Benny C. Chan , The College of New Jersey
The past year of funding has been very productive. Our research program has taken an interesting turn to develop new thermoelectric materials. We used the Cs-Bi-Te thermoelectric materials discovered by Kanatzidis as our model system and began to logically modify the structures. The Cs in the structure disrupted the NaCl-type structure of the Bi-Te, which decreases the translation of phonons and the conduction of heat through the material. We hypothesized that the addition of another element, a rare earth, with its higher coordination environment may further disrupt the structure. In our previous report, we discovered the compound K2La2Bi2Se9 (herein called 2229). We had found some initially curious XPS data that indicated the Bi was in the +3 and +5 oxidation states. Crystallographically, the Bi should all be in the +3 oxidation states. We were missing two electrons. We have further developed our project to a fully formed hypothesis driven research.
This year, we have made the Ce, Nd, and Yb analogs of the original 2229 compound and characterized them crystallographically. We discovered that the structure changed from the Pbam space group in the La analog to the non-centrosymmetric P21212 space group in the Ce, Nd, and Yb analogs. We believe the structure change is caused by the contraction of the lanthanide cation size between La and Ce. The rare earth environments are actually the same in the structures with a slight contraction of the RE-Se bond lengths. The contraction caused a large shift in the Bi sites that removed the symmetry of the glide planes in the Ce, Nd, and Yb analogs. We are quantitatively analyzing the structure to show the change. We may also try to measure non-linear optics properties as we have a new physicist at TCNJ who specializes in NLO measurements. We are unsure whether the black nature of these compounds will impede NLO measurements.
The compounds were finally made as pure powders as detected by We have continued the characterization of the four analogs using magnetic susceptibility with the help of Princeton University. The Ce, Nd, and Yb analogs should all have +3 oxidation states for the rare earth and the susceptibility data proved the effective magnetic moment was as expected in the Ce analog. The Ce and Yb were indeed in the +3 oxidation states. In addition, the magnetic data indicated the Ce and Yb analogs contained temperature independent paramagnetism (TIP), which indicates the materials have electrons that are in the conduction band. The electrons did not come from the rare earth, so our original curious result of +3 and +5 Bi is now answered. We believe that some of the 6s electrons in Bi have moved into the conduction band in these compounds giving rise to the TIP. We also found the La analog is diamagnetic under high magnetic fields as anticipated. Under lower fields, we have discovered that the La analog also exhibits TIP. We believe we have discovered the potential mechanism by which these compounds
We are now preparing a manuscript for Inorganic Chemistry to present the La and Ce structure and magnetic data to be submitted in early 2012. We are continuing the synthesis of the Tb and Ho analogs to complete the rare earth series. We are continuing the XPS measurements of the Ce, Nd, and Yb analogs and also ensuring the rare earth and se oxidation states are as anticipated. Once the final XPS data is obtained and continue resistivity and thermopower measurements, we anticipate a full paper on the entire series in the next year to be published in Materials Chemistry, Inorganic Chemistry, or Journal of Solid State Chemistry.
We also now hypothesize that we can disrupt the NaCl structure by adding an element with lower coordination environment than Bi, the coinage metals, Co, Ni, and Zn. We now have preliminary results that have indicated we have made a new compound that contains K-M-Bi-Se (M=Co, Ni). The unit cells are the same in both compounds and the coordination of the M site is trigonal planar. We are studying the literature to compare our coordination environments are typical. We will continue our typical measurements including magnetism, resistivity, and thermopower.
Impacts of the PRF grant
This year has been very exciting as we have been able to fully develop a research program on the alteration of the Bi2Te3 structure to discover new potential thermoelectric materials.
The project has provided three student projects directly, Matthew Kita (2012, applying to graduate schools), Vincent Wu (2014, plans undecided but considering graduate school), and Lisa Kennedy (2014, interest in medical school) over a summer research program. During the academic year, the project has impacted three other students Melanie Hutnick (Hispanic, 2012, applying to graduate school), Rachel Roesch (2012, graduate school), and Eddie Ndiche (African American, 2013, plans to apply to medical school).
We anticipate 2 more publications to be directly funded by PRF, the communication and the full paper. The K-M-Bi-Se project and extensions of the 2229 project will become part of a new grant application to NSF. The PRF grant has allowed for me to fully formulate a hypothesis driven research program.