Energy relavant materials: Investigations based on first principles

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: Energy production, storage and efficient usage are all crucial factors for environmentally sound and sustainable future technologies. One important question concerns the refrigeration industry, where the energy efficiency of the presently used technologies is at best 40% of the theoretical Carnot limit. Magnetic refrigerators offer a modern low-energy demand and environmentally friendly alternative. The diiron phosphide-based materials have been proposed to be amongst the most promising candidates for working body of magnetic refrigerators. Hydrogen is one of the most promising sources of renewable energy. Considerable international research focuses on finding good solid state materials for hydrogen storage. On the other hand, hydrogen gas is obtained from hydrogen containing chemical compounds, which after breaking the chemical bonds usually yield to a mixture of different gases. Palladiumsilver alloys are frequently used for hydrogen separation membranes for producing purified hydrogen gas. All these applications need a fundamental understanding of the structural, magnetic, chemical and thermophysical properties of the involved solid state materials. In this thesis ab initio electronic structure methods are used to study the magnetic and crystallographic properties of Fe2P based magneto-caloric compounds and the thermophysical properties of Pd-Ag binary alloys.The nature of magnetism and the strong sensitivity of the Curie temperature of the Fe2P1?xTx (T = boron, silicon and arsenic) are investigated. Using first principles theory, the change in the average magnetic exchange interactions upon doping is decomposed into chemical and structural contributions, the latter including the c/a and vol-ume effects. It is demonstrated that for the investigated alloys the structural effect can´be ascribed mainly to the c/a ratio that strengthens the magnetic exchange interactions between the two Fe sublattices. On the other hand, it is shown that the two types of Fe atoms have a very complicated co-dependency, which manifests in a metamagnetic behavior of the FeI sublattice. This behavior is strongly influenced by doping the P sites.Lattice stability of pure Fe2P and the effect of Si doping on the phase stability are pre-sented. In contrast to the observation, for the ferromagnetic state the hexagonal structure (hex, space group P62m) has no the lowest energy. For the paramagnetic state, the hex structure is shown to be the stable phase and the computed total energy versuscomposition indicates a hex to bco (body centered orthorhombic, space group Imm2)crystallographic phase transition with increasing Si content. The mechanisms responsi-ble for the structural phase transition are discussed in details.The magnetic properties of Fe2P can be subtly tailored by Mn doping. It was shown experimentally that Mn atoms preferentially occupy one of the two different Fe sites of Fe2P. Theoretical results for the Mn site occupancy in MnFeP1?xSix are presented.The single crystal elastic constants, the polycrystalline elastic moduli and the Debye temperature of disordered Pd1?xAgx binary alloys are calculated for the whole range of concentration, 0 ? x ? 1. It is shown that the variation of the elastic parameters of Pd-Ag alloys with chemical composition strongly deviates from the simple linear trend. The complex electronic origin of these anomalies is demonstrated. The effect of long range order on the single crystal elastic constants of Pd0.5Ag0.5 alloy is also investigated.Within this thesis most of the calculations were performed using the Exact Muffin-Tin Orbitals method. The chemical and magnetic disorder are treated via the Coherent Potential Approximation. The paramagnetic phase is modeled by the Disordered Local Magnetic Moments approach.