformation between the two ferroelectric states.
In our paper recently published in JPCM, we use density functional theory to calculate the unstable phonon modes of the paraelectric phase of LaBGeO5, and then use the eigenvectors of these modes to determine the atomic displacements driving the ferroelectric phase transition. By reversing the direction of the atomic displacements, we were able to construct a continuous path between the two structures with oppositely-oriented polarization vectors. An illustration of the BO4 chain displacement, which is the major contributor to the polarization change, along this path can be seen in the left-hand side of the figure. After relaxing the structures along the switching path to their lowest energies, we determined the spontaneous polarization of LaBGeO5 by taking half the difference in polarization between the two endpoints. Polarization values along the full path are shown in the right-hand side of the figure.
Brian Demaske is a 3rd year PhD student in Materials Science and member of the Florida Laboratory of Advanced Materials Simulation (FLAMES) at the University of Florida. Aleksandr Chernatynskiy is a recent alumnus of the FLAMES group and now assistant professor of Physics at Missouri University of Science and Technology. Simon Phillpot is a professor of Materials Science at the University of Florida and co-leader of the FLAMES group.
This work is licensed under a Creative Commons Attribution 3.0 Unported License. Figure taken from B J Demaske et al 2016 J. Phys.: Condens. Matter 28 165901. © 2016 IOP Publishing Ltd. Photos of the authors provided and used with permission their permission.
Categories: Journal of Physics: Condensed Matter