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Lanthanum-III-pernitride is unique amongst pernitrides because of the seemingly misfit charge balance and stoichiometry. La+3N2 has been predicted to be stabilized by resonance between two mesomeric states of bonding in the pernitride group, which include a remarkable N‒N single bond limiting state. However, successful synthesis of this compound remained extant. Here, we report synthesis of LaN2 through shock-driven decomposition of lanthanum nitrate. The result is noteworthy as a first quantitative measure of the stability of N‒N single bonds in solids formed at GPa-level pressure and with respect to a possible cross-over of redox potentials in the nitrogen- and oxygen-systems at high pressure and temperature.

Premelting or virtual melting was proposed previously as an important deformation mechanism for high strain-rate loading. However, two questions remain outstanding: how premelting occurs exactly, and whether it plays a role in plastic deformation independent of, parasitic on, or synergetic with, dislocation motion. By virtue of double-shock compression, our large-scale molecular dynamics simulations reveal two yields in single-crystal copper, with the first yield achieved via dislocation motion, and the second, via dislocation-induced premelting as well. The clean capture of melting during dislocation motion suggests that premelting occurs on slip planes and at their intersections, facilitating gliding and leading to yield together with dislocation motion.

Superconductivity originates from the coupling between charge carriers and bosonic excitations of either phononic or electronic origin. Identifying the most relevant pairing glue is a key step towards a clear understanding of the unconventional superconductivity. Here, by applying the ultrafast optical spectroscopy on the electron-doped cuprates La$_{1.9}$Ce$_{0.1}$CuO$_{4\pm\delta}$, we discern a bosonic mode of electronic origin that has the strongest coupling with the charge carriers near $T_c$. We argue that this mode is associated with the two-dimensional antiferromagnetic spin fluctuations, and can fully account for the superconducting pairing. Our work may help to establish a quantitative relation between bosonic excitations and superconducting pairing in electron-doped cuprates, and pave the way for systematic exploration of superconductivity and other collective phenomena in all correlated materials.

It is well known that strain rate and size effects are both important in material failure, but the relationships between them are poorly understood. To establish this connection, we carry out molecular dynamics (MD) simulations of cavitation in Lennard-Jones and Cu liquids over a very broad range of size and strain rate. These studies confirm that temporal and spatial scales play equivalent roles in the tensile strengths of these two liquids. Predictions based on smallest-scale MD simulations of Cu for larger temporal and spatial scales are consistent with independent simulations, and comparable to experiments on liquid metals. We analyze these results in terms of classical nucleation theory and show that the equivalence arises from the role of both size and strain rate in the nucleation of a daughter phase. Such equivalence is expected to hold for a wide range of materials and processes and to be useful as a predictive bridging tool in multiscale studies.

Deformation twinning in pure aluminum has been considered to be a unique property of nanostructured aluminum. A lingering mystery is whether deformation twinning occurs in coarse-grained or single-crystal aluminum, at scales beyond nanotwins. Here, we present the first experimental demonstration of macro deformation twins in single-crystal aluminum formed under ultrahigh strain-rate ($\sim$10$^6$ s$^{-1}$), large shear strain (200$\%$) via dynamic equal channel angular pressing. Deformation twinning is rooted in the rate dependences of dislocation motion and twinning, which are coupled, complementary processes during severe plastic deformation under ultrahigh strain rates., 4 pages and 4 figures

We report on a time-resolved ultrafast optical spectroscopy study of the topological insulator Bi_{2}Se_{3}. We unravel that a net spin polarization cannot only be generated using circularly polarized light via interband transitions between topological surface states (SSs), but also via transitions between SSs and bulk states. Our experiment demonstrates that tuning photon energy or temperature can essentially allow for photoexcitation of spin-polarized electrons to unoccupied topological SSs with two distinct spin relaxation times (∼25 and ∼300 fs), depending on the coupling between SSs and bulk states. The intrinsic mechanism leading to such distinctive spin dynamics is the scattering in SSs and bulk states which is dominated by E_{g}^{2} and A_{1g}^{1} phonon modes, respectively. These findings are suggestive of novel ways to manipulate the photoinduced coherent spins in topological insulators.

