Molecular dynamics simulations of the melting curve of tantalum under pressure

Liu, Zhongli; Cai, Ling-Cang; Chen, Xiangrong; Jing, Fuqian

doi:10.1103/physrevb.77.024103

Cited by 70 publications

(57 citation statements)

References 65 publications

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“…Collectively, the Ta melt curves presented in Ta: Large-cell two-phase and slope, T m and dT m / dP , at P = 0 are 3222 K and 51 K/GPa, respectively -in good agreement with experimental measurements of 3270 K [7,11] and ~ 59 K/GPa [11] and also in accord with other recent theoretical results [18][19][20]. The calculated T m at the observed shock melt pressure of 295 GPa [15] is 10000 K and close to the value recently measured by pyrometry of ~ 9700 K. As a further point of interest in Fig.…”

Section: High-temperature Anharmonicitysupporting

confidence: 78%

Polymorphism and melt in high-pressure tantalum

2012

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Recent small-cell (< 150-atom) quantum molecular dynamics (QMD) simulations for Ta based on density functional theory (DFT) have predicted a hexagonal omega (hex-ω) phase more stable than the normal bcc phase at high temperature (T) and pressure (P) above 70 GPa [Burakovsky et al., Phys. Rev. Lett. 104, 255702 (2010)]. Here we examine possible high-T , P polymorphism in Ta with complementary DFT-based model generalized pseudopotential theory (MGPT) multi-ion interatomic potentials, which allow accurate treatment of much larger system sizes (up to ~ 80000 atoms). We focus on candidate bcc, A15, fcc, hcp, and hex-ω phases for the high-T , P phase diagram to 420 GPa, studying the mechanical and relative thermodynamic stability of these phases for both small and large computational cells. Our MGPT potentials fully capture the T = 0 DFT energetics of these phases, while MGPT-MD simulations demonstrate that the higher-energy fcc, hcp and hex-ω structures are only mechanically stabilized at high temperature by large, size-dependent, anharmonic vibrational effects, with the stability of the hex-ω phase also being found to be a sensitive function of its c / a ratio. Both twophase and Z-method melting techniques have been used in MGPT-MD simulations to determine relative phase stability and its size dependence. In the large-cell limit, the twophase method yields accurate equilibrium melt curves for all five phases, with bcc producing the highest melt temperatures at all pressures and hence being the most stable phase of those considered. The two-phase bcc melt curve is also in good agreement with dynamic experimental data as well as with the MGPT melt curve calculated from bcc and 2 liquid free energies. In contrast, we find that the Z method produces only an upper bound to the equilibrium melt curve in the large-cell limit. For the bcc and hex-ω structures, however, this is a close upper bound within 5% of the two-phase results, although for the A15, fcc, and hcp structures, the Z-melt curves are 25-35% higher in temperature than the two-phase results. Nonetheless, the Z method has allowed us to study melt size effects in detail. We find these effects to be either small or modest for the cubic bcc, A15, and fcc structures, but to have a large impact on the hexagonal hcp and hex-ω melt curves, which are dramatically pushed above that of bcc for simulation cells less than 150 atoms. The melt size effects are driven by and closely correlated with similar size effects on the mechanical stability and the vibrational anharmonicity. We further show that for the same simulation cell sizes and choice of c / a ratio, the MGPT-MD bcc and hex-ω melt curves are in good agreement with the QMD results, so the QMD prediction is confirmed in the small-cell limit. But in the large-cell limit, the MGPT-MD hex-ω melt curve is always lowered below that of bcc for any choice of c / a , so bcc is the most stable phase.We conclude that for the non-bcc Ta phases studied, one requires simulation cells of at least 250-500 atoms to be free ...

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Section: High-temperature Anharmonicitysupporting

confidence: 78%

Polymorphism and melt in high-pressure tantalum

2012

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“…The present experiments confirm that as pressure increases the melting slope of Ta decreases. Molecular dynamics (MD) (Liu et al, 2008), first-principles (FP) (Taioli et al, 2007), and quantum-based (QB) atomistic (Moriarty et al, 2002) calculations give, above 10 GPa, consistently a higher M T than the DAC experiments.…”

Section: Page 5 Of 16mentioning

confidence: 99%

“…In particular, the melting of the late transition metal iron at high pressures has wide scientific implications, including geophysical and geochemical modeling of the Earth's interior. This fact has triggered a substantial effort to determine the melting curve of Fe (for a review see: Boehler, R. et al, 2007) and other transition metals including Mo, Ta, and W (Belonoshko et al, 2008;Cazorla et al, 2008;Dai et al, 2009;Errandonea et al, 2001;Errandonea et al, 2003;Foata-Prestavoine et al, 2007;Hixon et al, 1989;Liu et al, 2008;Moriarty et al, 2002;Santamaría-Pérez et al, 2009;Taioli et al, 2007;Zhang et al, 2009). However, despite important advances in experimental and theoretical methods, considerable controversy exists about the pressure dependence of the melting temperatures ( M T ) of Mo, Ta, and W.…”

Section: Introductionmentioning

confidence: 99%

Microscopic evidence of a flat melting curve of tantalum

Ruiz‐Fuertes

Karandikar

Boehler

et al. 2010

Physics of the Earth and Planetary Interiors

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New data on the high-pressure melting curve of Ta up to 48 GPa are reported.Evidence of melting from changes in sample texture was found in five different experiments using scanning electron microscopy. The obtained melting temperatures are in excellent agreement with earlier measurements using x-ray diffraction or the laser-speckled method but are in contrast with several theoretical calculations. The results are also compared with shock-wave data.These findings are of geophysical relevance because they confirm the validity of earlier experimental techniques that resulted in low melting slopes of the transition metals measured in the diamond-anvil cell, including iron.

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“…The melting temperature of Nb at ambient pressure is 2750.6 K. 51 Compression to 75 GPa would increase it to about 4600 K, if estimated using the slope of the melting curve of Ta. 52 The variation of C44 as a function of temperature at 75 GPa is shown in the inset of Above calculation did not include phonon contribution. It is well known that the elastic constants can be affected by lattice vibrations, and thermal motion of ions is a common mechanism for material softening at high enough temperatures.…”

Section: Anomalies Contributed From Thermo-electronsmentioning

confidence: 99%

First-principles investigation of elastic anomalies in niobium at high pressure and temperature

Wang

Geng

et al. 2017

Journal of Applied Physics

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Niobium does not show any structure transition up to very high pressures. Nonetheless, by using density functional theory, we demonstrate in this work that it exhibits striking softening in elastic moduli C44 and C' at a pressure from 20 to 150 GPa. A novel anomaly softening in C44 from 275 to 400 GPa is also predicted. The physics behind these two anomalies is elaborated by electronic structure calculations, which revealed that they are actually different, with the first one directly relates to an underlying rhombohedral distortion whereas the latter originates in an electronic topological transition. The large magnitude of the softening leads to a remarkable elastic anisotropy in both the shear and the Young's moduli of Nb. Further investigation shows that thermo-electrons have an important role on these anomalies. This effect has not been noticed before. With increased electronic temperature, it is found that all anomalies (both the elastic softening and anisotropy) in Nb are gradually diminished, effectively giving rise to a temperature-induce hardening phenomenon.

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Molecular dynamics simulations of the melting curve of tantalum under pressure

Cited by 70 publications

References 65 publications

Polymorphism and melt in high-pressure tantalum

Polymorphism and melt in high-pressure tantalum

Microscopic evidence of a flat melting curve of tantalum

First-principles investigation of elastic anomalies in niobium at high pressure and temperature

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