Exploration of R2XM2 (R=Sc, Y, Ti, Zr, Hf, rare earth; X=main group element; M=transition metal, Si, Ge): structural motifs, the novel compound Gd2AlGe2 and analysis of the U3Si2 and Zr3Al2 structure types
Date of Award
Master of Science
In the process of exploring and understanding the influence of crystal structure on the system of compounds with the composition Gd5(Si[subscript x]Ge[subscript 1-x])4 several new compounds were synthesized with different crystal structures, but similar structural features. In Gd5(Si[subscript x]Ge[subscript 1-x])4, the main feature of interest is the magnetocaloric effect (MCE), which allows the material to be useful in magnetic refrigeration applications. The MCE is based on the magnetic interactions of the Gd atoms in the crystal structure, which varies with x (the amount of Si in the compound). The crystal structure of Gd5(Si[subscript x]Ge[subscript 1-x])4 can be thought of as being formed from two 32434 nets of Gd atoms, with additional Gd atoms in the cubic voids and Si/Ge atoms in the trigonal prismatic voids. Attempts were made to substitute nonmagnetic atoms for magnetic Gd using In, Mg and Al. Gd2MgGe2 and Gd2lnGe2 both possess the same 32434 nets of Gd atoms as Gd5(Si[subscript x]Ge[subscript 1-x])4, but these nets are connected differently, forming the Mo2FeB2 crystal structure. A search of the literature revealed that compounds with the composition R2XM2 (R=Sc, Y, Ti, Zr, Hf, rare earth; X=main group element; M=transition metal, Si, Ge) crystallize in one of four crystal structures: the Mo2FeB2, Zr3Al2, Mn2AlB2 and W2CoB2 crystal structures. These crystal structures are described, and the relationships between them are highlighted. Gd2AlGe2 forms an entirely new crystal structure, and the details of its synthesis and characterization are given. Electronic structure calculations are performed to understand the nature of bonding in this compound and how electrons can be accounted for. A series of electronic structure calculations were performed on models with the U3Si2 and Zr3Al2 structures, using Zr and Al as the building blocks. The starting point for these models was the U3Si2 structure, and models were created to simulate the transition from the idealized U3Si2 structure to the distorted Zr3Al2 structure. Analysis of the band structures of the models has shown that the transition from the U3Si2 structure to the Zr3Al2 structure lifts degeneracies along the [Gamma right pointing arrow Zeta] direction, indicating a Peierls-type mechanism for the displacement occurring in the positions of the Zr atoms.
Sean William McWhorter
McWhorter, Sean William, "Exploration of R2XM2 (R=Sc, Y, Ti, Zr, Hf, rare earth; X=main group element; M=transition metal, Si, Ge): structural motifs, the novel compound Gd2AlGe2 and analysis of the U3Si2 and Zr3Al2 structure types" (2006). Retrospective Theses and Dissertations. 19339.