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Water in the mantle transition zone and the core-mantle boundary plays a key role in Earth's stratification, volatile cycling, and core formation. If water transportation is actively running between the aforementioned layers, the lower mantle should contain water channels with distinctive seismic and/or electromagnetic signatures. Here, we investigated the electrical conductivity and sound velocity of ε-FeOOH up to 71 GPa and 1800 K and compared them with global tomography data. An abrupt three-order jump of electrical conductivity was observed above 50 GPa, reaching 1.24(12) × 10

Dicaesium octaiodide is composed of layers of zigzag polyiodide units (I8 2−) intercalated with caesium cations. Each I8 2− unit is built of two triiodides bridged with one diiodine molecules. This system was subjected to compression up to 5.9 GPa under hydrostatic conditions. Pressure alters the supramolecular architecture around I8 2−, leading to bending of the triiodide units away from their energetically preferred geometry (D ∞h). Short I2...I3 − contacts compress significantly, reaching lengths typical for the covalently bonded polyiodides. Unlike in reported structures at ambient conditions, pressure-induced catenation proceeds without symmetrization of the polyiodides, pointing to a different electron-transfer mechanism. The structure is shown to be half as compressible [B0 = 12.9 (4) GPa] than the similar CsI3 structure. The high bulk modulus is associated with higher I—I connectivity and a more compact cationic net, than in CsI3. The small discontinuity in the compressibility trend around 3 GPa suggests formation of more covalent I—I bonds. The potential sources of this discontinuity and its implication on the electronic properties of Cs2I8 are discussed.

The crystal structure of the high-pressure polymorph of meyerhofferite, ideally Ca2B6O6(OH)10·2(H2O), has been determined by means of single-crystal synchrotron X-ray diffraction data. Meyerhofferite undergoes a first-order isosymmetric phase transition to meyerhofferite-II, bracketed between 3.15 and 3.75 GPa, with a large volume discontinuity. The phase transition is marked by an increase in the coordination number of the boron B1 site, from III to IV, leading to a more interconnected and less compressible structure. The main structural differences between the two polymorphs and the P-induced deformation mechanisms at the atomic scale are discussed.

The compression of argon is measured between 10 K and 296 K up to 20 GPa and and up to 114 GPa at 296 K in diamond anvil cells. Three samples conditioning are used: (1) single crystal sample directly compressed between the anvils, (2) powder sample directly compressed between the anvils, (3) single crystal sample compressed in a pressure medium. A partial transformation of the face-centered cubic (fcc) phase to a hexagonal close-packed (hcp) structure is observed above 4.2–13 GPa. Hcp phase forms through stacking faults in fcc-Ar and its amount depends on pressurizing conditions and starting fcc-Ar microstructure. The quasi-hydrostatic equation of state of the fcc phase is well described by a quasi-harmonic Mie–Grüneisen–Debye formalism, with the following 0 K parameters for Rydberg-Vinet equation: $$V_0$$ V 0 = 38.0 Å$$^3$$ 3 /at, $$K_0$$ K 0 = 2.65 GPa, $$K'_0$$ K 0 ′ = 7.423. Under the current experimental conditions, non-hydrostaticity affects measured P–V points mostly at moderate pressure ($$\le$$ ≤ 20 GPa).

The direct chemical reactivity between phosphorus and nitrogen was induced under high-pressure and high-temperature conditions (9.1 GPa and 2000-2500 K), generated by a laser-heated diamond anvil cell and studied by synchrotron X-ray diffraction, Raman spectroscopy, and DFT calculations. α-P

Electric vehicles (EV) demand high charge/discharge rates creating potentially dangerous temperature rises. Lithium-ion cells are sealed during their manufacture, making internal temperatures challenging to probe1. Tracking current collector expansion using X-ray diffraction (XRD) permits non-destructive internal temperature measurements2; however, cylindrical cells are known to experience complex internal strain3,4. Here, we characterise the state-of-charge (SoC), mechanical strain, and temperature within Lithium-ion 18650 cells operated at high rates (>3C) via two advanced synchrotron XRD methods: firstly, as entire cross-sectional temperature maps during open-circuit cooling and secondly, single-point temperatures during charge/discharge cycling. We observed that a 20-minute discharge on an energy-optimised cell (3.5 Ah) resulted in internal temperatures >70 °C, whereas, a faster 12-minute discharge on a power-optimised cell (1.5 Ah) resulted in substantially lower temperatures (< 50 °C). However, when comparing the two cells under the same electrical current, the peak temperatures were similar, e.g., a 6A discharge resulted in 40°C peak temperatures for both cell types. We observe that the operando temperature rise is due to heat accumulation, strongly influenced by the charging protocol e.g., constant-current (CC) and/or constant-voltage (CV). Design mitigations for temperature-related battery issues should now be explored using this novel methodology to provide opportunities for improved thermal management during high-rate EV applications.

