The phonon entropy of transition metals and alloys : effects of impurities and of a martensitic phase transition

摘要

For a fixed configuration of ions on a given crystalline lattice, low energy excitations around the static average configuration can be thermally activated and will contribute to the entropy of the system. As such, phonons, spin-waves or electronic excitations have their own entropic contribution. This thesis investigates the entropic effects of lattice vibrations in transition metal alloys, both from experimental and computational points of view.Using inelastic neutron scattering, it is shown that a few percent of substitutional impurities from the transition metal series strongly affect the phonon density of states (DOS) of pure vanadium. Alloying with 6% Pt solutes produces a strong stiffening of the phonon DOS, inducing a large and negative vibrational entropy of mixing, which overcomes the increase in configurational entropy. A systematic study of chemical trends for different transition metal impurities was conducted. A previously unknown correlation is established between the vibrational entropy of alloying and the difference in electronegativity of the solute and the host. Density-functional theory calculations were conducted and confirmed the occurrence of systematic charge-transfers correlating with the electronegativity, which affect the interatomic force-constants and the phonons.The effect of impurities on the anomalous temperature-dependence of phonons in vanadium is investigated. It is found that the solutes which affect the phonon density of states most strongly at room temperature also suppress the anomalous temperature behavior. Electron-phonon and phonon-phonon couplings are examined as potential sources of this effect, through a careful accounting of their contributions to the heat capacity, based on inelastic neutron scattering experiments, calorimetry measurements and electronic structure calculations.Finally, the changes in the phonon DOS and the vibrational entropy across the low-temperature martensitic phase transformation in Fe71Ni29 are investigated. The respective contributions of the phonons and magnetism to the entropy of the direct and reverse transformation are evaluated from neutron scattering and differential scanning calorimetry measurements. A significant magnetic entropy is found in the reverse transformation, which is not present in the direct transformation. This result stresses the necessity to account for the respective contributions of all microscopic degrees of freedom in evaluating entropy changes in solid-solid phase transitions.