Supervisor: Professor Paul McMillan
The project will use experimental laser-heated diamond anvil cell combined with laboratory-based light scattering techniques and synchrotron X-ray diffraction to determine the vibrational excitations and melting relations in metals at pressures into the megabar range.
Metals crystallise with either hexagonal or cubic (face-centred or body-centred) structures. Hexagonal close-packed metals like Sn and Ti have Raman active vibrational modes at the Brillouin zone centre. We will study the pressure shifts of these vibrations into the megabar range. We will determine the anharmonicity of the Raman modes by combining the data on frequency changes with pressure and with temperature (at ambient P). These new results will constitute the first determination of anharmonic vibrational properties in these metal systems. They will provide an important constraint for theoretical studies of the thermodynamic properties of these metals. This study will constitute a first area of investigation in the PhD thesis.
Next we weill examine the vibrational properties of metals with cubic symmetry, such as Ta or Mo. These have no first-order Raman spectrum and so cannot normally be observed in high pressure experiments. However, observation of the vibrational density of states (VDOS) can be activated by alloy formation or introducing impurity atoms (e.g., N, C or Si) into the metal structure. For example, we previously obtained Raman data on the VDOS for cubic MoNx (x<0.5) that has vibrational properties similar to the pure metal. We will prepare suitable alloys and 'defective' metals using our high-P,T synthesis equipment and obtain VDOS spectra for comparison with available neutron scattering results at ambient conditions. Then we will obtain VDOS spectra at high pressure in the diamond anvil cell. These will be challenging experiments that will lead to the first determination of the vibrational properties of cubic metals at high pressures and the results will provide an important constraint on theoretical studies. They will also allow us to evaluate the phono contribution to the heat capacity.
Finally, the student will examine structural changes including melting that occur under high-P,T conditions for the same metals (either Sn or Ti, and Ta or Mo), using synchrotron X-ray diffraction combined with in situ laser heating in the diamond anvil cell. During these experiments we will study the changes that occur in the 'speckle' pattern caused by relectance from a polycrystalline sample by coherent (laser) light, that is commonly used as a laboratory diagnostic of melting. The experimental data will be compared with results of theoretical calculations obtained in a parallel PhD project and they will be used to link with shock wave data on melting at high pressure.