High pressure reactivity of phosphorus and hydrogen is relevant to fundamental chemistry, energy conversion and storage, and materials science. Here we report the synthesis of (PH3)2H2, a crystalline van der Waals (vdW) compound (I4cm) made of PH3 and H2 molecules, in a Diamond Anvil Cell by direct catalyst-free high pressure (1.2 GPa) and high temperature (T ≲ 1000 K) chemical reaction of black phosphorus and liquid hydrogen, followed by room T compression above 3.5 GPa. Group 15 elements were previously not known to form H2-containing vdW compounds of their molecular hydrides. The observation of (PH3)2H2, identified by synchrotron X-ray diffraction and vibrational spectroscopy (FTIR, Raman), therefore represents the discovery of a previously missing tile, specifically corresponding to P for pnictogens, in the ability of non-metallic elements to form such compounds. Significant chemical implications encompass reactivity of the elements under extreme conditions, with the observation of the P analogue of the Haber-Bosch reaction for N, fundamental bond theory, and predicted high pressure superconductivity in P-H systems., Chemical elements at high pressure may behave more consistently with their periodic properties than they do at ambient conditions. The authors report the synthesis of PH3 from black phosphorous and hydrogen, and the crystallization of the van der Waals compound (PH3)2H2 which fills a gap in the chemistry of adjacent elements in the periodic table.

International audience; Melting properties of the deep mantle remain controversial due to experimental difficulties; e.g., reports of solidus temperatures of mantle-relevant compositions span over ∼700 K at 2000 km depth. This situation limits our understanding of the thermochemical state of the Earth's interior. Using the laser heated diamond anvil cell (LH-DAC), we performed new experimental determination of the solidus profile of ultra-dry pyrolite and the solidus of two compositions of (Mg,Fe)(Si,Al)O3 bridgmanite (Bg). Melting was detected (i) from -the correlation between laser power and sample temperature, -changes of sample texture and -the level of visible light absorption, for all samples, (ii) using X-ray diffraction, for the MgSiO3 composition and (iii) after scanning electron microscope observations, for selected Fe-bearing samples. Special care was given to using ultra-dry experimental chambers and to determination of sample temperature. In particular, we discuss the wavelength-dependent thermal emission of silicate samples, which lowers the solidus by 100 to 300 K, compared to the grey-body assumption.The solidus of MgSiO3-Bg is in good agreement with previous reports using ab initio calculations and shock wave experiments. We observe a net decrease in the solid-liquid Clapeyron slope at 60(3) GPa and 4400(200) K, which can be related to rapid pressure-induced coordination change of Si in the melt. (Mg0.955,Fe0.045)(Si0.993,Al0.007)O3 Bg melts 600–800 K lower than MgSiO3-Bg. Its solidus evolves smoothly with pressure, suggesting progressive Si coordination change in the melt. In the pressure range investigated (24–135 GPa) Clapeyron slopes suggest rapid decrease of the volume of fusion, from 14 to 2% for MgSiO3 and from 9 to 3% for (Fe,Al)-bearing Bg, assuming congruent melting. By comparing the solidii of various silicates, it appears that the higher the number of cations, the less pronounced is the curvature of the solidus. This observation suggests that the relatively ordered structure of simple liquid compositions with a limited number of distinct network-modifying cations frustrates the coexistence of tetrahedrally and octahedrally coordinated Si polyhedral.The solidus of pyrolite presents a smooth evolution from 2200(100) K to 3950(200) K in the same pressure interval. This is very similar to our previous work on chondritic-type mantle. The new solidus is 200–300 K lower than that of KLB-1 peridotite, which can be related to more incompatible elements in pyrolite. It remains problematic that our solidus plots several 100 K higher than other recent measurements performed on pyrolite; we discuss the possibility of a higher water content in previous samples, compared to our experiments. Assuming a dry lowermost mantle, our results imply a core-mantle boundary temperature lower than 3950(200) K. Modeling the melting diagram at the core-mantle boundary suggests a pseudo-eutectic melt significantly depleted in SiO2, compared to the composition of the mean mantle.

