Orbital hybridization and Negative Gruneisen parameters of bcc-based intermetallic FeTi at high pressure

FeTi is a brittle intermetallic material that crystallizes in the bcc-based CsCl structure and is stable until it melts at 1600 K. We investigated its electronic band structure and phonon dispersion relations using DFT in which the majority of the phonon modes decrease in energy or remain unchanged with decreasing volume. This behavior is usually observed in invar materials, but unlike them, FeTi is nonmagnetic and there is negligible change in the Fermi surface with pressure. The behavior occurs more generally in materials that show negative thermal expansion, but unlike most of those materials, the crystal structure of FeTi is not particularly open and it is stable at high pressure. In this talk, we show experimental measurements of the phonon density-of-states curves performed via nuclear-resonant inelastic x-ray scattering in a diamond-anvil cell (DAC) at pressures up to 55 GPa and x-ray diffraction also in a DAC at pressures up to 25 GPa that are consistent with the volume range of the calculated negative Gruneisen parameters, as well as an analysis of the calculated force constants, frozen-phonons, charge densities, and band structures that preliminarily point towards orbital hybridization as the origin of the observed negative Gruneisen parameters.