Calculation of anomalous phonons and the hcp-bcc phase transition in zirconium

Pinsook, Udomsilp; Ackland, Graeme

doi:10.1103/physrevb.59.13642

Cited by 51 publications

(33 citation statements)

References 30 publications

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“…All of this has been applied in classical MD by numerous authors. 8 We now consider applying exactly the same process to an anharmonic MD. Modes with strong Raman/IR signals will still have strong Raman signals, since the polarizability ultimately depends on the motion of the atoms.…”

Section: A Finite Temperature Phononsmentioning

confidence: 99%

“…One solution to this is to extract vibrational frequencies from molecular dynamics (MD) data. [7][8][9][10] For simple structures this is relatively straightforward, bcc titanium and zirconium being nice examples. In these materials the soft T 1N phonon eigenvector is well defined, and its frequency and width can be calculated from projection of the MD (or Monte Carlo) trajectories onto the relevant mode eigenvector, followed by Fourier transformation (FT).…”

Section: Introductionmentioning

confidence: 99%

“…In these materials the soft T 1N phonon eigenvector is well defined, and its frequency and width can be calculated from projection of the MD (or Monte Carlo) trajectories onto the relevant mode eigenvector, followed by Fourier transformation (FT). 8,11 For molecular systems IR spectra can be calculated from dipole contributions from each molecule. 12 Direct calculation of Raman spectra requires second-derivative (polarizability) calculations, which are still more challenging.…”

Section: Introductionmentioning

confidence: 99%

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Identification of high-pressure phases III and IV in hydrogen: Simulating Raman spectra using molecular dynamics

2013

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We present a technique for extracting Raman intensities from ab initio molecular dynamics (MD) simulations at high temperature. The method is applied to the highly anharmonic case of dense hydrogen up to 500 K for pressures ranging from 180 to 300 GPa. On heating or pressurizing we find first-order phase transitions under the experimental conditions of the phase III-IV boundary. At even higher pressures, close to 350 GPa, we find a second phase transformation to the previously proposed Cmca-4. Our method enables, for the first time, a direct comparison of Raman vibrons between theory and experiment at finite temperature. This turns out to provide excellent discrimination between subtly different structures found in MD. We find candidate structures whose Raman spectra are in good agreement with experiment. The new phase obtained in hightemperature simulations adopts a dynamic, simple hexagonal structure with three layer types: freely rotating hydrogen molecules, static hexagonal trimers, and rotating hexagonal trimers. We show that previously calculated structures for phase IV are inconsistent with experiment, and their appearance in simulation is due to finite-size effects.

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Section: A Finite Temperature Phononsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Identification of high-pressure phases III and IV in hydrogen: Simulating Raman spectra using molecular dynamics

2013

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“…In particular, a cooperative kinetics of the transformation resulting in the formation of a coherent twin system was discussed for the cases of alloy NiAl [16] and elemental Zr [17,18].…”

Section: Introductionmentioning

confidence: 99%

Effect of magnetism on kinetics of γ–α transformation and pattern formation in iron

Razumov¹,

Gornostyrev²,

Katsnelson³

2013

J. Phys.: Condens. Matter

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Abstract. Kinetics of polymorphous  - transformation in Fe is studied numerically within a model taking into account both lattice and magnetic degrees of freedom, based on first-principle calculations of the total energy for different magnetic states. It is shown that magnetoelastic phenomena, namely, a strong sensitivity of the potential relief along the Bain deformation path to the magnetic state, are crucial for the picture of the transformation. With the temperature increase, a scenario of the phase transformation evolves from a homogeneous lattice instability at T < M S (M S is the temperature of the beginning of the martensitic transformation) to the growth and nucleation of embryos of the new phase at T > M S . In the latter case, a stage of formation of a tweed-like structure occurs, with a strong short-range order and slow interphase fluctuations.MSC: 74N10, 74N15, 82B26, 82B80, 82C26, 37K60

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“…Previous methods to obtain high-temperature phonons are based on the idea that the phonon eigenvectors are temperature independent 17,18 . However, Raman intensities are related to atomic motions, so in the classical approximation they can be extracted directly from linear combinations without explicitly evaluating eigenmodes.…”

Section: Extracting Phonons From MDmentioning

confidence: 99%

Efficacious calculation of Raman spectra in high pressure hydrogen

Ackland

Magdău

2014

High Pressure Research

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We present and evaluate an efficient method for simulating Raman spectra from molecular dynamics (MD) calculations without defining normal modes. We apply the method to high pressure hydrogen in the high-temperature "Phase IV": a plastic crystal in which the conventional picture of fixed phonon eigenmodes breaks down. Projecting trajectories onto in-phase molecular stretches is shown to be many orders of magnitude faster than polarisability calculations, allowing statistical averaging at high-temperature. The simulations are extended into metastable regimes and identify several regimes associated with symmetry-breaking on different timescales, which are shown to exhibit features in the Raman spectra at the current experimental limit of resolvability. In this paper we have concentrated on the methodology, a fuller description of the structure of Phase IV hydrogen is given in a previous paper 1 .

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Calculation of anomalous phonons and the hcp-bcc phase transition in zirconium

Cited by 51 publications

References 30 publications

Identification of high-pressure phases III and IV in hydrogen: Simulating Raman spectra using molecular dynamics

Identification of high-pressure phases III and IV in hydrogen: Simulating Raman spectra using molecular dynamics

Effect of magnetism on kinetics of γ–α transformation and pattern formation in iron

Efficacious calculation of Raman spectra in high pressure hydrogen

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