Highly brilliant synchrotron source is indispensable to track pressure-induced phenomena in confined crystalline samples in megabar range. In this article, a number of experimental variables affecting the quality high-pressure single-crystal x-ray diffraction data is discussed. An overview of the recent advancements in x-ray diffraction techniques at extreme conditions, in the frame of European Synchrotron Radiation Facility (ESRF)- Extremely Bright Source (EBS), is presented. Particularly, ID15b and ID27 beamlines have profited from the source upgrade, allowing for measurements of a few-micron crystals in megabar range. In case of ID27, a whole new beamline has been devised, including installation of double-multilayer mirrors and double crystal monochromator and construction of custom-made experimental stations. Two case studies from ID27 and ID15b are presented. Hypervalent CsI3 crystals, studied up to 24 GPa, have shown a series of phase transitions: Pnma → P-3c1→ Pm-3 n. First transition leads to formation of orthogonal linear iodine chains made of I3 -. Transformation to the cubic phase at around 21.7 GPa leads to equalization of interatomic I–I distances and formation of homoleptic In m- chains. The second study investigates elastic properties and structure of jadarite, which undergoes isosymmetric phase transition around 16.6 GPa. Despite a few-micron crystal size, twinning and dramatic loss of crystal quality, associated with pressure-induced phase transitions, crystal structures of both compounds have been determined in a straightforward matter, thanks to the recent developments within ESRF-EBS.

Chemical reactivity between As and N2 , leading to the synthesis of crystalline arsenic nitride, is here reported under high pressure and high temperature conditions generated by laser heating in a Diamond Anvil Cell. Single crystal synchrotron X-ray diffraction at different pressures between 30 and 40 GPa provides evidence for the synthesis of a covalent compound of AsN stoichiometry, crystallizing in a cubic P213 space group, in which each of the two elements is single-bonded to three atoms of the other and hosts an electron lone pair, in a tetrahedral anisotropic coordination. The identification of characteristic structural motifs highlights the key role played by the directional repulsive interactions between non-bonding electron lone pairs in the formation of the AsN structure. Additional data indicate the existence of AsN at room temperature from 9.8 up to 50 GPa. Implications concern fundamental aspects of pnictogens chemistry and the synthesis of innovative advanced materials.

International audience; We report a new technique for torsional testing of materials under giga-pascal pressures, which uses a shearing module in a large volume Paris-Edinburgh press in combination with highresolution fast radiographic X-ray imaging. The measurement of the relative amplitude and phase lag between the cyclic displacement in the sample and a standard material (Al2O3) provides the effective shear modulus and attenuation factor for the sample. The system can operate in the 0.001 to 0.01 Hz frequency range, and up to 5 GPa and 2000 K; although hightemperature measurements may be affected by grain growth and plastic strain. Preliminary experimental results on San Carlos olivine are in quantitative agreement with previously reported Q-1 factors at lower pressure. This cyclic torsional loading method opens new directions to quantify the viscoelastic properties of minerals/rocks at seismic frequencies and in pressure-temperature conditions relevant to the Earth's mantle, for a better interpretation of seismological data.

Journal of the American Chemical Society 143(21), 7920 - 7924 (2021). doi:10.1021/jacs.1c03765, Ge and Sn are unreactive at ambient conditions. Their significant promise for optoelectronic applications is thus largely confined to thin film investigations. We sought to remove barriers to reactivity here by accessing a unique pressure, 10 GPa, where the two elements can adopt the same crystal structure (tetragonal, $I4_1$/$amd$) and exhibit compatible atomic radii. The route to GeSn solid solution, however, even under these directed conditions, is different. Reaction upon heating at 10 GPa occurs between unlike crystal structures (Ge, $Fd3m$ and Sn, $I$4/$mmm$), which also have highly incompatible atomic radii. They should not react, but they do. A reconstructive transformation of $I$4/$mmm$ into the $I$4$_1$/$amd$ solid solution then follows. The new tetragonal GeSn solid solution ($I$4$_1$/$amda$ = 5.280(1) Å, c = 2.915(1) Å, Z = 4 at 9.9 GPa and 298 K) also constitutes the structural and electronic bridge between 4-fold and newly prepared 8-fold coordinated alloy cubic symmetries. Furthermore, using this high-pressure route, bulk cubic diamond-structured GeSn alloys can now be obtained at ambient pressure. The findings here remove confining conventional criteria on routes to synthesis. This opens innovative avenues to advanced materials development., Published by American Chemical Society, Washington, DC

