Theory of novel electroactive materials

Ferroelectric materials, which by de?nition possess a spontaneous electric polarization,are often also strong piezoelectrics. This means that they distort upon application of anelectrical voltage, or conversely, that they generate an electrical voltage upon applicationof mechanical stress. The goal of this work is to carry out realistic ?rst-principles calcu-lations of the structural, electronic, dielectric, and piezoelectric properties of ferroelectricand other electroactive materials. Such materials are of pressing technological interest forNavy sonar applications (acoustic projection and detection) as well as for other piezoelectric(e.g., medical ultrasound imaging, micromanipulation), memory storage, and optoelectronicapplications. In this contract period, we will continue our search for novel ferroelectric mate-rials with improved properties, including corundum derivatives, hexagonal ABC compounds,hybrid improper ferroelectrics, and charge-ordered ferroelectrics. We will also investigateexoelectricity, in which an electric polarization is induced by a strain gradient (to be com-pared with piezoelectricity, in which a polarization is induced by a homogeneous strain),using methods recently developed in our group. The work will be based on ?rst-principlesKohn-Sham density-functional calculations using a plane-wave pseudopotential approach,both in the absence and presence of electric ?elds. Theoretical materials search strategieswill be carried out using well-tested pseudopential libraries, and will include evaluation ofmaterials stability with respect to decomposition into other phases. We also plan to extendour studies of multiferroic materials, especially materials in which electric and magneticbehaviors are strongly cross-linked. Our overall objective is to develop a thorough atomisticunderstanding of the structural and electronic properties of novel ferroelectric and piezo-electric materials, so as to provide guidance in the search for new materials that may be ofimproved utility in technological applications.