Investigation of the Lutetium-Hydrogen system

The lutetium-hydrogen system is in many respects similar to the corresponding systems of the other heavier rare- earth metals (including Y and Sc) and hydrogen, but in a number of features different from the systems of the lighter rare-earths (like, e.g., La) with hydrogen. The Lu-H system shows a number of very interesting physical properties, which depend strongly on the hydrogen content. For low hydrogen concentrations a hexagonal solid solution phase (a phase) exist. At temperatures below ca. 440 K hydrogen ordering occurs whereby sine-shaped H- Lu-H arrangements along the c axis are observed experimentally. In the cubic ß phase the H atoms occupy the tetrahedral sites of the face-centred cubic Lu lattice such that LuH 2 has fluorite structure. For the superstoichiometric dihydrides some of the octahedral sites become filled. While the a and ß phases have metallic conductivity, the hexagonal phase (LuH3-x , x less than 0.01) is an insulator (semiconductor). The exact details of the crystal structure of the trihydride are still not known. The goal of this project is the ab-initio investigation of the Lu-H system as regards phase stability, structural properties, thermodynamic aspects as well as the optical properties of insolating phase. These investigations will be performed on three different levels from a methodical 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. In a third step, the configurational entropy will be calculated using the cluster-expansion method and Monte Carlo simulations in order to arrive at the phase diagram of the Lu-H system. These calculations will be accompanied by experiments (neutrons and nuclear-magnetic resonance) performed by my collaboration partners. This will make it possible to assess the quality of the calculations.

Density-functional theory can in a number of cases be successfully used to solve structural problems which cannot at all or at least not easily be tackled experimentally. This concerns particularly metal-hydrogen systems which quite often have very interesting hydrogen-ordering and defect structures. In a previous project the binary LaH system was investigated successfully, also with respect to the concentration-dependent reversible metal-insulator transition which can already be observed at room temperature and moderate pressures (switchable-mirror effect). The procedure which proved to be adequate for the LaH system was adopted for the follow-up project of a similar investigation of the lutetiumhydrogen system. Although both La and Lu belong to the lanthanoids, the phase diagrams and the structural details for the corresponding hydrides are quite different. This becomes also manifest in the fact that the atomic radius for Lu is noticeably shorter than that for La.The structure of elemental lanthanum is double hexaxagonal close-packed. When hydrogen is absorbed, it preferentially occupies the octahedral vacancies for spatially separated H atoms, and otherwise there is a tendency towards forming dumb-bell-like HLaH units, in which the H atoms are located at tetrahedral vacancy positions. Our computations show thatfor ordinary hexagonal Lu the situation is different, insofar as there is a pronounced preference for the occupation of tetrahedral sites for spatially separated H atoms and dumb-bell-like HLuH units with either two tetrahedral H atoms or a combination of one tetrahedral and two octahedral H atoms.The dihydrides of La and Lu as well as of many other rare-earth metals adopt the cubic fluorite structure. In this project the most favourable vacancy-ordering structures for the sub- and superstoichiometric dihydrides of Sc, Y, La, and Lu are obtained and compared. It is found, e. g., that for the substoichiometric dihydrides of Y, La, and Lu with two H vacancies per unit cell dumb-bell shaped units with a metal atom in the centre is obtained, while for Sc two H vacancies in the smallest possible distance correspond to the most stable configuration.For the stoichiometric trihydride of Lu the space group has been obtained as P63 by systematic symmetry lowering. Furthermore, the most favourable structure elements in the substoichiometric trihydride have been determined. Presently, computations for the hexagonal trihydride phase using the Cluster-Expansion Method are being performed (in cooperation with Prof. S. Müller and Dr. T. C. Kerscher, TUHH Hamburg) in order to complete the already available results.