Dynamic stabilization of cubic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Ca</mml:mi><mml:mi>Si</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>perovskite at high temperatures and pressures from<i>ab initio</i>molecular dynamics

Sun, Tao; Zhang, Dong-Bo; Wentzcovitch, Renata M.

doi:10.1103/physrevb.89.094109

Cited by 94 publications

(109 citation statements)

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“…Similarly, all other quasiparticle mode frequencies sampled by the 4x4x4 supercell are equally well defined. As previously indicated [30,31], these renormalized phonon frequencies plus the normal modes enable the calculation of the renormalized force constant matrix and complete phonon dispersions. This quantitative characterization of phonon quasiparticles and renormalized phonon dispersion provide a solid foundation for studying thermal properties.…”

Section: 83åmentioning

confidence: 83%

“…At P > 11 GPa, bcc Be is dynamically stabilized by pressure and the QHA might, in principle, be applied. However, the hcp/bcc boundary [23][24][25] predicted by the QHA does not agree with experiments [13], suggesting that anharmonic effects still play an important role at higher pressures.In this Letter, we report a new investigation of the phase stability of bcc Be and the associated hcp → bcc phase transition boundary up to 30 GPa and temperatures up to 2,000 K. We have used a recently developed hybrid approach [30,31] that combines first-principles molecular dynamics (MD) and lattice dynamics calculations to address anharmonic effects in the free energy. In this method, the concept of phonon quasiparticles offers a quantitative characterization of the effects of lattice anharmonicity [32,33].…”

mentioning

confidence: 99%

“…In this Letter, we report a new investigation of the phase stability of bcc Be and the associated hcp → bcc phase transition boundary up to 30 GPa and temperatures up to 2,000 K. We have used a recently developed hybrid approach [30,31] that combines first-principles molecular dynamics (MD) and lattice dynamics calculations to address anharmonic effects in the free energy. In this method, the concept of phonon quasiparticles offers a quantitative characterization of the effects of lattice anharmonicity [32,33].…”

mentioning

confidence: 99%

“…In the present approach, phonon quasiparticles are numerically characterized by the mode projected velocity autocorrelation function [30,31],…”

mentioning

confidence: 99%

“…should have a Lorentzian-type line shape [30,31]. The renormalized phonon frequency ω q,s , and the linewidth, Γ q,s can then be obtained as discussed in more details in the Supplemental Materials.…”

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confidence: 99%

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Premelting hcp to bcc Transition in Beryllium

Sun

Zhang

et al. 2017

Phys. Rev. Lett.

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Beryllium (Be) is an important material with wide applications ranging from aerospace components to X-ray equipments. Yet a precise understanding of its phase diagram remains elusive. We have investigated the phase stability of Be using a recently developed hybrid free energy computation method that accounts for anharmonic effects by invoking phonon quasiparticles. We find that the hcp → bcc transition occurs near the melting curve at 0 < P < 11 GPa with a positive Clapeyron slope of 41 ± 4 K/GPa. The bcc phase exists in a narrow temperature range that shrinks with increasing pressure, explaining the difficulty in observing this phase experimentally. This work also demonstrates the validity of this theoretical framework based on phonon quasiparticle to study structural stability and phase transitions in strongly anharmonic materials.PACS numbers: 63.20. Ry, 81.30.Bx, 61.50.Ks,Elemental solids usually undergo a series of phase transitions from ambient conditions to extreme conditions [1][2][3]. Knowledge of their phase diagrams is a prerequisite for establishing their equations of state (EOS), a fundamental relation for determining thermodynamics properties and processes at high pressures and temperatures (PT). However, resolving phase boundaries is challenging for experiments given the uncertainties from several sources, especially at very high PT. Beryllium (Be) is a typical system whose phase diagram remains an open problem despite intense investigations. It assumes a hexagonal close-packed (hcp) structure at relatively low T [1]. Near the melting temperature T M (∼ 1, 550 K at 0 GPa), a competing phase with the body-centered cubic symmetry (bcc) seems to emerge [4][5][6]. However, not all experiments [7][8][9][10][11][12][13] have observed this bcc phase, causing confusion and controversies. Be is important for both fundamental research [14][15][16][17] and practical applications. Being a strong and light-weight metal, it has been widely used in a broad range of technological applications in harsh environments and extreme PT conditions, e.g., up to T > 4,000 K and P > 200 GPa [18][19][20][21][22].The bcc phase of Be was directly observed [4,5] only at T > 1, 500 K around ambient pressure before melting. Measurements of the temperature dependent resistance suggested that bcc Be is a high pressure phase and the hcp/bcc phase boundary between 0 < P < 6 GPa has a negative Clapeyron slope (−52 ± 8 K/GPa) [6]. However, recent experiments have challenged this conclusion [8,9,[11][12][13]. For example, it was reported that bcc Be was not observed for 8 < P < 205 GPa and 300 < T < 4, 000 K [13]. On the theory side, the study of Be's phase diagram using conventional methods encounters significant difficulties. The lattice dynamics of bcc Be is highly anharmonic, and the widely used quasi-harmonic approximation (QHA) and Debye model are not able to capture such effect [23][24][25][26][27][28][29]. For this reason, bcc Be and the associated hcp/bcc phase transition remain poorly understood for P < 11 GPa (density < 2.1g/...

