A Review of the Melting Curves of Transition Metals at High Pressures Using Static Compression Techniques

International audienceThe accurate determination of melting curves for transition metals is an intense topic within high pressure research, both because of the technical challenges included as well as the controversial data obtained from various experiments. This review presents the main static techniques that are used for melting studies, with a strong focus on the diamond anvil cell; it also explores the state of the art of melting detection methods and analyzes the major reasons for discrepancies in the determination of the melting curves of transition metals. The physics of the melting transition is also discussed

Spectroscopic Raman study of the phase competition effects in MgB₂ superconductor

A systematic investigation of phase separation phenomena in the family of MgB₂ superconductors has been carried out by use of the Raman spectroscopy technique, together with the influence of various internal or external pertubations (chemical potential, temperature, magnetic field). In particular, the electron-doped Mg(B₁₋ₓCₓ)₂ and Mg₁₋ₓAlₓB₂ systems seem to present a mesoscopic phase separation in both structural and electronic terms. Especially in the case of Mg₁₋ₓAlₓB₂ the Raman data have been confirmed by extensive XRD measurements. The two observed phases correspond to rich and poor C or Al contents, and to different charge densities, which are reflected in the electron-phonon coupling strength of the E2g mode with σ- electronic states of boron. The basic characteristic of the Raman spectra is the appearance of two E2g phonon branches, in renormalized (~600 cm⁻¹) and non-renormalized (~850 cm⁻¹) frequencies. It seems that the Raman spectra and phase separation scenario can provide a sufficient explanation for the Tc-dependence on composition for low doping levels, while in the higher concentration region additional and more complex mechanisms are required to fully explain the underlying superconducting behavior. Hole doping has been also investigated by the Raman measurements on Mg₁₋ₓLiₓB₂ crystals, while from the double doped compound Mg₁₋ₓLiₓ(B₁₋yCy)₂ it is derived that holes and electrons couple with π- and σ- states respectively, without counterbalancing each other. Moreover, magnetic ions introduced by substituting Mg with isovalent Mn, leave the E2g phonon mainly unaffected, reducing though Tc very effectively according to Abrikosov-Gor’kov theory for magnetic impurities in superconductors. All the above systems do not seem to exhibit phase separation, and thus such effects are more likely to be induced by electron doping. Low-temperature measurements on various samples have given strong evidence on the anharmonic nature of the soft (~600 cm⁻¹) E2g vibrational mode. Moreover, interesting features have been observed in the electronic continuum of the Raman spectra, the most prominent being the 2Δσ peak, probably corresponding to the larger of the two superconducting order parameters, as well as an asymmetry, which could be correlated with the appearance of a new gap, as in many high-if superconductors. Finally, an introductory research on combined Raman and magnetic field measurements has been carried out, where some interesting spectral characteristics, such as the amplification of certain phonon modes, have been revealed.