We report molecular dynamics simulations of thermal spray coating of Cu nanodroplets on Cu substrates with different orientations, and show that the droplets crystallize by adding the closest-packed {111} planes regardless of the substrate orientations. Such preferential growth along the closest-packed planes may be common in a broad range of crystallization and melting processes.

We investigate deformation of pure Cu, pure Nb and 30 nm Cu/30 nm Nb nanolaminates induced by high strain rate shock loading. Abundant dislocation activities are observed in shocked pure Cu and Nb. In addition, a few deformation twins are found in the shocked pure Cu. In contrast, in shocked Cu/Nb nanolaminates, abundant deformation twins are found in the Cu layers, but only dislocations in the Nb layers. High resolution transmission electron microscopy reveals that the deformation twins in the Cu layers preferentially nucleate from the Cu(112)//Nb(112) interface habit planes rather than the predominant Cu(111)//Nb(110) interface planes. Our comparative study on the shock-induced plastic deformation of the pure metals (Cu and Nb) and the Cu/Nb nanolaminates underscores the critical role of heterogeneous phase interfaces in the dynamic deformation of multilayer materials.

Planar shock compression experiments are performed at 9–105 GPa on polycrystalline ferropericlase (Mg0.94Fe0.06)O to investigate its Fe2+ spin transition. Forward and reverse impact configurations are used to obtain Hugoniot and shock-state sound velocities. While wave profiles, shock velocity–particle velocity and pressure–density measurements show negligible/weak indications of a phase transition, the shock-state sound speed data clearly manifest a phase transition in the range of 36–62 GPa at the nanosecond time scales. These shock data reveal the phase transition as the spin transition identified in static compression experiments and first-principles calculations.

The dynamics of plasma grating contrast enhancement (PGCE) irradiated by an infrared laser pulse is investigated with one dimensional particle-in-cell simulation where field ionization and impact ionization are simultaneously considered for the first time. The numeric results show that the impact ionization dominates the PGCE process. Upon the interaction with the laser pulse, abundant free electrons are efficiently accelerated and subsequently triggered massive impact ionizations in the density ridges of the plasma grating for the higher local plasma energy density, which efficiently enhances the grating contrast. Besides the dynamic analysis of PGCE, we explore the parameter space of the incident infrared laser pulse to optimize the PGCE effect, which can provide useful guidance to experiments related to laser-plasma-grating interactions and may find applications in prolonging the duration of the plasma grating.

When an intense picosecond laser pulse is loaded upon a dense plasma, a high energy density plasma bunch, including electron bunch and ion bunch, can be generated in the target. We simulate this process through one-dimensional particle-in-cell simulation and find that the electron bunch generation is mainly due to a local high energy density electron sphere originated in the plasma skin layer. Once generated the sphere rapidly expands to compress the surrounding electrons and induce high density electron layer, coupled with that, hot electrons are efficiently triggered in the local sphere and traveling in the whole target. Under the compressions of light pressure, forward-running and backward-running hot electrons, a high energy density electron bunch generates. The bunch energy density is as high as TJ/m3 order of magnitude in our conditions, which is significant in laser driven dynamic high pressure generation and may find applications in high energy density physics.

