Pressure-induced phase transitions of simple metals

Structural changes of materials under high pressure are currently a field of topical interest. The proposed project will investigate pressure-induced phase transitions of simple metals, namely the elements Li, Na, K and Ba, which are of particular interest for modelling the high-pressure behaviour of materials. Up to now, a number of investigations have studied the high-pressure phases of these metals by x-ray powder diffraction, a method which is well-suited for the identification of the observed crystal structures, however, does not give further information on the transformation mechanisms. Theoretical studies have suggested two fundamenatally different mechanisms: changes in the occupation of electronic states under pressure (denoted as s->d or s->p transfer) or pressure- enhanced quasi-martensitic shear transformations. There are a few indications that the latter mechanism might occur in Li but the former in K and Ba. Experimentally, the details of the transformation mechanisms can only be clarified by the study of single crystals. Recent progress of pressure-cell design now allows single crystal investigations by neutron scattering techniques under pressures up to 12 GPa. This will be used for the following investigations on Li, Na, K and Ba: 1) inelastic neutron scattering: study of the lattice dynamics under pressure. 2) diffuse-elastic scattering: search for static precursors of the phase transitions, which will give information on the nucleation process. Investigation of stacking faults in the close-packed phases. 3) Bragg scattering: investigation of the crystallographic orientation relations between the phases, of the kinetics of the phase transitions, hysteresis effects and incubation times. 4) exploration of the temperature-pressure phase diagrams of Li, Na, K and Ba below room temperature. This is of particular interest since two of the elements, namely Li and Na, also exhibit low-temperature martensitic transformations at atmospheric pressure. The transition between these two different types of lattice instabilities is of essential interest in order to clarify the phase relations and the tranformation mechanisms in these metals. For the low-temperature investigations, a special cryostat compatible with the pressure-cells will be constructed.