Infrared spectroscopy of Fe2VAl Heusler compounds

Abstract

Heusler compounds are a very broad class of materials first discovered by Friedrich Heusler in 1903. They are intermetallic compounds which show a big variety of physical phenomena, such as superconductivity,shape memory behavior or topological insulators. Some of which are well understood and can be manufactured following specific rules. Additionally, some Heusler compounds have been found to posses a high efficiency for thermoelectric applications which makes them a major research interest.One of the main quests for thermoelectric applications is the challenge to improve the overall efficiency of the conversion from heat to electricity which is given by the thermoelectric figure of merit (ZT).Several material parameters play a vital role in ZT. Different approaches have been found in order to optimize the various material parameters and therefore maximize ZT. One such approach is approximated in the samples investigated in this thesis. Through the introduction of disorder by thermal quenching or by doping with aluminum an additional peak in the density of states has been predicted by density functional theory simulations. Such a peak near the Fermi energy would enhance the thermoelectirc figure of merit.This work investigates Heusler compounds based on Fe2VAl which were either doped with aluminum or quenched at higher temperatures.With the help of a Fourier-transformed infrared (FTIR) spectroscopy the optical reflectivity was recorded at room and low temperatures. Using the Kramers-Kronig relation the optical conductivity was calculatedand fitted with a Drude-Lorentz model. The changes in the optical conductivity were analyzed and discussed. Through fitting the data with a Drude-Lorentz model the optical response was separated into different components in order to gain a deeper insight into the thermoelectric transport.The main finding in this work was the shifting of the interband transition to lower frequencies with higher quenching temperature in the material. Due to the shifting of interband transition in the optical datathe pseudogap in the density of states should be decreased with higher quenching temperature. Materials exhibited a relative temperature-independent optical response which could be beneficial for their applicationsover the range of the investigated temperatures.