[100] CaF2 single crystals are shock-compressed via symmetric planar impact, and the flyer plate–target interface velocity histories are measured with a laser displacement interferometry. The shock loading is slightly above the Hugoniot elastic limit to investigate incipient plasticity and its kinetics, and its effects on optical properties and deformation inhomogeneity. Fringe patterns demonstrate different features in modulation of fringe amplitude, including birefringence and complicated modulations. The birefringence is attributed to local lattice rotation accompanying incipient plasticity. Spatially resolved measurements show inhomogeneity in deformation, birefringence, and fringe pattern evolutions, most likely caused by the inhomogeneity associated with lattice rotation and dislocation slip. Transiently overdriven elastic states are observed, and the incubation time for incipient plasticity decreases inversely with increasing overdrive by the elastic shock.[100] CaF2 single crystals are shock-compressed via symmetric planar impact, and the flyer plate–target interface velocity histories are measured with a laser displacement interferometry. The shock loading is slightly above the Hugoniot elastic limit to investigate incipient plasticity and its kinetics, and its effects on optical properties and deformation inhomogeneity. Fringe patterns demonstrate different features in modulation of fringe amplitude, including birefringence and complicated modulations. The birefringence is attributed to local lattice rotation accompanying incipient plasticity. Spatially resolved measurements show inhomogeneity in deformation, birefringence, and fringe pattern evolutions, most likely caused by the inhomogeneity associated with lattice rotation and dislocation slip. Transiently overdriven elastic states are observed, and the incubation time for incipient plasticity decreases inversely with increasing overdrive by the elastic shock.

We investigate physical properties of bromoform (liquid CHBr3) including compressibility and refractive index under dynamic extreme conditions of shock compression. Planar shock experiments are conducted along with high-speed laser interferometry. Our experiments and previous results establish a linear shock velocity−particle velocity relation for particle velocities below 1.77 km/s, as well as the Hugoniot and isentropic compression curves up to ∼21 GPa. Shock-state refractive indices of CHBr3 up to 2.3 GPa or ∼26% compression, as a function of density, can be described with a linear relation and follows the Gladstone-Dale relation. The velocity corrections for laser interferometry measurements at 1550 nm are also obtained.

We investigate grain boundary (GB) melting using molecular dynamics simulations on face-centered-cubic Cu bicrystals with symmetric 〈 1 1 0 〉 tilt GBs. Two representative types of GBs are explored: Σ = 11 / ( 113 ) / 50 . 48 ∘ (low GB energy) and Σ = 27 / ( 552 ) / 148 . 41 ∘ (high GB energy). The temperature and temporal evolutions of the Cu bicrystals under stepped heating are characterized in terms of order parameters and diffusion coefficients, as well as the nucleation and growth of melt. Within the GB region, continuous local melting precedes discontinuous bulk melting, while continuous solid state disordering may precede local melting. Premelting may occur for local melting but not for bulk melting. For Σ = 11 / ( 113 ) / 50 . 48 ∘ , premelting of the GB region is negligible, and local melting occurs near the thermodynamic melting temperature. The GB region as a whole is superheated by about 13% before its bulk melting. In the case of Σ = 27 / ( 552 ) / 148 . 41 ∘ , considerable premelting is observed for local melting, while the bulk melting occurs with negligible superheating. The exact melting behavior of a general GB depends on the GB energy, but is likely bracketed within these two cases.

Superplasticity can be achieved in nanoglasses but at the expense of strength, and such a loss can be mitigated via embedding stronger nanocrystals, i.e., forming nanoglass/nanocrystal composites. As an illustrative case, we investigate plastic deformation of Cu64Zr36 nanoglass/nanocrystalline Cu composites during uniaxial tension and nanoindentation tests with molecular dynamics simulations. With an increasing fraction of nanocrystalline grains, the tensile strength of the composite is enhanced, while its ductility decreases. The dominant interface type changes from a glass-glass interface to glass-crystal interface to grain boundary, corresponding to a failure mode transition from superplastic flow to shear banding to brittle intercrystal fracture, respectively. Accordingly, the indentation hardness increases continuously and strain localization beneath the indenter is more and more severe. For an appropriate fraction of nanocrystalline grains, a good balance among strength, hardness and ductility can be realized, which is useful for the synthesis of novel nanograined glass/crystalline composites with high strength, high hardness and superior ductility.

aB s T rac T In the present study, we summarize indications for the existence of kinked post-perovskite structures in the MAS system. X-ray diffraction data and Raman spectra of aluminous magnesium metasilicate post-perovskite are inconsistent with the CaIrO3 structure. Instead the observations are consistent with structures intermediate between the perovskite and the CaIrO3 structure. Ab initio calculations show that the enthalpies of the kinked structures are slightly higher than the CaIrO3 structure at 0 K. Finite temperature, minor element chemistry, kinetics of phase transformation, and actual stress regime are plausible reasons for the observed differences between the present and the previously reported postperovskite phases.

