Superionic State in Alumina Produced by Nonthermal Melting

Voronkov, R.; Medvedev, Nikita; Волков, А. Е.

doi:10.1002/pssr.201900641

Cited by 12 publications

(21 citation statements)

References 41 publications

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“…(see our previous work [25]), mixed-bonding crystals, the band gap collapses at doses high above their damage thresholds. Finally, in ionic NaCl, the band gap slightly shrinks but does not collapse at doses up to 7.2 eV/atom.…”

Section: Naclmentioning

confidence: 79%

Dependence of nonthermal metallization kinetics on bond ionicity of compounds

Voronkov

Medvedev

Волков

2020

Sci Rep

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it is known that covalently bonded materials undergo nonthermal structure transformations upon ultrafast excitation of an electronic system, whereas metals exhibit phonon hardening in the bulk. Here we study how ionic bonds react to electronic excitation. Density-functional molecular dynamics predicts that ionic crystals may melt nonthermally, however, into an electronically insulating state, in contrast to covalent materials. We demonstrate that the band gap behavior during nonthermal transitions depends on a bonding type: it is harder to collapse the band gap in more ionic compounds, which is illustrated by transformations in Y 2 o 3 vs. nacl, Lif and KBr.

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Section: Naclmentioning

confidence: 79%

Dependence of nonthermal metallization kinetics on bond ionicity of compounds

Voronkov

Medvedev

Волков

2020

Sci Rep

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“…This limit usually serves as a disordering structural criterion and the time evolution of the average atomic displacements is needed to unambiguously asses melting [19,20]. Bismuth Peierls-distorted crystal lattice [21,22] and mixed ionic-covalent bonded solids [23,24] are other types of crystalline solids concerned by non-thermal lattice softening.…”

Section: Introductionmentioning

confidence: 99%

Structural instability of transition metals upon ultrafast laser irradiation

et al. 2021

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Precision in laser material structuring is critically defined by the energy flow during the irradiation process, particularly in ultrafast regimes. Alternative to thermal evolutions, nonequilibrium electronic excitation can exercise a direct influence on the atomic bonding delivering a potentially rapid destructuring process. In this context, the atomic disordering of transition metals (Cr, Ni and Ti) induced by non-equilibrium electronic excitation typical for ultrafast laser processing is studied with an emphasis on the role of the d-band filling and crystalline structure. The density functional theory is used to obtain, from first principles, structural stability criteria and non-thermal disordering pathways on timescales shorter than picosecond electron-phonon dynamics. We show that the hot electrons distort charge distribution in the cold crystalline arrangement and increase the entropy of the system thus driving the mechanical expansion. The calculated nonequilibrium freeenergy potentials indicate that a solid destabilization is possible when electron temperature reaches a universal value of around 2 eV for all considered metals. Under a uniaxial lattice relaxation expected in the laser ablation of surface layers the charge redistribution and loss of lattice stability is shown to be oriented along specific crystal directions. Moreover, we show that the interatomic potential destabilization is energetically more favorable for Ni than for Cr and Ti. Lattice dynamics is affected by space charge redistribution induced by Fermi smearing associated with anisotropic expansion due to electron pressure gradients. This ultrafast destructuring mechanism is shown to be a general feature of partially-filled-d-band transition metals.

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“…Since the original work by Recoules et al 13 , it is generally accepted in the laser-mater interaction community that metals do not melt nonthermally. Indeed, it was shown that bulk metals typically exhibit hardening of phonon modes under intense electronic excitation, in contrast to covalently bonded materials 12 – 14 , materials with mixed ionic-covalent bonds 15 , 16 , or with Peierls distortions 17 . Absence of nonthermal melting in metals was confirmed in numerous subsequent ab-initio simulations for various metals, see e.g.…”

Section: Introductionmentioning

confidence: 99%

Nonthermal phase transitions in metals

Medvedev

Milov

2020

Sci Rep

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It is well known that sufficiently thick metals irradiated with ultrafast laser pulses exhibit phonon hardening, in contrast to ultrafast nonthermal melting in covalently bonded materials. It is still an open question how finite size metals react to irradiation. We show theoretically that generally metals, under high electronic excitation, undergo nonthermal phase transitions if material expansion is allowed (e.g. in finite samples). The nonthermal phase transitions are induced via an increase of the electronic pressure which leads to metal expansion. This, in turn, destabilizes the lattice triggering a phase transition without a thermal electron-ion coupling mechanism involved. We find that hexagonal close-packed metals exhibit a diffusionless transition into a cubic phase, whereas metals with a cubic lattice melt. In contrast to covalent solids, nonthermal phase transitions in metals are not ultrafast, predicative on the lattice expansion. Ultrafast deposition of high energy density into a solid target creates far-from-equilibrium states of matter with unusual properties that are not achievable in equilibrium states 1-3. Such a material with dynamically changing properties poses fundamental difficulties for a theoretical description, as it occupies the gap in standard methods between the solid state, plasma, chemical physics, and other disciplines 4,5. Such energy deposition may be realized via irradiation of the target with intense femtosecond laser pulses, in particular from X-ray free-electron lasers (XFEL) such as e.g. LCLS 6 , SACLA 7 , EuXFEL 8 , etc. In an irradiated solid, energy deposition by photons excites primarily the electronic system 9. Excited electrons transfer their energy to the ions, which may result in material modifications 10. Upon ultrafast energy deposition into an electronic system of a material, there are two typical scenarios of material damage. Energy transfer via electronphonon (or, more generally, electron-ion) coupling leads to heating of the atomic system, which, when exceeding the melting threshold, leads to atomic disordering. Alternatively, electronic excitation may affect the interatomic potential and destabilize the lattice without any electron-ion coupling involved. Such a mechanism is known as nonthermal melting, and has been demonstrated to take place in highly excited covalently bonded materials 1,11,12. Since the original work by Recoules et al. 13 , it is generally accepted in the laser-mater interaction community that metals do not melt nonthermally. Indeed, it was shown that bulk metals typically exhibit hardening of phonon modes under intense electronic excitation, in contrast to covalently bonded materials 12-14 , materials with mixed ionic-covalent bonds 15,16 , or with Peierls distortions 17. Absence of nonthermal melting in metals was confirmed in numerous subsequent ab-initio simulations for various metals, see e.g. Ref. 18. Experimental evidence supported this conclusion 19,20. On the other hand, softening of phonon modes in gold nanospheres was reported in R...

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Superionic State in Alumina Produced by Nonthermal Melting

Cited by 12 publications

References 41 publications

Dependence of nonthermal metallization kinetics on bond ionicity of compounds

Dependence of nonthermal metallization kinetics on bond ionicity of compounds

Structural instability of transition metals upon ultrafast laser irradiation

Nonthermal phase transitions in metals

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