Σκοπός αυτής της εργασίας είναι η συστηματική διερεύνηση φαινομένων ανταγωνισμού φάσεων σε υλικά της οικογένειας του MgB₂, όπως αυτά γίνονται αισθητά με την τεχνική της φασματοσκοπίας Raman, καθώς και η μελέτη της επίδρασης διάφορων εσωτερικών και εξωτερικών διαταραχών (χημικό δυναμικό, θερμοκρασία, μαγνητικό πεδίο). Συγκεκριμένα, τα συστήματα εμπλουτισμού ηλεκτρονίων Mg(B₁₋ₓCₓ)₂ και Mg₁₋ₓAlₓB₂ βρέθηκαν να παρουσιάζουν μεσοσκοπικό διαχωρισμό φάσης, τόσο σε δομικό όσο και σε ηλεκτρονικό επίπεδο. Στην περίπτωση του Mg₁₋ₓAlₓB₂, ο παραπάνω ισχυρισμός ενισχύεται και από τις διεξοδικές μετρήσεις XRD. Οι δυο παρατηρούμενες φάσεις αντιστοιχούν σε περιοχές πλουσιότερες και φτωχότερες σε συγκέντρωση προσμίξεων, και πιθανότατα σε διαφορετικές πυκνότητες ηλεκτρονιακού φορτίου, οι οποίες αντικατοπτρίζονται στη διαφορετική τιμή της σύζευξης ηλεκτρονίου-φωνονίου του E2g ρυθμού με τις σ-ηλεκτρονικές καταστάσεις. Βασικό χαρακτηριστικό των φασμάτων Raman αποτελεί η εμφάνιση δύο κλάδων για το E2g φωνόνιο, τόσο σε επανακανονικοποιημένη (~600 cm⁻¹) όσο και σε μη-επανακανονικοποιημένη (~850 cm⁻¹) συχνότητα. Τα φάσματα Raman παρέχουν μια ικανοποιητική εξήγηση για την εξάρτηση της Tc με την αντικατάσταση, μέσω της εξίσωσης McMillan, σε χαμηλά επίπεδα συγκεντρώσεων, ενώ για μεγαλύτερες συγκεντρώσεις περισσότερο πολύπλοκα φαινόμενα φαίνεται να διέπουν τον υπεραγώγιμο μηχανισμό του συστήματος. Ο εμπλουτισμός οπών επίσης μελετήθηκε μέσω της ένωσης Mg₁₋ₓLiₓB₂, ενώ από τη διπλή αντικατάσταση Mg₁₋ₓLiₓ(B₁₋yCy)₂ προκύπτει ότι οπές και ηλεκτρόνια αλληλεπιδρούν με τις π- και σ- καταστάσεις αντίστοιχα, χωρίς οι επιδράσεις τους στο χημικό δυναμικό να είναι αθροιστικές. Επιπλέον, η εισαγωγή μαγνητικών ιόντων, αντικαθιστώντας το Mg με το ισοηλεκτρονικό του Μn, αφήνει το E2g φωνόνιο σχεδόν ανεπηρέαστο, μειώνει όμως δραματικά την Tc, σύμφωνα με τη θεωρία των Abrikosov- Gor’kov για τις μαγνητικές προσμίξεις σε υπεραγωγούς. Τα άνωθι συστήματα δε φαίνεται να παρουσιάζουν ανταγωνισμό φάσεων, δείχνοντας ότι ανάλογα φαινόμενα προκύπτουν κατά κύριο λόγο εξαιτίας του εμπλουτισμού ηλεκτρονίων. Οι μετρήσεις χαμηλών θερμοκρασιών σε διάφορα υλικά έδωσαν ισχυρές ενδείξεις για την ύπαρξη αναρμονικότητας του μαλακού (~600 cm⁻¹) τρόπου ταλάντωσης. Επιπλέον, παρατηρήθηκαν ενδιαφέροντα στοιχεία στο ηλεκτρονικό συνεχές των φασμάτων Raman, τα σημαντικότερα των οποίων είναι η εμφάνιση της κορυφής 2Δσ, η οποία πιθανόν να αντιστοιχεί στο μεγαλύτερο εκ των δύο υπεραγώγιμων χασμάτων, και μία ασυμμετρία, πιθανά συσχετιζόμενη με την εμφάνιση ενός νέου χάσματος, όπως συμβαίνει σε αρκετούς υπεραγωγούς υψηλών θερμοκρασιών. Τέλος, έλαβε χώρα μία πρωταρχική έρευνα συνδυασμένων μετρήσεων Raman με μαγνητικό πεδίο, όπου παρατηρήθηκαν κάποια ενδιαφέροντα νέα φασματικά χαρακτηριστικά, όπως η ενίσχυση συγκεκριμένων τρόπων ταλάντωσης

Melting curve of elemental zirconium

International audienceMelting experiments require rapid data acquisition due to instabilities of the molten sample and optical drifting due to the high required laser power. In this work, the melting curve of zirconium has been determined up to 80 GPa and 4000 K using in situ fast x-ray diffraction (XRD) in a laser-heated diamond anvil cell. The main method used for melt detection was the direct observance of liquid diffuse scattering in the XRD patterns and it has been proven to be a reliable melting diagnostic. The effectiveness of other melting criteria such as the appearance of temperature plateaus with increasing laser power is also discussed

