Electronic and Vibrational Properties of Fe2NiAl and Co2NiAl Full Heusler Alloys: A First-Principles Comparison

<p>In this work, we report on a systematic first-principles study of the structural, electronic, vibrational and thermodynamic properties of the cubic Fe₂NiAl and tetragonally distorted Co₂NiAl full Heusler compounds. We discuss systematically the competition between the inverse Heusler structure and a T<i>^p</i>-type layered atomic ordering formed by the alternating planes of (Fe,Co) and Ni atoms in terms of the electronic and vibrational density of states.</p> <p>Such an arrangement is predicted to be the ground state of Fe₂NiAl. For Co₂NiAl, layered ordering has slightly higher energy in comparison with the inverse one, however, we show that it might be stabilized at rather low temperatures.</p> <p> </p> <p>Due to the broken symmetry, layered T<i>^p</i>-Fe₂NiAl possesses a large MAE of the same order as tetrataenite FeNi - even in a phase with cubic lattice parameters, which makes a T<i>^p</i>-type layered order an interesting feature for rare-earth free permanent magnets in Heusler-type compounds.</p>

Electronic and Vibrational Properties of Fe2NiAl and Co2NiAl Full Heusler Alloys: A First-Principles Comparison

<p>In this work, we report on a systematic first-principles study of the structural, electronic, vibrational and thermodynamic properties of the cubic Fe₂NiAl and tetragonally distorted Co₂NiAl full Heusler compounds. We discuss systematically the competition between the inverse Heusler structure and a T<i>^p</i>-type layered atomic ordering formed by the alternating planes of (Fe,Co) and Ni atoms in terms of the electronic and vibrational density of states.</p> <p>Such an arrangement is predicted to be the ground state of Fe₂NiAl. For Co₂NiAl, layered ordering has slightly higher energy in comparison with the inverse one, however, we show that it might be stabilized at rather low temperatures.</p> <p> </p> <p>Due to the broken symmetry, layered T<i>^p</i>-Fe₂NiAl possesses a large MAE of the same order as tetrataenite FeNi - even in a phase with cubic lattice parameters, which makes a T<i>^p</i>-type layered order an interesting feature for rare-earth free permanent magnets in Heusler-type compounds.</p>

Heat capacity anomalies of the molecular crystal 1-fluoro-adamantane at low temperatures

Disorder–disorder phase transitions are rare in nature. Here, we present a comprehensive low-temperature experimental and theoretical study of the heat capacity and vibrational density of states of 1-fluoro-adamantane (C10H15F), an intriguing molecular crystal that presents a continuous disorder–disorder phase transition at T = 180 K and a low-temperature tetragonal phase that exhibits fractional fluorine occupancy. It is shown that fluorine occupancy disorder in the low-T phase of 1-fluoro-adamantane gives rise to the appearance of low-temperature glassy features in the corresponding specific heat (i.e., “boson peak” -BP-) and vibrational density of states. We identify the inflation of low-energy optical modes as the main responsible for the appearance of such glassy heat-capacity features and propose a straightforward correlation between the first localized optical mode and maximum BP temperature for disordered molecular crystals (either occupational or orientational). Thus, the present study provides new physical insights into the possible origins of the BP appearing in disordered materials and expands the set of molecular crystals in which “glassy-like” heat-capacity features have been observed.

Universal law for the vibrational density of states of liquids

An analytical derivation of the vibrational density of states (DOS) of liquids, and, in particular, of its characteristic linear in frequency low-energy regime, has always been elusive because of the presence of an infinite set of purely imaginary modes—the instantaneous normal modes (INMs). By combining an analytic continuation of the Plemelj identity to the complex plane with the overdamped dynamics of the INMs, we derive a closed-form analytic expression for the low-frequency DOS of liquids. The obtained result explains, from first principles, the widely observed linear in frequency term of the DOS in liquids, whose slope appears to increase with the average lifetime of the INMs. The analytic results are robustly confirmed by fitting simulations data for Lennard-Jones liquids, and they also recover the Arrhenius law for the average relaxation time of the INMs, as expected.

Lattice Dynamics of CrW Dilute Alloy Using Modified Embedded Atom Method Potential

A modified embedded atom method (MEAM) has been used to study the lattice dynamics and vibrational properties of CrW alloy. Using the MEAM potential the force-constants up to second neighbours for pure Cr and its dilute alloy with small concentration of W as substitutional impurity are calculated. The Phonon dispersions for dilute CrW alloy at 0.3%, 0.8% and 1.6 % concentration of W substitutional impurity have been computed and the obtained results are compared with the available experimental data. We have obtained a very good agreement with the experimentally measured results of phonon dispersions. With the application of obtained force-constants from MEAM potential, the local vibrational density of states in ideal crystal and its alloys using Green’s function method has been calculated. On the basis of the results of local vibrational density of states, the condition of resonance modes has been investigated. Using the calculated vibrational local density of states, the mean square thermal displacements of impurity atoms in CrW alloys are also calculated.

Electronic-vibrational density evolution in a perylene bisimide dimer: mechanistic insights into excitation energy transfer

Mechanistic picture of excitation energy transfer through the time evolution of electronic-vibrational densities in a perylene bisimide dimer from all-mode real-time path integral calculations.

Proton Dynamics in Palladium–Silver: An Inelastic Neutron Scattering Investigation

Proton dynamics in Pd77Ag23 membranes is investigated by means of various neutron spectroscopic techniques, namely Quasi Elastic Neutron Scattering, Incoherent Inelastic Neutron Scattering, Neutron Transmission, and Deep Inelastic Neutron Scattering. Measurements carried out at the ISIS spallation neutron source using OSIRIS, MARI and VESUVIO spectrometers were performed at pressures of 1, 2, and 4 bar, and temperatures in the 330–673 K range. The energy interval spanned by the different instruments provides information on the proton dynamics in a time scale ranging from about 102 to 10−4 ps. The main finding is that the macroscopic diffusion process is determined by microscopic jump diffusion. In addition, the vibrational density of states of the H atoms in the metal lattice has been determined for a number of H concentrations and temperatures. These measurements follow a series of neutron diffraction experiments performed on the same sample and thus provide a complementary information for a thorough description of structural and dynamical properties of H-loaded Pd-Ag membranes.