RUI: Developing Insight and Control of Polarity in the Pyrochlore Lattice

Bates College, Lewiston ME

Investigators

Abstract

Non-Technical Summary Many energy and technology-driven applications rely on materials that utilize the chemical properties of lead. However, the known toxicity of lead and subsequent regulations prompt the investigation of alternative elements for technological applications. With joint support from the Solid State and Materials Chemistry program in the Division of Materials Research and the Established Program to Stimulate Competitive Research (EPSCoR), this project seeks to understand the arrangement and interactions of atoms that give rise to the desired physical properties for applications, and will focus on a widely studied yet poorly understood family of materials called pyrochlore oxides. The principles discovered through this work will deepen the understanding of pyrochlore oxides, offering new knowledge for achieving properties that offer alternatives to lead-based technologies. Undergraduate education and exposure to solid state chemistry is at the heart of this research, and providing an experience that promotes excitement and is inclusive to all is of paramount importance. Taking place at Bates College, a primarily undergraduate institution, this work is well-suited for students interested in chemistry, materials science, and sustainable approaches for energy and electronic applications. Through participation in this research, students will be exposed to techniques not typically covered in undergraduate chemistry curricula, as well as perform cutting-edge experiments at facilities that are often underutilized by undergraduate institutions. In addition to independent research, this work will promote professional development of participants through purposeful discussion of career paths, interaction with outside scientists, and reflection on activities performed throughout their involvement. Technical Summary Polar crystal structures comprise a class of materials with a broad range of technological applications, particularly as components in electronic devices. While widespread in their use, these materials often rely on the electron lone-pair chemistry of Pb, and finding alternative Pb-free materials that do not sacrifice performance is of considerable importance. Current research focuses broadly on perovskite oxides due to high temperature stability, facile preparation, and a wide range of chemical substituents. However, the vast literature on perovskite oxides provides no clear way to get around the use of Pb. The ability to alter the metal cations and anion stoichiometry in pyrochlore oxides provides a versatile platform for obtaining Pb-free polar crystal structures. Pyrochlores have been less studied than perovskites due to their complex crystal structure, but recent developments in measurement and computational techniques make it possible to study and determine the causes of long-range polarity in these materials. This research, supported by the Solid State and Materials Chemistry program in the Division of Materials Research and the Established Program to Stimulate Competitive Research (EPSCoR), will apply knowledge of the causes of polarity in perovskites to systematically investigate and determine the fundamental drivers of polarity in pyrochlores. The interpenetrating networks found in the pyrochlore structure will be decoupled through systematic substitution of different metal cations, namely those that can cause second-order Jahn-Teller distortions (s2 or d0 electronic configurations). Comparing detailed structures of materials with metal ions that can cause structural distortions to materials with cations that will not cause distortions will provide insights into the underlying chemistry that leads to long-range polarity. In addition to cation substitution, the work will investigate the influence of anion stoichiometry on structure, local distortions, and overall polarity. Structural observations will be correlated to physical property measurements through characterization of the dielectric constant and nonlinear optical behavior. Undergraduate student researchers will be the primary participants in on-site preparation and characterization of samples at Bates College, a primarily undergraduate institution, and off-site characterization at national laboratory user facilities. Through collaboration, the group will obtain computational calculations to support experimental measurements. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.