To constrain the influence of impact shock on water and hydrogen isotope signatures of Martian meteorite kaersutites, we conducted shock recovery experiments on three terrestrial kaersutite crystals. Homogeneous impact shock to 32 GPa, commensurate with shock levels experienced by Martian meteorite kaersutites, led to increases in kaersutite water contents (ΔH_2O = 0.25–0.89 wt.%), decreases in Fe^3+/ΣFe (4–20%), and enrichments in hydrogen isotope composition (ΔD = + 66 to + 87‰) relative to pre-shock values. The latter values represent the largest shock-induced hydrogen isotope fractionations measured to date. These observations are explained most completely by a two-step shock process. First, shock-induced devolatilization led to hydrogen isotope enrichment through preferential loss of H relative to D. Second, reaction of the kaersutite with the ambient atmosphere led to increased water contents and reduced Fe. Fe reduction and water addition via the reaction Fe^2+ + OH^− ↔ Fe^3+ + O_2− + ½H_2 explain the Fe^3+/ΣFe data and some of the water data. Further water addition mechanisms (irreversible adsorption, shock implantation) are necessary to fully explain the increased water contents. Addition of water from the terrestrial atmosphere, which is isotopically light relative to the experimental kaersutite compositions, means the measured hydrogen isotope enrichments are likely minima. The measured (minimum) levels of hydrogen isotope enrichment are relevant to the hydrogen isotope variability within and among Martian kaersutites, but are minor relative to their absolute δD values. Alternatively, addition of water from the enriched Martian atmosphere could explain both Martian kaersutite hydrogen isotope variability and absolute δD values. However, the low Martian kaersutite water contents leave little room for significant water addition. The importance of the ambient atmosphere to the outcome of the shock experiments makes it difficult to translate our results to Mars given the unknown influence of its more tenuous atmosphere on the processes observed in the experiments. Our results suggest that shock is a feasible mechanism for influencing Martian kaersutite water contents and hydrogen isotope compositions but that its complex signature precludes precise determination of that influence.

We conducted nanoindentation to explore the hardness and elastic properties of silica stishovite, synthesized at high pressure and quenched to ambient conditions. A total of 10 crystallographic orientations were examined on selected grains with a maximum load of 4 or 20 mN. We observed discontinuity in the load-displacement curve (pop-in) for the [2 5 over(1, -)] and [6 2 over(1, -)] grains subjected to a maximum load of 20 mN. The single-crystal hardness at high plastic deformation is quasi-isotropic with an average of 32 ± 1 GPa, similar to the polycrystalline hardness reported earlier; the theoretical hardness determined from the experiments is about 54 ± 3 GPa. These two hardnesses suggest that stishovite is one of the hardest oxides. The measured indentation moduli are close to the predictions at low load (minor plasticity) but are considerably lower at high load (high plasticity). Both indentation hardness and modulus decrease with increasing plasticity. Our results underscore the necessity of considering the degree of plastic deformation when interpreting hardness and elastic moduli from indentation experiments.

