Theoretical and Experimental Studies of Electronic Structure, Magnetic, and Hyperfine Interaction Properties of Novel Compounds

Title: Theoretical and Experimental Studies of Electronic Structure, Magnetic, and Hyperfine Interaction Properties of Novel Compounds
Authors: Nejadsattari, Farshad
Date: 2016
Abstract: This thesis consists of the results of theoretical and experimental studies on the electronic structure, magnetic properties and hyperfine-interaction parameters of five compounds. The theoretical studies are based on ab-initio calculations, and the experimental studies are based on Mössbauer spectroscopy, magnetic measurements, and X-ray powder diffraction measurements. The calculated hyperfine-interaction parameters are shown to be in good agreement with the corresponding experimental parameters for all studied compounds. Al13Fe4 crystallizes in the monoclinic space group 𝐶2/𝑚, in which Fe atoms are located at five inequivalent crystallographic sites. It is shown that the zero-field Mössbauer spectra can be decomposed into three quadrupole doublets. It is found that the shape of the Mössbauer spectrum measured in an external magnetic field can be accounted for with five component subspectra generated using the calculated EFG parameters at five inequivalent Fe sites. A pseudogap is observed in the density of states (DOS) in the vicinity of the Fermi level. The recently discovered layered iron oxyselenide Na2Fe2Se2O is confirmed to be a Mott insulator. It is demonstrated that Na2Fe2Se2O orders antiferromagnetically. The Debye temperature of Na2Fe2Se2O is found to be 274(3) K. The Fe2NiGa compound crystallizes in the cubic space group 𝐹4 ̅3𝑚 and is predicted to be half-metallic with covalent chemical bonding. It orders ferromagnetically. The saturation magnetization per formula unit and the estimated Fe magnetic moments at the A and B sites are given. It is observed that the magnetic properties of Fe2NiGa are very strongly dependent on their heat treatment. The presence of a mixture of strong covalent and weak ionic chemical bonding and of metallic characteristics for the (Li0.8Fe0.2)OHFeSe superconductor is shown. The Mössbauer spectra consist of two quadrupole-doublet patterns. It is demonstrated that there is no magnetic ordering of the 2𝑎-site Fe magnetic moments down to 2.0 K. The final compound is the Al76Ni9Fe15 approximant. The existence of a pseudogap in the calculated electronic density of states slightly above the Fermi level suggests electronic stabilization according to the Hume-Rothery-type mechanism. High metallicity is predicted. Both the Mössbauer spectra and magnetic susceptibility data indicate that Al76Ni9Fe15 is a paramagnet down to 2.0 K.
CollectionThèses, 2011 - // Theses, 2011 -