High-pressure structural anomalies and electronic transitions in the topological Kondo insulator SmB

We have scrupulously investigated the high-pressure structural behaviour of the topological Kondo insulator SmB6 through synchrotron powder x-ray diffraction. Lattice modifications are observed at a critical pressure of $\sim 7\ \text{GPa}$ for three separate compression experiments at ambient and low temperatures. The initially subtle compression of the Sm-B bond length shows a sharp kink downward at the critical pressure. These anomalies coincide with the phase diagram boundaries for the pressure-induced metallization and the concurrent appearance of long-range magnetic ordering in the compound. We propose a scenario of delocalization of 4f electrons and the gradual filling of 5d conduction bands, which can also provide some clarification on the valence dependence of Sm with pressure

High pressure antiferrodistortive phase transition in mixed crystals of EuTiO3 and SrTiO3

We report a detailed high pressure study on Eu1−xSrxTiO3 polycrystalline samples using synchrotron x-ray diffraction. We have observed a second-order antiferrodistortive phase transition for all doping levels which corresponds to the transition that has been previously explored as a function of temperature. The analysis of the compression mechanism by calculating the lattice parameters, spontaneous strains and tilt angles of the TiO6 octahedra leads to a high pressure phase diagram for Eu1−xSrxTiO3

Sound velocity and equation of state of liquid Cesium at high pressure and high temperature

International audienceLiquid cesium (l-Cs) sound velocity at high densities was investigated along a 500 K isotherm using high-pressure picosecond acoustics measurements. At 2.0 GPa, the liquid sound velocity goes through a maximum versus pressure without any change on the reflectivity and interferometry acoustic signals. Upon further compression, a softening of the l-Cs visco-elastic properties is observed from 2.0 up to 4.0 GPa, pressure at which the reflectometric signal is abruptly reversed whereas the interferometric signal remains qualitatively the same. This anomalous behaviour could be related to an electronic transformation within the l-Cs state, which here again could reflect what happens at lower temperature within the solid state. If so, such liquid-liquid transition may be driven by the progressive collapse of the 6s electronic orbital onto the 5d ones. Above 4.0 GPa, the l-Cs sound velocity starts again to increase as commonly expected upon compression

Picosecond Acoustics Technique to Measure the Sound Velocities of Fe-Si Alloys and Si Single-Crystals at High Pressure

International audienceWe describe here a time resolved pump-probe laser technique-picosecond interferometry-which has been combined with diamond anvil cells (DAC). This method enables the measurement of the longitudinal sound velocity up to Mbar pressure for any kind of material (solids, liquids, metals, insulators). We also provide a description of picosecond acoustics data analysis in order to determine the complete set of elastic constants for single crystals. To illustrate such capabilities, results are given on the pressure dependence of the acoustic properties for prototypical cases: polycrystal (hcp-Fe-5 wt% Si up to 115 GPa) and single-crystal (Si up to 10 GPa)

Determination of indium melting curve at high pressure by picosecond acoustics

International audiencePicosecond acoustics combined with a diamond anvil cell is used to study liquid indium and to determine with high accuracy both the sound velocity and the melting curve over an extended pressure and temperature range. The sound velocities, determined by phonon surface imaging, complement previous inelastic x-ray scattering determinations and are in good agreement with estimations according to a thermodynamic model. Based on exact thermodynamic relations, the equation of state of the liquid phase is obtained using the isothermal bulk modulus BT,0 and its first pressure derivative B′T. These quantities are derived from the precise experimental determination of the variation of the sound velocity as a function of pressure. Melting is determined via the detection of abrupt changes in the elastic properties between solid and liquid phases and through the monitoring of the solid-liquid coexistence. The melting curve constrained up to 6 GPa and 673 K is shown to be well described by the Simon-Glatzel equation in the full (p,T) range explored

