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International audience; A 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.

International audience; X-ray absorption spectroscopy (XAS) is a widely used technique to probe the local environment around specific atomic species. Applied to samples under extreme pressure and temperature conditions, XAS is sensitive to phase transitions, including melting, and allows gathering insights on compositional variations and electronic changes occurring during such transitions. These characteristics can be exploited for studies of prime interest in geophysics and fundamental high-pressure physics. Here, we investigated the melting curve and the eutectic composition of four geophysically relevant iron binary systems: Fe-C, Fe-O, Fe-S and Fe-Si. Our results show that all these systems present the same spectroscopic signatures upon melting, common to those observed for other pure late 3d transition metals. The presented melting criterion seems to be general for late 3d metals bearing systems. Additionally, we demonstrate the suitability of XAS to extract melt compositional information in situ, such as the evolution of the concentration of light elements with increasing temperature. Diagnostics presented in this work can be applied to studies over an even larger pressure range exploiting the upgraded synchrotron machines, and directly transferred to time-resolved extreme condition studies using dynamic compression (ns) or fast laser heating (ms). The coupling of X-ray absorption spectroscopy (XAS) with laser heated-diamond anvil cell (LH-DAC) is a challenging technical development only recently implemented in synchrotron facilities 1-3. The opportunities that stem from this combination are of great interest in physics, chemistry and planetary science. The element selectivity and the local structure sensitivity are two of the distinctive characteristics that make XAS a very informative and multipurpose technique. Combined together, they provide the possibility of microscopic investigation of the local atomic environment of a specific element in a compound or an alloy, irrespectively of the solid, liquid or amorphous state. The X-ray absorption near edge spectroscopy (XANES) region of the XAS spectrum, also probes the unoccupied electron density of states right above the Fermi level 4-7. Recent high pressure/high temperature absorption experiments 6,8-14 and theoretical calculations 8,9,11,15-19 on pure metals directly correlate changes in the XANES spectra and/or in the pre-edge to phase transitions, but to which extent these findings could be generalized to multi-component systems remains to be assessed. XANES sensitivity to the electronic structure further grants the possibility to probe the chemical environment, through relatively simple approaches like the linear combination analysis (LCA), as demonstrated in many studies at ambient conditions (Benfatto et al. 20 , among others), but never attempted under extreme high-pressure, high-temperate conditions. open

International audience; Picosecond acoustics is an optical pump-probe technique allowing to access thermoelastic properties and sound velocities of a large variety of materials under extreme conditions. Coupled with diamond anvil cells and laser heating, picosecond acoustics measurements offer the possibility to probe materials over a pressure and temperature range directly pertinent for the deep planetary interiors. In this paper we highlight the capabilities and versatility of this technique by presenting some recent applications on materials of geophysical interest. All the independent components of the elastic tensor of MgO are simultaneously determined by measurements on a single crystal at ambient conditions. Compressional sound velocity is measured at high pressure on an iron-carbon alloy and on polycrystalline argon. First laser heating test measurements performed on molybdenum at high pressure are also presented. These examples demonstrate that picosecond acoustics is a valuable alternative to already existing techniques for determining the physical properties of samples under extreme pressure and temperature conditions.

BiS 2 was synthesized using a multi-anvil large volume press at a pressure and temperatures of 5.5 GPa and 1250 °C, respectively, and was then recovered at ambient conditions. Using data collection from single crystal synchrotron X-ray diffraction experiments, the crystal structure was found to consist of two distinct distorted square-based pyramidal BiS 5 -units. Synchrotron powder X-ray diffraction in the 85–300 K range shows smooth thermal expansion with a modest anisotropy. Physical property measurements reveals an optical band gap of 1.10 eV and a heat capacity with no anomalies in the 2–300 K range. Debye temperatures, determined by both heat capacity and thermal motion analysis, agree well with values of around 114 and 107 K for Bi1 and Bi2 atoms, respectively. Furthermore, theoretical calculations of the electronic band structure, by density functional theory, confirm the gapped state and reveal a small degree of band inversion at the Γ-point, but calculation of parity eigenvalues show BiS 2 to possess a trivial topology. The high pressure behavior up to 60 GPa was investigated by powder diffraction in a diamond anvil cell; the structure is retained to at least 35 GPa while indications of a structure transition are observed afterwards. Fitting a 3rd order Birch-Murnaghan equation of state for pressures up to 30 GPa gives a bulk modulus of K 0 = 35.5(8) GPa and K 0 ′ = 7.37(18). Finally, a short structural comparison between the high pressure phase of BiS 2 and BiS 2 -based superconductors is presented.

