First principles force field for metallic tantalum

Strachan, Alejandro; Çağın, Tahir; Gülseren, Oğuz; Mukherjee, Sonali; Cohen, R. E.; Goddard, William A.

doi:10.1088/0965-0393/12/4/s10

Cited by 37 publications

(30 citation statements)

References 61 publications

(122 reference statements)

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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: 90%

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: 90%

Polymorphism and melt in high-pressure tantalum

2012

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“…6b, the present full-theory result is in good accord with the measured data of Touloukian et al [47] up to about 400 K and still in reasonable agreement to 1000 K, again capturing the observed behavior in both the quantum and classical temperature regimes. Above 1000 K, however, the data has large error bars and begins to diverge from the theory, and only provisional values of thermal expansion were measured above 2000 K. In a similar manner, the steep rise in the Touloukian et al data above 2000 K was found to move rapidly above the corresponding DFT-based result of Cohen and Gülseren [42] as well as the EAM-MD calculations of β in Ta by Strachan et al [48].…”

Section: B Qhld MD and Vpt Implementation Of Mgpt Potentialsmentioning

confidence: 72%

Efficient wide-range calculation of free energies in solids and liquids using reversible-scaling molecular dynamics

Moriarty

Haskins

2014

Phys. Rev. B

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We elaborate a novel and efficient method to obtain multiphase Helmholtz free energies from molecular dynamics (MD) simulations over wide ranges of volume and temperature in materials that can be described by temperature-independent ion forces, with both higher accuracy and order-of-magnitude cost savings compared to direct thermodynamicintegration techniques. Our method leverages and significantly extends the technique of reversible scaling molecular dynamics (RSMD) proposed by de Koning et al. [Phys. Rev. Lett. 83, 3973 (1999)], which allows a free-energy difference in a given phase at constant volume to be calculated as a function of temperature from a single MD simulation. In mechanically stable solid phases, our approach carefully combines quasiharmonic lattice dynamics at low temperatures with an accurate and fully isolated RSMD simulation of the anharmonic vibrational free energy at high temperatures to produce a seamless free energy from zero temperature to above melt along constant-volume isochores. In the liquid, we combine a unique calculation of the free energy along a high-temperature reference isotherm with isochoric RSMD simulations from that temperature to below melt. In metastable solid phases that are mechanically unstable at low temperature, we use two-phase MD melt simulations together with the liquid free energy to obtain the solid free energy along the solidus melt line and then perform isochoric RSMD simulations to temperatures above and below that point. While our free-energy method is general, we have specifically adapted it here to the case of metals in which the ion forces are well described by model generalized pseudopotential theory (MGPT) multi-ion interatomic potentials, and additive electron-thermal free-energy contributions can be 2 included. Then using refined Ta6.8x MGPT potentials, we have converged total free energies and their components to very high and unprecedented sub−milli-Rydberg (mRy) numerical accuracy in the stable-bcc, liquid, and metastable-fcc phases of tantalum for volumes ranging from up to 26% expansion to nearly two-fold compression and for temperatures to 25,000 K. In turn, we have successfully used the free energies so obtained to calculate physically accurate thermodynamic properties and gain new insight into their behavior, including sensitive thermodynamic derivatives, bcc and fcc melt curves, and a multiphase equation of state for Ta over the same temperature range and for pressures as high as 600 GPa. We show that the anharmonic free-energy component in the bcc solid, although only 1-5 mRy in magnitude for Ta, can have a significant (15-20%) effect on thermal expansivity, the Grüneisen parameter, and melt temperatures. We further show that the electron-thermal free-energy component can similarly impact the specific heat and thermal expansivity in both the solid and the liquid, while only minimally affecting (to ≤ 3%) the bcc and fcc melt curves.

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“…Two EAM potentials are of particular interest: one by R. Ravelo et al (this proceedings) simply denoted as EAM, and the other by Strachan et al denoted as qEAM [1]. We show that both potentials reproduce the experimental Hugoniots, and reveal the anisotropic deformation and damage processes in single crystal Ta.…”

Section: Introductionmentioning

confidence: 68%

“…The Ta EAM potential adopts the standard EAM form (R. Ravelo et al, this proceedings), while for the qEAM potential, the electron density depends on the local volume or mass density [1]. Such volume dependence may be a concern for the later stage of spallation (large voids), so we choose not to perform direct spall simulations with qEAM.…”

Section: Methodsmentioning

confidence: 99%