104 research outputs found
Temperature dependent effective potential method for accurate free energy calculations of solids
We have developed a thorough and accurate method of determining anharmonic
free energies, the temperature dependent effective potential technique (TDEP).
It is based on \emph{ab initio} molecular dynamics followed by a mapping onto a
model Hamiltonian that describes the lattice dynamics. The formalism and the
numerical aspects of the technique are described in details. A number of
practical examples are given, and results are presented, which confirm the
usefulness of TDEP within \emph{ab initio} and classical molecular dynamics
frameworks. In particular, we examine from first-principles the behavior of
force constants upon the dynamical stabilization of body centered phase of Zr,
and show that they become more localized. We also calculate phase diagram for
He modeled with the Aziz \emph{et al.} potential and obtain results which
are in favorable agreement both with respect to experiment and established
techniques
Ab initio Determination of Phase Stabilities of Dynamically Disordered Solids: rotational C2 disorder in Li2C2
The temperature-induced orthorhombic to cubic phase transition in Li2C2is a
prototypical ex-ample of a solid to solid phase transformation between an
ordered phase, which is well describedwithin the phonon theory, and a
dynamically disordered phase with rotating molecules, for which the standard
phonon theory is not applicable. The transformation in Li2C2 happens from a
phase with directionally ordered C2 dimers to a structure, where they are
dynamically disordered. We provide a description of this transition within the
recently developed method (Klarbring et al.,Phys.Rev. Lett. 121, 225702 (2018))
employing ab initio molecular dynamics (AIMD) based stress-strain thermodynamic
integration on a deformation path that connects the ordered and dynamically
disordered phases. The free energy difference between the two phases is
obtained. The entropy that stabilizes the dynamically disordered cubic phase is
captured by the behavior of the stress on the deformation path
Na–Ni–H phase formation at high pressures and high temperatures: hydrido complexes [NiH5]3– versus the perovskite NaNiH3
The Na-Ni-H system was investigated by in situ synchrotron diffraction studies of reaction mixtures NaH-Ni-H-2 at around 5, 10, and 12 GPa. The existence of ternary hydrogen-rich hydrides with compositions Na3NiH5 and NaNiH3, where Ni attains the oxidation state II, is demonstrated. Upon heating at similar to 5 GPa, face-centered cubic (fcc) Na3NiH5 forms above 430 degrees C. Upon cooling, it undergoes a rapid and reversible phase transition at 330 degrees C to an orthorhombic (Cmcm) form. Upon pressure release, Na3NiH5 further transforms into its recoverable Pnma form whose structure was elucidated from synchrotron powder diffraction data, aided by first-principles density functional theory (DFT) calculations. Na3NiH5 features previously unknown square pyramidal 18- electron complexes NiH53-. In the high temperature fcc form, metal atoms are arranged as in the Heusler structure, and ab initio molecular dynamics simulations suggest that the complexes are dynamically disordered. The Heusler-type metal partial structure is essentially maintained in the low temperature Cmcm form, in which NiH53- complexes are ordered. It is considerably rearranged in the low pressure Pnma form. Experiments at 10 GPa showed an initial formation of fcc Na3NiH5 followed by the addition of the perovskite hydride NaNiH3, in which Ni(II) attains an octahedral environment by H atoms. NaNiH3 is recoverable at ambient pressures and represents the sole product of 12 GPa experiments. DFT calculations show that the decomposition of Na3NiH5 = NaNiH3 + 2 NaH is enthalpically favored at all pressures, suggesting that Na3NiH5 is metastable and its formation is kinetically favored. Ni-H bonding in metallic NaNiH3 is considered covalent, as in electron precise Na3NiH5, but delocalized in the polyanion [NiH3](-).Funding Agencies|Swedish Research Council (VR)Swedish Research Council [2019-05551]; Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at at Linkoping University (Faculty Grant SFO-Mat-LiU) [200900971]; Carl Tryggers Stiftelse (CTS) [16:198, 17:206]</p
Low viscosity of the Earth’s inner core
The Earth’s solid inner core is a highly attenuating medium. It consists mainly of iron. The high attenuation of sound wave propagation in the inner core is at odds with the widely accepted paradigm of hexagonal close-packed phase stability under inner core conditions, because sound waves propagate through the hexagonal iron without energy dissipation. Here we show by first-principles molecular dynamics that the body-centered cubic phase of iron, recently demonstrated to be thermodynamically stable under the inner core conditions, is considerably less elastic than the hexagonal phase. Being a crystalline phase, the bodycentered cubic phase of iron possesses the viscosity close to that of a liquid iron. The high attenuation of sound in the inner core is due to the unique diffusion characteristic of the body-centered cubic phase. The low viscosity of iron in the inner core enables the convection and resolves a number of controversies
Phonon thermal conductivity of scandium nitride for thermoelectrics from first-principles calculations and thin-film growth
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