Lattice dynamics and melting features of Li and Na

Lepeshkin, Sergey; Magnitskaya, M. V.; Maksimov, Evgenii G.

doi:10.1134/s0021364009110137

Cited by 22 publications

(16 citation statements)

References 22 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In this respect Li behaves similarly to Na, where it has been shown experimentally [12] that the melting line of the bcc phase attains a maximum at $31 GPa with a value close to 1000 K, and then decreases reaching values close to room temperature at about 100 GPa. Several studies have attempted to uncover the cause of the negative slope in the melting line of Na, some arguing for the higher compressibility of the liquid as the explanation [20,35], while others have drawn attention to a softening of phonon modes with increasing pressure, resulting in a relative destabilization of the solid phase [36][37][38]. We note that these explanations are not mutually exclusive, and it is likely that both contribute to the observed negative slope in the melting line of sodium.…”

Section: Prl 104 185701 (2010) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 85%

First-Principles Simulations of Lithium Melting: Stability of the bcc Phase Close to Melting

et al. 2010

View full text Add to dashboard Cite

We report large-scale first-principles simulations of melting of four different phases of Li at pressures ranging from 0 to 50 GPa. We find excellent agreement with existing experimental data at low pressures, and confirm that above 10 GPa the melting line develops a negative slope, in parallel to what occurs for Na at 30 GPa. Surprisingly, our results indicate that the melting temperature of the bcc phase is always higher than that of fcc Li, suggesting the intriguing possibility of the existence of a narrow field of bcc stability separating the fcc and liquid phases, as predicted by Alexander and McTague [Phys. Rev. Lett. 41, 702 (1978)]. DOI: 10.1103/PhysRevLett.104.185701 PACS numbers: 64.70.dj, 62.50.Àp, 65.20.Àw, 65.40.Àb The alkali elements Li and Na have long been described as simple metals, due to their prototypical atomic and electronic structure. At ambient conditions they adopt the bcc lattice arrangement, transforming into the fcc at higher pressure (at $7:5 GPa and room temperature in the case of Li, at 65 GPa in the case of Na), while their electronic structure is nearly free electronlike. However, recent theoretical and experimental studies have shown that these materials display unexpectedly complex behavior at high pressures. Neaton and Ashcroft [1,2] reported ab initio calculations predicting that pressure induces phase transitions to less symmetric, lower coordinated structures, associated to electronic localization [3,4], a rather counterintuitive prediction. Shortly after, Hanfland et al.[5] reported an experimental study of Li at high pressures, and although the actual sequence of high-pressure phases found by these authors was different than that predicted theoretically [1], they nevertheless confirmed the tendency to adopt low symmetry phases at high pressures. Other experimental efforts have shown that these high-pressure structural transformations are accompanied by changes in the electronic structure. Indeed, Struzhkin et al. [6] and Shimizu and coworkers [7] have reported the observation of superconductivity in Li at pressures above 20 GPa (at ambient pressure T c ¼ 0:4 mK [8]), with transition temperatures varying with pressure, but ranging between 9 and up to 20 K, one of the highest measured for any element. Matsuoka and Shimizu [9] have reported a metal-tosemiconductor transition in Li near 80 GPa of pressure. Similar results have been reported for Na at 118 GPa, where Na is found to transform to an orthorhombic phase behaving as a poor metal [10]. At even higher pressures (c.a. 200 GPa) Ma and coworkers [11] have reported the observation of a sixfold coordinated hexagonal structure of Na which happens to be transparent, implying an insulator band gap in the electronic structure.In an effort to further characterize the thermodynamic behavior of alkali metals at high pressures, Gregoryanz et al. [12] have measured the melting temperature of Na up to pressures of 130 GPa. It was found that its melting curve reaches a maximum at $31 GPa, and decreases at higher pressures. The n...

show abstract

Section: Prl 104 185701 (2010) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 85%

First-Principles Simulations of Lithium Melting: Stability of the bcc Phase Close to Melting

et al. 2010

View full text Add to dashboard Cite

show abstract

“…Although previous ab initio studies [5][6][7][8][9][10] have reproduced the reentrant melting behaviour of sodium the physical origin of this anomaly still remains unclear. The hypothesis of structural and electronic transitions in liquid sodium proposed in one of the studies 6 has not been confirmed by other authors 5 .…”

mentioning

confidence: 98%

“…One of the most puzzling phenomena in the behaviour of dense sodium is the pressureinduced drop in its melting temperature, which extends from 1000 K at ∼30 GPa to as low as room temperature at ∼120 GPa 1 . Despite significant theoretical effort to understand the anomalous melting [5][6][7][8][9][10] its origins have remained unclear. In this work, we reconstruct the sodium phase diagram using an ab-initio-quality neural-network potential 11,12 .…”