Pressure‐Induced Conversion of a Paramagnetic FeCo Complex into a Molecular Magnetic Switch with Tuneable Hysteresis

International audienceA key challenge in the design of magnetic molecular switches is to obtain bistability at room temperature. Here, we show that application of moderate pressure makes it possible to convert a paramagnetic FeIII2CoII2 square complex into a molecular switch exhibiting a full dia‐ to paramagnetic transition: FeIICoIII ⇔ FeIIICoII. Moreover, the complex follows a rare behavior: the higher the pressure, the broader the magnetic hysteresis. Thus, the application of an adequate pressure allows inducing a magnetic bistability at room temperature with predictable hysteresis width. The structural studies at different pressures suggest that the pressure‐enhanced bistability is due to the strengthening of intermolecular interactions upon pressure increase. An original microscopic Ising‐like model including pressure effects is developed to simulate this unprecedented behavior. Overall, this study shows that FeCo complexes could be very sensitive piezo switches with potential use as sensors

Toward a coherent model for the melting behavior of the deep Earth’s mantle

Co-auteur étrangerInternational audienceKnowledge of melting properties is critical to predict the nature and the fate of melts produced in the deep mantle. Early in the Earth’s history, melting properties controlled the magma ocean crystallization, which potentially induced chemical segregation in distinct reservoirs. Today, partial melting most probably occurs in the lowermost mantle as well as at mid upper-mantle depths, which control important aspects of mantle dynamics, including some types of volcanism. Unfortunately, despite major experimental and theoretical efforts, major controversies remain about several aspects of mantle melting. For example, the liquidus of the mantle was reported (for peridotitic or chondritic-type composition) with a temperature difference of ∼1000 K at high mantle depths. Also, the Fe partitioning coefficient (DFeBg/melt) between bridgmanite (Bg, the major lower mantle mineral) and a melt was reported between ∼0.1 and ∼0.5, for a mantle depth of ∼2000 km. Until now, these uncertainties had prevented the construction of a coherent picture of the melting behavior of the deep mantle.In this article, we perform a critical review of previous works and develop a coherent, semi-quantitative, model. We first address the melting curve of Bg with the help of original experimental measurements, which yields a constraint on the volume change upon melting (ΔVm). Secondly, we apply a basic thermodynamical approach to discuss the melting behavior of mineralogical assemblages made of fractions of Bg, CaSiO3-perovskite and (Mg,Fe)O-ferropericlase. Our analysis yields quantitative constraints on the SiO2-content in the pseudo-eutectic melt and the degree of partial melting (F) as a function of pressure, temperature and mantle composition; For examples, we find that F could be more than 40% at the solidus temperature, except if the presence of volatile elements induces incipient melting. We then discuss the melt buoyancy in a partial molten lower mantle as a function of pressure, F and DFeBg/melt. In the lower mantle, density inversions (i.e. sinking melts) appear to be restricted to low F values and highest mantle pressures.The coherent melting model has direct geophysical implications: (i) in the early Earth, the magma ocean crystallization could not occur for a core temperature higher than ∼5400 K at the core-mantle boundary (CMB). This temperature corresponds to the melting of pure Bg at 135 GPa. For a mantle composition more realistic than pure Bg, the right CMB temperature for magma ocean crystallization could have been as low as ∼4400 K. (ii) There are converging arguments for the formation of a relatively homogeneous mantle after magma ocean crystallization. In particular, we predict the bulk crystallization of a relatively large mantle fraction, when the temperature becomes lower than the pseudo-eutectic temperature. Some chemical segregation could still be possible as a result of some Bg segregation in the lowermost mantle during the first stage of the magma ocean crystallization, and due to a much later descent of very low F, Fe-enriched, melts toward the CMB. (iii) The descent of such melts could still take place today. There formation should to be related to incipient mantle melting due to the presence of volatile elements. Even though, these melts can only be denser than the mantle (at high mantle depths) if the controversial value of DFeBg/melt is indeed as low as suggested by some experimental studies. This type of melts could contribute to produce ultra-low seismic velocity anomalies in the lowermost mantle