The aim of this research program, which is supported by the National Science Foundation, Solid State and Materials Chemistry Program, is to use flux techniques to synthesize lanthanide chalcogenides with both the known Th3P4 structure and the novel M2YbCuQ5 (M = La, Ce, Pr, Nd, Sm; Q = S, Se) structure. Magnetic and transport properties of a unique rare earth chalcogenide compound system, Ln3-xVACxQ4, employing several low temperature, high magnetic field, and high-pressure experiments will be investigated. In Ln3-xVACxQ4, Ln represents a trivalent rare earth, VAC represents a cation vacancy with a concentration range from x=0 to x=1/3, and Q represents a divalent chalcogenide anion, normally S or Se. For x=1/3 there is one vacancy per nine Ln atoms. All Ln3-xVACxQ4 compositions in this study have the generic cubic Th3P4 structure. By merely varying the vacancy concentration and/or the rare earth species in Ln3-xVACxQ4, it is thus possible to obtain a remarkably broad array of magnetic and electrical properties: from semiconducting to superconducting and from diamagnetic to ferromagnetic. In the pseudobinary continuum of compositional space between these extremes lie dilute magnetic semiconductors, magnetically doped superconductors, metallic spin glasses, possible quantum critical points, and very likely some highly correlated electron systems. Ln2YbCuQ5 was recently discovered, and in addition to adopting a novel structure type, these compounds display magnetic properties that range from simple Curie-Weiss paramagnetism to short-range antiferromagnetic coupling to long-range ordering in Sm2YbCuS5. In this latter phase negative susceptibility and substantial diamagnetism is observed in low-temperature magnetism experiments, perhaps indicating the presence of a superconducting component. Large single crystals are needed for a variety of studies to better understand the magnetic behavior of this unusual compound that will include neutron scattering/diffraction experiments, magnetic anisotropy, and resistivity measurements.
NON-TECHNICAL SUMMARY The preparation and characterization of new magnetic materials that utilize the abundant and readily available lanthanide elements is important in the development of novel technologies. This research program is aimed at training the next generation of materials scientists and solid state chemists to deeply probe materials synthesis and characterization, and to discover how novel properties can be used to solve technological problems. Specific classes continue to be developed to provide a fundamental education in this area of research, as well as to determine how best to capitalize on the knowledge of the unusual behavior of lanthanide elements. There is a strong focus in this program on training underrepresented groups, particularly African-Americans, and a strong track-record in recruiting and retaining these individuals has been established and will be significantly enhanced by the support from the National Science Foundation, Solid State and Materials Chemistry Program.
The goal of this research project was to prepare novel magnetic materials containing rare-earth elements and to understand how the atomic structure of these solids correlates with atypical magnetic interactions. In particular, we focused on the preparation of new compounds that contained two different rare earth metals that were able to magnetically communicate with one another. The most important discovery during this three-year project was a rare class of materials that exhibit magnetization reversal as the magnitude of the external magnetic field was changed. In addition, we discovered a class a family of materials that exhibited ferromagnetic coupling between rare earth metal centers even though the distance between the metal ions was quite large. In total more than 50 new materials were prepared and characterized; leading a significant expansion of our knowledge of the structure, bonding, and physical properties of complex, rare-earth compounds. Diversity is a very important aspect of the PI's research program and several graduate students from HBCU's were recruited into this program. This project also involved high school students including one from a minority-serving school. The lead investigator of the project was also heavily involved in science outreach and participated in several radio talk shows that are oriented toward helping the public understand the science and societal impact of heavy elements.
