In search of novel magnetocaloric materials, a number of rare earth-based phases were designed, synthesized and investigated. These compounds were prepared by arc-melting or sintering, followed by annealing at high temperature to obtain phase-pure materials. Single crystal and powder X-ray diffraction were employed for phase identification, purity assessment, structure solution and refinement. Energy dispersive X-ray spectroscopy (EDS) was used to determine sample compositions. A Quantum Design SQUID magnetometer equipped with an alternating current (ac) transport controller (model 7100) was employed to measure magnetic data and evaluate magnetocaloric properties. The crystal structure and physical properties were analyzed via electronic structure calculations.
In this thesis work, the RE5Ga3 and RECo2 (RE = rare earth) materials were chosen as a starting point for structural modifications. Specifically, substitution of Co for Ga (and vice versa) or rare earth replacement was used to design new materials. In total, four families were investigated: Ho5Ga3-xCox (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1), Er5Ga3-x(Fe/Co)x (x = 0, 0.4), RE(Co0.667Ga0.333)2 (RE = Gd, Tb, Dy, Ho, and Er), and Gd(Co1-xGax)2 (x = 0, 1/6, 1/3, 1/2, 2/3, 5/6, and 1). The samples were prepared by arc-melting, wrapped in Ta foil, sealed in evacuated silica tubes and annealed at specific temperatures.
The Ho5Ga3-xCox and Er5Ga3-x(Fe/Co)x systems features a Mn5Si3-type-to-Cr5B3-type structural transformation, driven by geometric factors. On the other hand, the structural transformation in the RE(Co0.667Ga0.333)2 and Gd(Co1-xGax)2 systems appears to be controlled by the valence electron count (VEC). The RE(Co0.667Ga0.333)2 (RE = Gd, Tb, Dy, Ho, and Er) phases adopt a hexagonal MgZn2-type structure (P63/mmc). Structural and magnetic properties of the MgZn2-type RE(Co0.667Ga0.333)2 materials were investigated via single crystal and powder X-ray diffraction, powder neutron diffraction (PND), and magnetic measurements. In addition to the hexagonal MgZn2-type structure, four other structures were discovered in the Gd(Co1-xGax)2 system: cubic MgCu2- (Fd3 ̅m), orthorhombic MgSrSi- (Pnma), orthorhombic CeCu2- (Imma), and hexagonal AlB2-type structure (P6/mmm). When Ga content increases, the structure moves from a “condensed cluster-based arrangement” to a “3D Network” to a “2D Network”. Meanwhile, coordination number (CN) of Co or Ga atoms changes from 6 to 4, and then to 3.
Magnetic properties of many of the RE-based phases were evaluated via temperature- and field-dependent magnetization measurement. Materials exhibited a sharp ferromagnetic transition and their MCE in terms of the isothermal magnetic entropy change, was explored. / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/21241 |
| Date | January 2017 |
| Creators | Yuan, Fang |
| Contributors | Mozharivskyj, Yurij, Chemistry and Chemical Biology |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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