Negative Gruneisen Parameters at High Pressure in FeTi from Density Functional Theory

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Munoz, Jorge
Khamala, Bethuel

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Density functional theory (DFT) calculations are a pillar of modern materials physics and quantum chemistry research and the execution of DFT codes represent a significant fraction of the overall utilization of Texas Advanced Computing Center (TACC) resources. We present results of DFT and DFT-based calculations on FeTi, 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 at different specific volumes and uncovered a volume range 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 will show ancillary 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 confirm that the Gruneisen parameters are indeed negative in the predicted specific volumes. We also show an analysis of the calculated force constants, charge densities, and band structures that preliminarily point towards orbital hybridization as the origin of the observed negative Gruneisen parameters.




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