U 1.33 T 4 Al 8 Si 2 (T = Ni, Co): Complex Uranium Silicides Grown from Aluminum/Gallium Flux Mixtures Ashini S. Jayasinghe, † You Lai, ‡ Ryan Baumbach, ‡ and Susan E. Latturner* ,† † Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States ‡ Department of Physics, Florida State University and National High Magnetic Field Laboratory, Tallahassee, Florida 32310, United States * S Supporting Information ABSTRACT: Two new quaternary analogs of the Gd 1+x Fe 4 Si 10‑y structure type were grown from the reaction of uranium, silicon, and a transition metal (nickel or cobalt) in an excess of aluminum/gallium flux. The use of a mixed flux was found to be necessary for the formation of U 1.33 T 4 Al 8 Si 2 (T = Ni, Co). Single crystal X-ray diffraction data shows the presence of disordered U/Si layers that are characteristic of this structure type; precession photographs indicate partial formation of a superstructure and stacking disorder along the c-axis. This disorder may be the cause of the spin glass behavior that is particularly evident in the nickel analog, which exhibits a spin freezing transition at T F = 7 K. These compounds are resistant to chemical attack and oxidation and may be potential waste forms. ■ INTRODUCTION Electricity has become a vital part of modern life, and around 20% of U.S. energy demand is supplied by nuclear reactors. 1−3 However, this leads to the generation of large quantities of radioactive waste; there are roughly 300 million L of radioactive waste in underground tanks at the Savannah River and Hanford sites alone. 2,4 This necessitates research into effective waste storage methods and creation of stable, storable actinide compounds. 4 Due to their high stability and refractory properties, uranium silicides are potential candidates for radioactive waste storage. 5−7 While silicides of transition metals and lanthanides are very well investigated, complex actinide silicides have been scarcely explored. 8−11 The study of the properties of multinary uranium silicide materials is vital to advancing the knowledge base of these compounds. In addition to their refractory properties, these compounds may also exhibit unusual magnetic behavior. This is particularly likely for those that also contain transition metals; coupling of delocalized d-orbitals with relatively localized f-orbitals of uranium can lead to exotic magnetic phenomena such as spin glass behavior, superconductivity, heavy fermion behavior, and the Kondo effect. 12−17 The growth of materials as single crystals greatly facilitates characterization of structural and electronic properties. Tradi- tional solid-state synthesis typically requires temperatures over 1000 °C and often results in polycrystalline products. Flux synthesis using a large excess of molten metal (or a mixture of metals) as a reaction medium has proven to be an effective method to synthesize novel metal silicide single crystals. This technique allows for lower reaction temperatures, enabling isolation of complex kinetically stabilized products instead of the most commonly formed thermodynamically stable products. 18−21 This is exemplified by the growth of quaternary Th 2 (Au x Si 1−x )[AuAl 2 ] n Si 2 (n = 1, 2, 4) 22 and RE 0.67 T 2 Ga 5‑x Tt x (RE = Y, Sm, Gd−Tm; T = Ni, Co; Tt = Si, Ge) 23 crystals in Al and Ga flux, respectively. In this work two new U 1.33 T 4 Al 8 Si 2 (T = Ni/Co) compounds were synthesized in aluminum/gallium flux mixtures. These phases form in the Gd 1+x Fe 4 Si 10‑y structure type and exhibit the structural disorder that is inherent to this family of compounds. 23−28 Both compounds are stable to water, brine solutions, and heating in air up to 900 °C. Magnetic susceptibility measurements on single crystals reveal anisotropic Curie−Weiss behavior at elevated temperatures, indicating that the uranium ions carry an f-moment. Magnetic transitions are observed for both compounds at low temper- atures, with the Co analog exhibiting fragile antiferromagnet- ism below 5 K. The Ni analog exhibits more complex behavior suggestive of spin frustration or glassy magnetism below 7 K. Heat capacity data for both compounds feature an enhanced electronic coefficient of the heat capacity at low temperature which relates to Kondo hybridization between the f- and conduction electron states. Finally, both compounds exhibit weak temperature dependences of the electrical resistivity that Received: June 2, 2019 Article pubs.acs.org/IC Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.9b01627 Inorg. Chem. XXXX, XXX, XXX−XXX Downloaded via STOCKHOLM UNIV on August 29, 2019 at 00:39:54 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.