First-principles investigations of the La-H system

The lanthanum-hydrogen system shows a number of very interesting features, of which many are not fully understood up to now. In principle, the structure of the ß phase, which exists at stoichiometries between the dihydride and the trihydride, is derived from the cubic fluorite structure (for LaH 2 ) where the H atoms occupy the tetrahedral sites of the face-centred cubic La lattice and, at higher H concentrations, also the octahedral sites become filled. While generally there is no long-range order for the hydrogen atoms, at certain compositions, however, ordering has been observed, e. g., for LaH 2.25 and LaH 2.50. In reality the structural situation is much more complicated. It has been known for some time that octahedral H atoms are displaced considerably towards tetrahedral lattice sites. While LaH 2+x is a metal for 0 < x < 0.8, it becomes an insulator (semiconductor) for higher H concentrations. Neither is the mechanism for this metal-insulator transition understood, nor the fact that it occurs at high vacancy concentrations. The goal of this project is the ab-initio investigation of the La-H system as regards phase stability, structural properties and thermodynamic aspects and to investigate the mechanism of the metal-insulator transition. This will be performed on three different levels from a methodic point of view. First, electronic-structure calculations and structure optimizations by means of the Vienna ab-initio simulation package (VASP) shall be performed. Secondly, lattice vibrations will be taken into account (using MedeA-Phonon) to calculate the zero-point energies and the temperature dependence of the thermodynamic functions. Thirdly, the configurational entropy will also be included in order to obtain the phase diagram of the La-H system. These calculations will be accompanied by experiments performed by my collaboration partners. This will make it possible to assess the quality of the calculations.

In binary metal-hydrogen systems the crystal structure is in many cases not known exactly. This is due to the fact that the localization of H atoms in solids is a difficult problem. In X-ray diffraction investigations this is caused by the low mass of the H and D atoms. Neutron experiments, on the other hand, often yield results which are compatible with several different structure models, although the H and D nuclei are good diffraction centres. Therefore the combination of different experimental and computational methods provides a successful strategy in such cases. The first-principles calculations performed in this project provide answers to a series of open questions concerning the structures of the different phases in the La-H system. Possibly in connection with structural aspects a metal-insulator transition is observed for LaH 2.80. This transition occurring at ambient temperatures and pressures can be carried out reversibly ("switchable mirrors"). Its mechanism has also been investigated in this project. For elemental La and H concentrations less than a few atom percents the stability of the double hexagonal structure with a close-packed arrangement of the La atoms (a phase) could be confirmed. Increase of the H content up to 6 at.% causes a destabilisation of the hexagonal structure with doubled c axis in favour of a structure with a cubic densest packing of the La atoms. The metal lattices of both structures have tetrahedral as well as octahedral voids which can be filled by H atoms. It follows from our computations that contrary to what has been assumed up to now the occupation of the octahedral empty sites is favoured if the H atoms are spatially separated. Also, the formation of H-La-H arrangements is energetically favoured if these arrangements are linear or almost linear. For compositions near the trihydride pairs of H vacancies on octahedral sites in the shortest possible distance (3.9 Å) have turned out as the most stable configuration. For stoichiometric LaH 3 a hitherto unknown orthorhombic distortion of the ideal cubic structure has been found which is energetically favourable and shows a band gap. A band gap has also been found, e. g., for the energetically favourable structure with composition LaH 2.94. Its appearance can be understood by the formation of strong La d-d bonds across the vacancy sites which lead to a lowering of the respective La d bands and to an opening of a band gap. At present calculations with the cluster- expansion method are performed (with Prof. S. Müller and T. Kerscher) which should lead to a theoretical phase diagram for the La-H system. Furthermore elastic and inelastic neutron experiments (with Prof. G. Krexner) for low H content and the trihydride are performed.