Thermal Properties of Phase-Change Materials From Lattice Dynamics to Thermoelectricity

Phase-change materials (PCMs) form a unique collection of materials that is defined by its extraordinary property portfolio. The attribute "phase-change" readily summarizes the most prominent feature of a representative of this material class: PCMs can be rapidly and reversibly switched between the crystalline and the amorphous phase. Alongside with the phase transition drastic changes occur in optical reflectivity and electrical conductivity, which predestine these compounds for technological utilization in modern data storage devices. The large contrast in reflectivity leveraged the employment of PCMs in optical data storage devices such as DVD-RWs or the Blu-Ray disc, while the contrast in resistivity makes them promising contenders for a new class of non-volatile, electronic data storage devices (so called PRAMs or PCRAMs) that bear the potential to replace FLASH technology in the near future.The aim of this thesis is to support the ongoing development of PCM-based technology by enhancing the understanding of its physical foundation. Here, the focus is set on a thorough investigation of the thermal properties of GeSbTe-based phase-change materials (GSTs). Thermal properties determine several critical parameters of phase-change technology e.g. the power consumption of the switching process. Furthermore, thermal properties are strongly related to the peculiar physical effects which have been reported for phase-change materials such as the unconventional bonding in the crystalline phase (resonance bonding) or structural disorder that strongly affects transport properties. In order to investigate these miscellaneous relationships, the study at hand is divided into three parts. In the first part the focus is set on microscopic and macroscopic thermal properties that are associated with lattice vibrations. These are the phonon density of states (PDOS) measured by inelastic neutron scattering and the specific heat of GSTs determined by differential scanning calorimetry. It is demonstrated that crystalline GSTs exhibit signs of a pronounced anharmonic atomic interaction potential, which is a predicted consequence of resonant bonding.The second part covers thermal transport properties. Here, low temperature measurements of the thermal conductivity from room temperature down to 50 K (performed with a custom-designed 3omega setup) enable valuable insights into the phonon scattering mechanisms. It is demonstrated that thermal transport is strongly affected by the degree of configurational disorder - similar to electrical transport. In combination with the aforementioned finding of pronounced anharmonicity a model evolves, in which thermal transport in GSTs is strongly reduced by an interplay of anharmonicity and disorder. Furthermore, it is demonstrated, that the degree of disorder can be used to tailor the thermal and electrical transport properties.The topic of transport control is further expanded within the third part of the thesis where the focus moves to the thermoelectric properties of GSTs. The question arises whether the tunability of the electric and the thermal conductivity enables the approach of a so-called phonon-glass, electron crystal property combination. Such a combination is highly desirable in the field of thermoelectrics, where a thermoelectric composition converts heat into electricity and vice versa. In this part of the investigation the thermal and electrical conductivities are complemented with the corresponding Seebeck data to calculate the thermoelectric figure of merit ZT of GSTs. Elevated ZT values around 0.8 are found for GeTe-rich GST compositions and GSTs close to the metal-insulator transition. This may open up further technological utilization of these versatile materials in the field of thermoelectrics and underlines the high relevance of the thermal characterization of phase-change materials