X-ray diffraction measurements of Mo melting to 119 GPa and the high pressure phase diagram

Santamarı́a-Pérez, D.; Ross, Marvin; Errandonea, Daniel; Mukherjee, Goutam Dev; Mézouar, M.; Boehler, R.

doi:10.1063/1.3082030

Cited by 85 publications

(95 citation statements)

References 46 publications

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“…As a result, there is currently an ongoing experimental effort to repeat these measurements more accurately [49]. The correct interpretation of the flat DAC melting curves in bcc metals [7][8][9] is also an open question.…”

Section: B Z-methods Melt Curves For Small and Large Cellsmentioning

confidence: 99%

“…More recently, research interest in high-temperature polymorphism in the bcc transition metals has re-emerged in connection with unexpected results from static high-pressure melting experiments, where laser-heated diamond-anvil-cell (DAC) measurements on V [7], Ta [7,8], Mo [7,9] and W [7] have indicated only small increases in the melting temperature T m with increasing melting pressure P m up to 100 GPa. These flat T m (P m ) melt curves strongly conflict with the steep melt curves indicated by dynamic experimental data, namely, the large melting slopes dT m / dP m obtained for Nb [10], Ta demonstrated with first-principles DFT calculations, where the bcc structure is predicted to be the stable phase to at least 1000 GPa [22,32], while alternate A15, hex-ω, fcc, hcp and dhcp structures all have calculated imaginary quasi-harmonic phonon frequencies over this pressure range [22].…”

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confidence: 99%

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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: B Z-methods Melt Curves For Small and Large Cellsmentioning

confidence: 99%

mentioning

confidence: 99%

Polymorphism and melt in high-pressure tantalum

2012

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“…3). This melting criterion has been applied in many previous investigations to determine melting in a wide variety of substances [20][21][22] and it has been directly correlated with the appearance of liquid S(Q) during in situ x-ray diffraction experiments in the LH-DAC [14,20].…”

Section: B Thermal Signal Processingmentioning

confidence: 99%

“…Laserheating experiments and the determination of melting criteria from x-ray diffraction measurements are further discussed in Refs. [17][18][19][20]. Initial pressures were determined from the room temperature equation of state of NaCl before later being refined using the room temperature equation of state of Sn [5].…”

Section: A In Situ X-ray Diffractionmentioning

confidence: 99%

High-pressure melting behavior of tin up to 105 GPa

et al. 2017

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The melting curve of Sn initially rises steeply as a function of pressure but exhibits a decrease in slope (dT m /dP ) above 40 GPa to become nearly flat above 50 GPa. Previous studies have argued that a body-centered tetragonal (bct) to cubic (bcc) phase transition occurs in this range at room temperature. However, our investigations have shown that the phase behavior is more complex in this region with orthorhombic (bco) splitting of reflections occurring in the x-ray diffraction pattern above 32 GPa and coexisting diffraction signatures of bco and bcc structures are observed between 40 and 70 GPa. Here we have documented the simultaneous presence of bco and bcc reflections up to the melting point, negating the possibility that their coexistence might indicate a kinetically hindered first-order phase transformation. In this paper we have extended the observation of Sn melting relations into the megabar (P > 100 GPa) range using the appearance of liquid diffuse scattering in x-ray diffraction patterns and discontinuities during thermal signal processing to diagnose the occurrence of melting. Both techniques yield consistent results that indicate the melting line maintains the same low slope up to the highest pressure examined and does not flatten. The results below approximately 40 GPa agree well with the melting relations produced recently using a multiphase equation of state fitted to available or assumed data. Above this pressure the experimental melting points lie increasingly below the predicted crystal-liquid phase boundary, but above the flat melting from past studies, indicating that the thermodynamic properties of the body-centered "γ "-Sn structure remain to be clarified.

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“…In recent year its phase stability, equation of state, and melting behavior have been the subject of intensive investigation [1][2][3][4][5][6][7][8][9] . The equation of state of molybdenum has been well studied using static compression techniques [10][11][12][13][14] with recent work 15 demonstrating that Mo remains in the body centered cubic (BCC) phase until at least 410 GPa at room temperature.…”

Section: Introductionmentioning

confidence: 99%

X-ray diffraction of molybdenum under shock compression to 450 GPa

et al. 2015

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Molybdenum (Mo) is a body-centered-cubic (BCC) transition metal that has widespread technological applications. While the BCC transition elements are used as test cases for understanding the behavior of metals under extreme conditions, the melting curves and phase transitions of these elements have been the subject of stark disagreements in recent years. Here we use X-ray diffraction to examine the phase stability and melting behavior of Mo under shock loading to 450 GPa. The BCC phase of Mo remains stable along the Hugoniot until 380 GPa. Our results do not support previous claims of a shallow melting curve for molybdenum.

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X-ray diffraction measurements of Mo melting to 119 GPa and the high pressure phase diagram

Cited by 85 publications

References 46 publications

Polymorphism and melt in high-pressure tantalum

Polymorphism and melt in high-pressure tantalum

High-pressure melting behavior of tin up to 105 GPa

X-ray diffraction of molybdenum under shock compression to 450 GPa

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