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Section: 83åmentioning

confidence: 83%

mentioning

confidence: 99%

mentioning

confidence: 99%

“…In the present approach, phonon quasiparticles are numerically characterized by the mode projected velocity autocorrelation function [30,31],…”

mentioning

confidence: 99%

mentioning

confidence: 99%

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Premelting hcp to bcc Transition in Beryllium

Sun

Zhang

et al. 2017

Phys. Rev. Lett.

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P‐V‐T equation of state of cubic CaSiO₃ perovskite from first‐principles computation

Kawai

Tsuchiya

2014

JGR Solid Earth

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Ca-perovskite (Pv) is considered to be one of the most abundant minerals in the Earth's lower mantle (LM) with an ideal cubic structure at LM pressures and temperatures. In this study, a pressure-volume-temperature (P-V-T) equation of state model for Ca-Pv is constructed using density functional first-principles molecular dynamics simulations. The calculated P-V-T data yield K /formula unit, γ 0 = 1.576, and q = 0.96 within the framework of the Mie-Grüneisen-Debye formulation. We compare the density and bulk sound velocity of Ca-Pv with those of iron-bearing Mg-Pv and seismological values. Along an adiabatic temperature gradient, Ca-Pv has~2.5% higher density and~0.7% faster bulk sound velocity than the preliminary reference Earth model, while it has~3.8% higher density and~2.7% slower bulk sound velocity than iron-bearing Mg-Pv. Our results indicate that a possible lateral variation in the Ca-Pv fraction in the LM could produce an anticorrelation between V Φ and ρ.

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Liquid iron‐sulfur alloys at outer core conditions by first‐principles calculations

Umemoto

Hirose

Imada

et al. 2014

Geophysical Research Letters

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We perform first-principles calculations to investigate liquid iron-sulfur alloys (Fe, Fe 56 S 8 , Fe 52 S 12 , and Fe 48 S 16 ) under high-pressure and high-temperature (150-300 GPa and 4000-6000 K) conditions corresponding to the Earth's outer core. Considering only the density profile, the best match with the preliminary reference Earth model is by liquid Fe-14 wt % S (Fe 50 S 14 ), assuming sulfur is the only light element. However, its bulk sound velocity is too high, in particular in the deep outer core, suggesting that another light component such as oxygen is required. An experimental check using inelastic X-ray scattering shows good agreement with the calculations. In addition, a present study demonstrates that the Birch's law does not hold for liquid iron-sulfur alloy, consistent with a previous report on pure liquid iron.

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Dynamic stabilization of cubicCaSiO3perovskite at high temperatures and pressures fromab initiomolecular dynamics

Cited by 94 publications

References 46 publications

Premelting hcp to bcc Transition in Beryllium

Premelting hcp to bcc Transition in Beryllium

P‐V‐T equation of state of cubic CaSiO₃ perovskite from first‐principles computation

Liquid iron‐sulfur alloys at outer core conditions by first‐principles calculations

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Dynamic stabilization of cubicCaSiO3perovskite at high temperatures and pressures fromab initiomolecular dynamics

Cited by 94 publications

References 46 publications

Premelting hcp to bcc Transition in Beryllium

Premelting hcp to bcc Transition in Beryllium

P‐V‐T equation of state of cubic CaSiO3 perovskite from first‐principles computation

Liquid iron‐sulfur alloys at outer core conditions by first‐principles calculations

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Product

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P‐V‐T equation of state of cubic CaSiO₃ perovskite from first‐principles computation