Effect of pressure on zone-center phonons in hexagonal-close-packed metals

We report on studies of several hexagonal-close-packed (hcp) metals by Raman scattering techniques in the diamond anvil cell for pressures up to 60 GPa. The pressure response of the observed transverse-optical (TO) zone-center phonon mode includes positive pressure shifts as well as anomalies, such as mode softening in connection with phase transitions. It is shown that the phonon frequencies and their pressure dependences are related to macroscopic elastic parameters. More general, these results show that the measurement of Raman-active phonons provides a direct probe of bonding in metals, and agreement with theoretical models gives additional confidence in ab initio techniques.