The superconducting phase of the $\mathrm{HgBa}_2\mathrm{CuO}_{4+\delta}$ (Hg-1201) and $\mathrm{HgBa}_2\mathrm{Ca}_2\mathrm{Cu}_3\mathrm{O}_{8+\delta}$ (Hg-1223) cuprates has been investigated by Raman spectroscopy under hydrostatic pressure. Our analysis reveals that the increase of $T_c$ with pressure is slower in Hg-1223 cuprate compared to the Hg-1201 due to a charge carrier concentration imbalance (accentuated by pressure) between the $\mathrm{CuO}_2$ layers of Hg-1223. We find that the energy variation under pressure of the apical oxygen mode from which the charge carriers are transferred to the $\mathrm{CuO}_2$ layers, is the same for both the Hg-1223 and Hg-1223 cuprates and it is controlled by the inter-layer compressibility. At last, we show that the binding energy of the Cooper pairs related to the maximum amplitude of the $d-$ wave superconducting gap at the anti-nodes, does not follow $T_c$ with pressure. It decreases while $T_c$ increases. In the particular case of Hg-1201, the binding energy collapses from 10 to 2 $K_B T_c$ as the pressure increases up to 10 GPa. These direct spectroscopic observations joined to the fact that the binding energy of the Cooper pairs at the anti-nodes does not follow $T_c$ either with doping, raises the question of its link with the pseudogap energy scale which follows the same trend with doping., Comment: 15 pages, 18 figures

Lead hafnate single crystals were characterized using single crystal x-ray diffraction under simultaneous application of hydrostatic pressure and high temperatures. The information on the structure of two intermediate phases, situated between antiferroelectric and paraelectric phases in the pressure-temperature phase diagram, has been obtained. The lower-temperature intermediate phase is characterized by incommensurate displacive modulations in Pb sublattice. The higher-temperature intermediate phase is characterized by oxygen framework distortion, primarily in the form of anti-phase tilts of the oxygen octahedra, which is also present in the lower-temperature intermediate phase.

We report the high-temperature and high-pressure single-crystal x-ray diffraction study of PbHfO3. The increase of pressure enables an additional intermediate phase, located between the cubic and incommensurate phases, and characterized by superstructure reflections indicating the presence of anti-phase oxygen octahedral tilts. The temperature-dependent diffuse peak at an incommensurate position is much better pronounced at higher pressures (~ 2 GPa) than at lower pressures (~ 1.1 GPa), which we interpret as enhancing of the incommensurate soft mode on pressure increase. Several peculiarities in the behavior of superstructural reflections at M- and X- points are reported and discussed.

Melting 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 for the first time up to 80 GPa and 4000 K using in-situ fast x-ray diffraction (XRD) in a laser-heated diamond anvil cell (LH-DAC). The main method used for melt detection was the direct observance of liquid diffuse scattering (LDS) 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 temperature plateaus with increasing laser power is also discussed.

The high pressure behaviors of the two perovskite structures (hexagonal 6H-SrIrO3 and orthorhombic 3C-SrIrO3) have been studied in diamond anvil cells to 43 and 60 GPa, respectively, using synchrotron powder X-ray diffraction. 6H-SrIrO3 was first synthesized at ambient pressure and subsequently transformed into 3C-SrIrO3 in a large volume press at 8.8 GPa and 1000 °C. Both polymorphs were found to retain the initial symmetry up to the highest pressures measured, but in the case of 6H-SrIrO3, two anomalies were identified: a change in the axial compressibilities at 24 GPa and a change in both the axial and volume compressibilities at 32 GPa. Fitting a 3rd order Birch-Murnaghan equation of state to the obtained P-V data yielded bulk moduli of K0=151.5(12) GPa (fitted range 0

International audience; Liquid 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.