We have investigated modifications of sapphire (0 0 0 1) surface with and without coating, induced by a single laser pulse with a 1054 nm wavelength, 2.2 µs sources duration, 7.75 mm spot and energy of 20–110 J. A holographic optical element was used for smoothing the drive beam spatially, but it induced small hotspots which initiated damage on the uncoated and coated surfaces. The individual damage effects of hotspots became less pronounced at high fluences. Due to high temperature and elevated non-hydrostatic stresses upon laser irradiation, damage occurred as fracture, spallation, basal and rhombohedral twinning, melting, vitrification, the formation of nanocrystalline phases, and solid–solid phase transition. The extent of damage increased with laser fluences. The formation of regular linear patterns with three-fold symmetry (1 1 2^[bar] 0) upon fracture was due to rhombohedral twinning. Nanocrystalline α-Al_2O_3 formed possibly from vapor deposition on the coated surface and manifested linear, triangular and spiral growth patterns. Glass and minor amounts of γ-Al_2O_3 also formed from rapid quenching of the melt on this side. The α- to γ-Al_2O_3 transition was observed on the uncoated surface in some partially spalled alumina, presumably caused by shearing. The nominal threshold for laser-induced damage is about 47 J cm^(−2) for these laser pulses, and it is about 94 J cm^(−2) at the hotspots.

The issues related to high-pressure melting of Ta are discussed within the context of diamond-anvil cell (DAC) and shock wave experiments, theoretical calculations and common melting models. The discrepancies between the extrapolations of the DAC melting curve and the melting point inferred from shock wave experiments, cannot be reconciled either by superheating or solid–solid phase transition. The failure to reproduce low-pressure DAC melting curve by melting models such as dislocation-mediated melting and the Lindemann law, and molecular dynamics and quantum mechanics-based calculations, undermines their predictions at moderate and high pressures. Despite claims to the contrary, the melting curve of Ta (as well as Mo and W) remains inconclusive at high pressures.

The origin of compression-induced failure in brittle solids has been a subject of debate. Using in situ, high-speed, strain field mapping of a representative material, polymethylmethacrylate, we reveal that shock loading leads to heterogeneity in a compressive strain field, which in turn gives rise to localized lateral tension and shear through Poisson's effects, and, subsequently, localized microdamage. A failure wave nucleates from the impact surface and its propagation into the microdamage zone is self-sustained, triggering interior failure. Its velocity increases with increasing shock strength and eventually approaches the shock velocity. The seemingly puzzling phenomena observed in previous experiments, including incubation time, failure wave velocity variations, and surface roughness effects, can all be explained consistently with the nucleation and growth of the microdamage, and the effects of loading strength and preexisting defects.

We demonstrate the stochastic nature of cavitation in a binary metallic liquid Cu46Zr54 during hydrostatic expansion by employing molecular dynamics (MD) simulations using a quantum mechanics (QM)-derived potential. The activation volume is obtained from MD simulations and transition-state theory. Extrapolation of the pressure dependence of the activation volume from our MD simulations to low tensile pressure agrees remarkably with macroscale cavitation experiments. We find that classical nucleation theory can predict the cavitation rate if we incorporate the Tolman length derived from the MD simulations.

Shock-induced freezing in liquids has long been a subject of interest as well as mystery. With large-scale molecular dynamics simulations, we demonstrate that homogeneous crystal nucleation in liquid Cu can be realized under effective supercooling $(\ensuremath{\theta})$, via quasi-isentropic compression or ramp wave loading with a particle velocity ${u}_{\mathrm{p}}$ achieved within a ramp time $\ensuremath{\tau}$. The simulations yield the $\ensuremath{\tau}\text{\ensuremath{-}}{u}_{\mathrm{p}}\text{\ensuremath{-}}\ensuremath{\theta}$ relations for homogeneous crystallization at the spatial and temporal scales of molecular dynamics; a ramp wave loading with $\ensuremath{\tau}\ensuremath{\sim}100$ ps is essentially isentropic for liquid Cu. Based on classical nucleation theory, we predict $\ensuremath{\theta}$, and thus ${u}_{\mathrm{p}}$, as required for homogenous nucleation in experiments with larger length and time scales. Homogeneous nucleation can also be achieved with shock loading in initially supercooled liquids.

