Synthetic exploration of ternary rare earth metal-rich tellurides and novel magnetic properties of gadolinium halide clusters.

摘要

This dissertation focuses on the exploratory synthesis of rare earth metal-rich ternary tellurides and the synthesis and magnetic properties of gadolinium halide cluster compounds. “Polar-intermetallic” bonding between the early- and late-transition metals has been exploited as a strategy for the synthesis of ternary metal-rich compounds. Compounds discovered herein all contain late transition metal elements as interstitials in the rare-earth metal framework.; A group of telluride compounds with the empirical compositions R ₆MTe₂, R₇M₂Te₂ and R ₅M₂Ta₂ were synthesized in high-temperature solid-state reactions and were found in four different structure types. GdsMTe₂ (M = Co, Ni) and Er₆RuTe₂ adopt the Zr₆CoAl ₂-type structure. Y₆MoTe₂ crystallizes in a ternary derivative of SC₂Te-type structure, which is different than the above 6-1-2 compounds. Er₇M₂Te₂ (M = Co, Ni) adopt a new structure type in space group Imm2. All the above three structures are constituted of M-centered, tricapped trigonal prisms of rare-earth atoms, regular or distorted, as the basic building blocks, and these structural motifs further condense in various fashions. Er₅M ₂Te₂ (M = Co, Ni) are unique in that they possess appreciably short M-M contacts that are not present in the previous structures.; Extended Hiickel band structure calculations were performed on Er ₇Ni₂Te₂ and indicate this compound is metallic with optimized Er-Ni bonding that is a stabilizing factor for the structure. Magnetic susceptibility measurements of polycrystalline Er₇Ni ₂Te₂ are consistent with discrete Er3+ in the high-temperature region while magnetic ordering occurs at 16.5 K. This is the first compound in the series that has exhibited a magnetic transition.; Reinvestigation of the well-known Gd₇X₁₂Z phases led to the preparation and structure determination of new compounds, Gd ₇Br₁₂Fe, Ca_0.80Gd_0.20Gd₆I ₁₂Co, Ca_0.73Gd_0.27Gd₆I₁₂Ni and Gd₇I₁₂C. The former three compounds crystallize in space group R 3¯ while the latter in R3. Magnetic measurements of Gd[Gd₆Fel₁₂] suggest that the open-5d-shell [Gd₆FeI₁₂]3− clusters exhibit strong intracluster ferromagnetic coupling of the 4f moments. The closed-5d-shell Gd[Gd₆CoI₁₂] offers a control to support our hypothesis that exchange interactions between the 4f electrons and unpaired delocalized cluster electrons provide the mechanism for this robust coupling. This discovery raises the possibility of developing a new class of molecular magnets based on open-5d-shell rare-earth clusters. A series of known cluster compounds will be investigated to further test our hypothesis.