We investigated the atomic structure of liquid Rb along an isothermal path at 573 K, up to 23 GPa, by X-ray diffraction measurements. By raising the pressure, we observed a liquid–liquid transformation from a simple metallic liquid to a complex one. The transition occurs at 7.5 ± 1 GPa which is slightly above the first maximum of the T–P melting line. This transformation is traced back to the density-induced hybridization of highest electronic orbitals leading to the accumulation of valence electrons between Rb atoms and to the formation of interstitial atomic shells, a behavior that Rb shares with Cs and is likely to be common to all alkali metals.

We report studies of pressure-induced phase transitions of ordered and disordered ternary tetradymite-like Bi2Te2Se by synchrotron powder X-ray diffraction (PXRD) in diamond anvil cells (DACs) for pressures up to 59 and 49 GPa, respectively. The first sample (SB) was prepared from a single crystal with ordered Se/Te sites while the second sample (Q) was prepared from a quenched melt resulting in disordered Se/Te. This allows for an investigation of the effect of disorder on the phase transitions and the equation of states (EoS) of the tetradymite-like α phase. Fitting of a third order Birch-Murnaghan EoS to the α phases yielded bulk moduli K0 of 34.5(10) and 38.3(17) GPa and K' of 6.2(3) and 5.0(5) for the SB and Q samples, respectively. An electronic topological transition (ETT) was identified in both samples at pressures of 4.4 and 3.1 GPa, respectively. This was followed by a transition near 11 GPa to a phase that is isostructural with the β phase of Bi2Te3. The Se/Te ordering only affects the transition pressure to a small extent. A cubic phase that resembles the δ phase observed in high-pressure studies of Bi2Te3 appears at 17-20 GPa, but the ternary composition leads to a more complex structure. The presence of a low angle diffraction peak in the δ phase demonstrates that the true structure is not simply body-centred cubic. In this way the samples resemble Bi2Se3 where Bi and Se show a high degree of ordering, but the proposed structure of δ-Bi2Se3 also does not fully describe the data for δ-Bi2Te2Se.

Co-auteur étranger; International audience; Knowledge 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.

Phase transitions in lead hafnate (PbHfO3) at high temperature and high pressure were studied by single-crystal diffraction and diffuse scattering of synchrotron radiation, using heated diamond anvil cell. Measurements were carried out in the temperature range 182 ° С

The French National Agency for Research ANR “MODS” (ANR-12-BS10-0005) is thanked for its financial support. An extended series of MIL-53(M)_X metal–organic frameworks (MIL = Materials of Institute Lavoisier; M = Al, Cr, Fe, Sc; X = Cl, CH3, NO2) have been systematically investiaged to explore the impact of the nature of both the metal centre and the functions grafted on to the organic linker on the mechanical behaviour of this family of highly flexible hybrid porous frameworks under the application of an external pressure of up to 3 GPa. The high-pressure X-ray diffraction measurements allowed the characterization of the pressure-induced phase transitions of the hydrated structures, the associated volume changes/pressure transitions, and their mechanical resilience through the determination of their bulk moduli. Postprint Postprint

International audience; We present a new continuously-loaded high-pressure cell for neutron diffraction made from TiZr 'null-matrix' alloy that combines high mechanical resistance below 100 °C and negligible coherent neutron scattering. This cell operates at a maximum pressure of 700 MPa down to a temperature of 1.5 K. A sapphire optical window allows simultaneous illumination of the sample over the broad wavelength range ~0.4 to almost 5 μm. The pressure is applied with Fluorinert or helium gas to ensure the best possible hydrostatic conditions at cryogenic temperatures.