We investigate using molecular dynamics, the tensile failure of single-crystal and nanocrystalline Lennard-Jones solids under uniaxial strain. Stresses are relaxed by plasticity and tensile failure, which are induced via stacking faults, twin planes, void nucleation and growth, and their interactions. Stacking faults and twin planes as well as multiple nanovoids are nucleated at grain boundaries in a nanocrystalline solid. Void formation is characterized by under-coordinated atoms with coordination number ≤ 7, and the critical void size is comparable to a vacancy. For a single crystal, the number of stacking faults and twin planes decreases during failure mostly due to the absorption by nanovoids. In contrast, it increases with strain monotonically for a nanocrystalline solid, where abundant grain boundaries help the nucleation and deter the propagation and absorption of the stacking faults and twin planes, inducing effective dislocation pile-ups at grain boundaries even in the presence of nanovoids.

We show that bulk amorphous silica recovered from shock-wave experiments on quartz to 57 GPa is not a true glass but rather keeps a large degree of long-range structural information that can be recovered by static cold recompression to 13 GPa. At this pressure, shock-retrieved silica assumes the structure of crystalline stishovite. A minor amount of material recovers the structure of a recently discovered new silica polymorph.

We investigate vacancy-induced densification of silica glass using molecular dynamics simulations. Equilibration of defective glasses initially with various concentrations of vacancies yields glasses denser than the intact glass. The structural and vibrational properties of the densified glasses are characterized. Densification is related to structural changes induced by atomic rearrangement near vacancies, and increases with the concentration of vacancies. Vacancies may cluster and form voids, and the maximum densification for void-structured glasses occurs at a critical radius of about 0.44 nm. The glasses densified by vacancies and by simulated UV-laser irradiation display nearly identical structural and vibrational properties. These results appear to support the Douillard–Duraud point defect model as a common mechanism for radiation densification of silica glass.

Molecular dynamics simulations are conducted to investigate disordering of stishovite (a high-pressure polymorph of silica) and the role of defects including grain boundaries, vacancies and free surfaces. It is shown that pre-existent defects initiate or facilitate solid-state disordering and melting. As illustrated in the case of vacancies, melting precedes solid-state disordering for stishovite with low defect concentrations, while defect-rich stishovite may transform into high-density vitreous silica which then undergoes a (quasi-)continuous transition into melt. In sharp contrast, melting of ordinary glass at the same heating rate is sluggish yet evidently first order. Disordering on free surfaces is anisotropic, being more pronounced on (101), (011) and (111) than on (100), (010), (001) and (110) crystallographic planes.

We investigate spontaneous structural disordering of nm-grain-sized polycrystals and clusters of stishovite at ambient conditions using molecular dynamics, and compare to disordering in stishovite nanocrystals and single-crystal α-quartz at high pressures. For nanocrystals and clusters at ambient conditions and sub-ns time scales, the extent and rate of disordering increase with the surface-area-to-volume ratio, and the structure becomes completely amorphous if the size is less than about 2 nm. Within the stability field of stishovite, disordering at the grain boundaries is less pronounced. Low-pressure disordering of stishovite structure involves Si coordination number reduction from six to five and finally to four, a process opposite to the pressure-induced amorphization of quartz.

We present a general method to determine the solid?liquid interfacial energy (?sl) from the maximum supercooling (or superheating), and apply it to the water?ice system. For solid?liquid phase transitions, the nucleation-theory-based systematics of maximum superheating and supercooling relate a dimensionless nucleation barrier to the superheating (supercooling) and heating (cooling) rates. Given superheating (or supercooling) values from either experiments or simulations, ?sl can then be deduced from the dimensionless nucleation barrier, equilibrium melting temperature and enthalpy of fusion. We demonstrate the accuracy of this approach using molecular dynamics (MD) simulations of the Lennard?Jones system: our predictions of ?sl at various pressures are in excellent agreement with independent, direct MD simulations. With this approach, we predict ?sl for the water?ice (Ih and III) system using experimental supercooling values in the pressure range of 0?0.3?GPa. The predicted value (28 ? 0.8?mJ?m?2) agrees with measurements on H2O?Ih at ambient pressure.

