Abstracts Earth and Environmental Sciences

Exploratory Flux Growth and Properties of Rare Earth Silicides and Gallides

by Jo Haddock

Flux synthesis is a very powerful and versatile technique for the exploratory synthesis of rare earth intermetallics. Reactions of rare earth metals and alkali earth elements with silicon in a Mg/Al flux or a Mg/Zn flux have yielded single crystals of a series of new complex intermetallic phases. The structures of products were determined by single crystal X-ray diffraction. Identical reactions were explored in both fluxes; both yielded large crystals suitable for study using single crystal X-ray diffraction and magnetic susceptibility measurements, although Mg/Zn reactions produced higher yields and larger crystals. While elemental analysis indicates that the product formed in the Mg/Al melts contains traces of aluminum, no incorporation of zinc was observed in compounds formed in the Mg/Zn flux. Reactions of Eu, Ca, and Si in either Mg/Zn or Mg/Al yield products with two structure types depending on the ratio of europium to calcium in the final product. Calcium-rich reactions produce Ca1-xEuxMgSi (0 < x < 0.7) compounds with the orthorhombic TiNiSi structure type in space group Pnma. Increasing europium content led to the prevalence of Eu8-xCaxMg16Si12 (0 < y < 3.4) products with the hexagonal Ho5Ni19P12 structure type in space group P-62m. In all the products, Eu is expected to be divalent, however there may be some switching to a trivalent state in the TiNiSi type compound due to chemical pressure. In all cases these compounds appear to order ferromagnetically. Ba6R1.8Mg18.2Si13 (R = Dy, Y) grow as large crystals from reactions of barium, silicon, and either dysprosium or yttrium in Mg/Al or Mg/Zn fluxes. The presence of the trivalent Y or Dy in the reaction mixture promotes the formation of Ba6R1.8Mg18.2Si13 in the Ho6Ni20P13 structure type (hexagonal P-6 space group) instead of the expected Ba5Mg19Si12 (Ho5Ni19P12 type). The trivalent cations are incorporated at a specific amount in the compound, mixing on one particular Mg site in the structure. Magnetic susceptibility measurements on Ba6Dy1.8Mg18.2Si13 confirm the presence of Dy3+ and indicate magnetic ordering at 9 K. The low melting point of gallium flux enabled an in-situ neutron diffraction study of the reaction of Ce and Ni in excess gallium. Initial formation of CeNi0.74Ga3.26 was seen at high temperature; this transforms into Ce2NiGa10 and Ce2NiGa12 below 600°C. This information enabled isolation of the high temperature phase by quenching the flux reaction at high temperature. CeNi0.74Ga3.26 is a strongly anisotropic ferromagnet below 7 K. A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy. July 7, 2022. Susan Latturner, Professor Directing Dissertation; Peng Xiong, University Representative; Michael Shatruk, Committee Member; Geoffrey Strouse, Committee Member.