The structural transformation of multiferroic ${\mathrm{EuTiO}}_{3}$ has been intensively investigated by synchrotron x-ray diffraction at pressures up to 50.3 GPa and temperatures from 50 to 500 K. A pressure-induced antiferrodistortive phase transition from cubic $Pm\overline{3}m$ to tetragonal $I4/mcm$ space group has been observed, identical to the one that has been previously explored at ambient pressure and low temperatures. Several compression/decompression cycles at different temperatures have been carried out to accurately map the transition, and as a result a $P\text{\ensuremath{-}}T$ phase diagram for ${\mathrm{EuTiO}}_{3}$ has been constructed. The observed phase transition exhibits a positive $d{P}_{c}/dT$ (${P}_{c}$ being the critical pressure of the transition) and has many similarities with isostructural ${\mathrm{SrTiO}}_{3}$, although the absence of magnetoelectric interactions in the latter accounts for the different phase boundaries between the two materials.

$\mathrm{SrMn}{\mathrm{O}}_{3}$ is a perovskite compound which, unlike most perovskites, can be synthesized in three different but closely related polymorphs. In this paper, an experimental equation of state of the 6H polymorph is reported. The experimentally determined bulk modulus of $\mathrm{SrMn}{\mathrm{O}}_{3}$ increases from 115.6(11) GPa in the 4H polymorph to 143.7(17) GPa in the 6H polymorph, while density functional theory (DFT) calculations predict a further increase to 172.5(4) GPa in the 3C polymorph. In situ observations of transformations between the three known polymorphs, under high pressure and high temperature conditions, are also reported. The results are compared with extensive DFT calculations and literature, and it is demonstrated that the 6H polymorph is the thermodynamically stable phase between 5.9(3) and 18.1(2) GPa at 0 K. The effect of possible oxygen substoichiometry is also explored, using DFT. Finally, the findings are combined with the existing knowledge of the phase behavior in this system to outline where further knowledge needs to be collected before a pressure/temperature (PT) phase diagram can be constructed for this system.

SrMnO 3 is a perovskite compound which, unlike most perovskites, can be synthesized in three different but closely related polymorphs. In this paper, an experimental equation of state of the 6H polymorph is reported. The experimentally determined bulk modulus of SrMnO 3 increases from 115.6(11) GPa in the 4H polymorph to 143.7(17) GPa in the 6H polymorph, while density functional theory (DFT) calculations predict a further increase to 172.5(4) GPa in the 3C polymorph. In situ observations of transformations between the three known polymorphs, under high pressure and high temperature conditions, are also reported. The results are compared with extensive DFT calculations and literature, and it is demonstrated that the 6H polymorph is the thermodynamically stable phase between 5.9(3) and 18.1(2) GPa at 0 K. The effect of possible oxygen substoichiometry is also explored, using DFT. Finally, the findings are combined with the existing knowledge of the phase behavior in this system to outline where further knowledge needs to be collected before a pressure/temperature (PT) phase diagram can be constructed for this system.

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 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.

The exploration of multi-dimensional phase diagrams is a topical subject. However, the simultaneous variation of several control parameters such as temperature, pressure and light irradiation requires a suitably optimized sample environment, particularly when the aim of the experiment is to obtain structural information. We report on a new such sample environment, developed in the context of neutron diffraction measurements, in which the sample can be submitted to pressures up to 7 kbar, temperatures down to 1.7 K and light irradiation in the 660 to 852nm wavelength range, simultaneously. The Ti-Zr alloy pressure cell combines a high mechanical resistance over a wide temperature range with an acceptable neutron background level. The pressure medium is helium gas, ensuring the best possible hydrostatic conditions over a very broad temperature range. The low-temperature environment is obtained from an ILL-type `orange cryostat'. After focusing into an optical fiber, laser light is transmitted to the sample through a sapphire optical window implemented in the pressure cell. The laser flux density at the sample position is of ~30mW/cm2. The geometry of the set-up is optimized to offer a wide optical access (+/- 500vertical, +/-1650horizontal), particularly well suited for Laue neutron diffraction techniques. First results obtained on the pressure-photo-induced spin crossover of a model coordination complex will be presented.