Magnetic and Electronic Properties in Rattling Systems, an Experimental and Theoretical Study
Rodriguez Robles, Sergio
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The search for heat regenerators is currently very important due to the amount of wasted heat produced in different human activities. Thermoelectric materials have emerged as a possible solution to the world?s demand and reuse of energy. Recent advances have included the development of materials with tailored phonon properties, including localized "rattling" oscillator modes. In addition a number of interesting physical properties have emerged in rattling systems. This dissertation reports a study of several such systems, experimentally and computationally. Experiments performed include XRD, electron micro-probe, electrical and thermal conductivity, Seebeck coefficient measurements, dc magnetization, dc susceptibility and NMR. In the computational side several ab-initio models have been considered to understand the structural, vibrational and magnetic properties observed in these compounds. Among the studied compounds, the Fe-Al-Zn materials showed interesting magnetic properties combined with anomalous vibrational behavior in a chain geometry. Computational results indicated that the moment is affected by Fe antisites, but also the neighbor configuration contributes to it. Al-V-La is an example of a classical Einstein oscillator material. These properties are related to the existence of loose atoms inside the material. A purely computational study on these materials denoted the existence of two weakly bonded sites. The clathrate structural results from first-principles considerations elucidated the preferred structural configurations in several clathrates. This included Ba-Cu-Ge clathrates, where it was confirmed that the compound follows the Zintl electron counting balance. Also the bonding inside these materials was studied to address the binding of the local-oscillator atoms within the material. For Ba-Ga-Sn clathrates an unusual dimorphism was studied, with both of the two different types of structures investigated. For type-I Ba8Ga16Sn30 the preferred configuration was obtained from NMR lineshape simulations and energy considerations. For the type-VIII Ba8Ga16Sn30 the experimental thermoelectric properties were analyzed in conjunction with computational modeling. Finally in Ba-Al-Ge clathrates the local environments, preferred configuration and vacancy formation were clarified. This included an extensive experimental and computational study on Ba8AlxGe46-x-y2(box)y systems. The different local Al environments were elucidated, with the location of vacancies influencing the surroundings. Also the correlation between the Al substitution and number of vacancies was studied.