mentioning

confidence: 99%

See 1 more Smart Citation

Microscopic Origins of the Anomalous Melting Behavior of Sodium under High Pressure

et al. 2012

View full text Add to dashboard Cite

Recent experiments have shown that sodium, a prototype simple metal at ambient conditions, exhibits unexpected complexity under high pressure [1][2][3][4] . One of the most puzzling phenomena in the behaviour of dense sodium is the pressureinduced drop in its melting temperature, which extends from 1000 K at ∼30 GPa to as low as room temperature at ∼120 GPa 1 . Despite significant theoretical effort to understand the anomalous melting 5-10 its origins have remained unclear. In this work, we reconstruct the sodium phase diagram using an ab-initio-quality neural-network potential 11,12 . We demonstrate that the reentrant behaviour results from the screening of interionic interactions by conduction electrons, which at high pressure induces a softening in the shortrange repulsion. It is expected that such an effect plays an important role in governing the behaviour of a wide range of metals and alloys.At ambient conditions, sodium crystallizes in the bodycentered cubic (bcc) structure, which transforms to the face-centered cubic (fcc) phase upon compression to 65 GPa 13 . At 105 GPa, the fcc phase undergoes a transformation to the cI16 phase -a distorted variant of the bcc structure 2 . Further compression results in the formation of a large variety of complex phases 3,4 , the existence of which is quite intriguing. However, the focus of this work is one of the most striking features of the sodium phase diagram -the anomalous melting curve. Diffraction measurements have revealed a maximum in melting temperature around 1000 K at ∼30 GPa followed by a pressure-induced drop, which extends to nearly room temperature at ∼120 GPa 1 and spans the regions of stability of three solid phases (Fig. 1a). This anomaly implies that in this range the liquid is denser than the solid.Although previous ab initio studies 5-10 have reproduced the reentrant melting behaviour of sodium the physical origin of this anomaly still remains unclear. The hypothesis of structural and electronic transitions in liquid sodium proposed in one of the studies 6 has not been confirmed by other authors 5 . The discrepancy between these simulation results has been attributed to a different treatment of the semicore states 8 .It is important to point out that due to the high computational cost of ab initio MD all simulations so far re- ported 5-8 have used small simulation cells and estimated melting temperatures (T m ) by heating the solid phase until it melts. In such simulations, the absence of nucleation centers and fast heating schedules delay melting relative to the thermodynamic coexistence point and, therefore, only an upper bound on T m can be obtained. A proper reconstruction of the melting curve requires comparing the free energies of the liquid and solid phases and involves long (∼10 ns) MD simulations on large systems (∼ 10 3 atoms). Performing such simulations with direct ab initio methods is computationally too expensive at present.

show abstract

“…For lithium, on the other hand, because of the difficulties in containing the sample under high pressure, the knowledge of its melting curve has long been confined to be less than 8 GPa 8,9 until a recent differential thermal analysis (DTA) measurement 10 , which extended the melting line of Li up to 15 GPa and reported a maximum at about 10 GPa. Theoretically, a Lindermann model curve of Li was calculated 7 to give an obvious discontinuity near the bcc-fcc-liquid triple point, which was not supported by experimental data 10 . More directly, Tamblyn et al 11 and Hernádez et al 12 have from first-principles molecular dynamics (FPMD) simulations determined the melting temperature of Li over a broad pressure range.…”

Section: Introductionmentioning

confidence: 84%

Quantum molecular dynamics simulations of lithium melting using Z-method

Li¹,

Zhang²,

Yan³

2010

Preprint

View full text Add to dashboard Cite

We performed first-principles molecular dynamics calculations for lithium using the projector augmented waves method and the generalized gradient approximation as exchange-correlation energy. The melting curve of lithium was computed using the Z -method technique for pressures up to 30 GPa, which agrees well with the experimental and two-phase simulated results. The change of the melting line slope from positive to negative was predicted by the characteristic shape inversion of the Z curve at about 8.2 GPa. Through analyzing the static properties, we conclude that no liquid-liquid phase transition accompanies the occurrence of the melting line maximum, which is caused by the higher compressibility of the liquid phase compared to the solid phase. In addition, we systematically studied the dynamic and optical properties of lithium near melting curve at critical superheating and melting temperatures. It was suggested that spectra difference at critical superheating and melting temperature may be able to diagnose the homogeneous melting.

show abstract

Lattice dynamics and melting features of Li and Na

Cited by 22 publications

References 22 publications

First-Principles Simulations of Lithium Melting: Stability of the bcc Phase Close to Melting

First-Principles Simulations of Lithium Melting: Stability of the bcc Phase Close to Melting

Microscopic Origins of the Anomalous Melting Behavior of Sodium under High Pressure

Quantum molecular dynamics simulations of lithium melting using Z-method

Contact Info

Product

Resources

About