Upon compression, many materials undergo major reconstructions of their structure and bonding, including increases in coordination of atoms and changes in bonding character. While transforming, the materials pass through intermediate states, which are often too transient to be captured and examined. Here we discuss the coordination change in silica as an example of a system where such interesting intermediate structural states have been quenched from shock-experiments. On the basis of these results we suggest a relation between the formation of one of these phases and the extension of the liquid–liquid transition boundary into the stability field of solid silica. We report Raman spectra of shock-retrieved vitreous silica which indicate different compression mechanisms for shock-generated amorphous silica and vitreous silica compressed at 300 K. Static recompression of shock-generated glass leads to an amorphous-crystal transition above 13 GPa.

Laser-induced shock waves in condensed matter have important applications in dynamic material studies and high pressure physics. We briefly review some techniques in laser-induced shock waves, including direct laser drive, laser-launched flyer plate, quasi-isentropic loading, point and line imaging velocity interferometry, transient X-ray diffraction, spectroscopy and shock recovery, and their applications to study of equation of state, spallation, and phase transitions.

A thermodynamically complete ab initio equation of state (EOS) for MgO was obtained using electron density functional theory and the quasiharmonic phonon approximation, and adjusted to match the ambient density. This EOS was demonstrated to be consistent with isotherm, thermal expansivity, heat capacity and melting curve measured in static experiments, and reproduced density and temperature measurements under shock wave loading of bulk and porous periclase. The Gruneisen parameter of periclase at a given density was shown to be weakly dependent on temperature. The B1–B2 phase change was calculated to occur near 320 GPa on the principal Hugoniot. The melting locus of periclase, relevant to the Earth's lower mantle pressures, was predicted to be accessible by shock wave loading of porous periclase, which could also put pressure and temperature bounds on B1–B2 transitions.

Shock-state temperature and sound-speed measurements on crystalline materials, demonstrate superheating-melting behavior distinct from equilibrium melting. Shocked solid can be superheated to the maximum temperature, T_c′. At slightly higher pressure, P_c, shock melting occurs, and induces a lower shock temperature, T_c. The Hugoniot state, (P_c,T_c), is inferred to lie along the equilibrium melting curve. The amount of superheating achieved on Hugoniot is, Θ_H+=T_c′/T_c−1. Shock-induced superheating for a number of silicates, alkali halides and metals agrees closely with the predictions of a systematic framework describing superheating at various heating rates [Appl. Phys. Lett. 82 (12) (2003) 1836]. High-pressure melting curves are constructed by integration from (P_c,T_c) based on the Lindemann law. We calculate the volume and entropy changes upon melting at (P_c,T_c) assuming the R ln 2 rule (R is the gas constant) for the disordering entropy of melting [J. Chem. Phys. 19 (1951) 93; Sov. Phys. Usp. 117 (1975) 625; Poirier, J.P., 1991. Introduction to the Physics of the Earth’s Interior. Cambridge University Press, Cambridge, 102 pp.]. (P_c,T_c) and the Lindemann melting curves are in excellent accord with diamond-anvil cell (DAC) results for NaCl, KBr and stishovite. But significant discrepancies exist for transition metals. If we extrapolate the DAC melting data [Phys. Rev. B 63 (2001) 132104] for transition metals (Fe, V, Mo, W and Ta) to 200–400 GPa where shock melting occurs, shock temperature measurement and calculation would indicate Θ_H+∼0.7–2.0. These large values of superheating are not consistent with the superheating systematics. The discrepancies could be reconciled by possible solid–solid phase transitions at high pressures. In particular, this work suggests that Fe undergoes a possible solid–solid phase transition at ∼200 GPa and melts at ∼270 GPa upon shock wave loading, and the melting temperature is ∼6300 K at 330 GPa.