The Effects of High Pressure on the Vibrational and Magnetic Properties of Iron-Based Materials

Experimental methods are demonstrated for studying pressure-dependent material properties and solid phases unattainable at ambient pressure with high-pressure synchrotron nuclear resonance techniques. Pressure presents an intriguing experimental parameter for investigating the problem of excess low-energy (<12 meV) vibrational modes in nanocrystalline materials. A satisfactory explanation for the origin of these modes has yet to be put forth, and until now, the volume dependence of the interatomic forces responsible for these modes had never been measured. The phonon density of states (DOS) of nanocrystalline Fe was measured under pressures up to 28 gigapascals (2.8× 10 atm) using the nuclear resonant inelastic x-ray scattering (NRIXS) technique. The nanocrystalline material exhibited an enhancement in its DOS at low energies by a factor of 2.2. This enhancement persisted throughout the entire pressure range, and was unaffected by the pressure-induced phase transformation (from bcc to hcp) at 13 GPa. At higher energies, the van Hove singularities observed in the samples were coincident in energy and remained so at all pressures, indicating that the forces conjugate to the normal coordinates of the nanocrystalline materials are similar to the interatomic potentials of bulk crystals. Subsequent neutron inelastic scattering measurments at ultra-low energies (2–18 μeV) also observed enhancement in the vibrational spectrum of the nanocrystalline material. A portion of this enhancement is attributed to novel microstructural modes, characterized by the cooperative dynamics of individual crystallites. The high pressure, hcp ( ) phase of iron and its alloys is thought to comprise