International audience; The pressure-induced structural changes in LiBH4 and in NaBH4 have been investigated experimentally up to 290 GPa by coupling Raman spectroscopy, infrared absorption spectroscopy and synchrotron X-ray diffraction. This data set is also analysed in the light of Density Functional Theory calculations performed up to 600 GPa. The [BH4](-) unit appears to be remarkably resistant under pressure. NaBH4 remains stable in the known Pnma gamma-phase up to 200 GPa and calculations predict a transition to a metallic polymeric C2/c phase at about 480 GPa. LiBH4 is confirmed to exhibit a richer polymorphism. A new Pnma orthorhombic phase VI is found to be stable above 60 GPa and there are hints of a possible phase VII above 160 GPa. DFT calculations predict that two other high pressure LiBH4 phases should appear at about 290 and 428 GPa. A very slight solubility of H-2 inside phases II, III and V of LiBH4 is observed. A NaBH4(H-2)(0.5) complex is predicted to be stable above 150 GPa.

The insertion of oxygen and nitrogen molecules in the one-dimensional (1D) pore system of the zeolite TON was studied at high pressure by vibrational spectroscopy, X-ray diffraction, and Monte Carlo (MC) molecular modeling. Rietveld refinements and MC modeling indicate that, on average, six diatomic molecules per unit cell enter the pores of the zeolite. This induces changes in compressibility and distortion related to the Cmc21-to-Pbn21 phase transition compared to the empty-pore material. The filling behavior with N2 and O2 under pressure is similar to that of argon, suggesting that the kinetic diameter, which is very close in these three systems, plays a major role. The orientation of the diatomic molecules appears to have a rather minor effect on filling occurring at a slightly higher pressure for N2, which has a larger kinetic diameter. Both inserted molecules, initially not showing any marked orientation, begin to exhibit a degree of orientational order above 2 GPa.

Physical review / B 103(13), 134103 (2021). doi:10.1103/PhysRevB.103.134103, Vanadium is reported to undergo a pressure-induced bcc-rhombohedral phase transition at 30–70 GPa, with a transition pressure that is sensitive to the hydrostaticity of the sample environment. However, the experimental evidence for the structure of the high-pressure phase being rhombohedral is surprisingly weak. We have restudied vanadium under pressure to 154 GPa using both polycrystalline and single-crystal samples, and a variety of different pressure transmitting media (PTM). We find that only when using single-crystal samples does one observe a rhombohedral high-pressure phase; the high-pressure diffraction profiles from the polycrystalline samples do not fit a rhombohedral lattice, irrespective of the PTM used. The single-crystal samples reveal two rhombohedral phases, with a continuous transition between them, and distortions from cubic symmetry are much smaller than previously calculated., Published by Inst., Woodbury, NY

The high-pressure behaviour of meyerhofferite [ideally Ca2B6O6(OH)10·2(H2O), with a ~ 6.63 A, b ~ 8.34 A, c ~ 6.47 A, α = 90.8°, β = 102°, γ = 86.8°, Sp. Gr. P $$\overline{1}$$ ], a B-bearing raw material (with B2O3 ≈ 46 wt%) and a potential B-rich aggregate, has been studied by single-crystal synchrotron X-ray diffraction up to 9 GPa, under hydrostatic conditions. Meyerhofferite undergoes a first-order phase transition to meyerhofferite-II, likely iso-symmetric, bracketed between 3.05 and 3.48 GPa, with a spectacular unit-cell volume discontinuity (i.e., ΔV ~ 30 A3). The isothermal bulk modulus (KV0 = β−1P0,T0, where βP0,T0 is the volume compressibility coefficient) of meyerhofferite, was found to be KV0 = 31.6(5) GPa, and a marked anisotropic compressional pattern, with K(a)0: K(b)0: K(c)0 ~ 1.5:1:3, was observed. The bulk modulus of meyerhofferite-II increases to 55(2) GPa and, differently to the majority of the borates studied at high pressure so far, the anisotropic compressional pattern of meyerhofferite decreases markedly in the high-pressure form. The P-induced deformation mechanisms controlling, at the atomic scale, the bulk compression of meyerhofferite are here described. Considerations about the use of meyerhofferite as a potential B-based aggregate in concretes, mortars or resins, are provided.

Herein we show that ${\mathrm{Mo}}_{4}{\mathrm{Ce}}_{4}{\mathrm{Al}}_{7}{\mathrm{C}}_{3}$, a recently discovered nanolamellar compound displaying mixed valence, combines Kondo lattice behavior with ferromagnetism. A sizeable magnetization is carried by $3p$ states of Al as evidenced by a strong x-ray magnetic circular dichroism signal at the $K$ edge of aluminum, whereas no detectable signal was observed at the $K$ edge of carbon and ${L}_{2,3}$ edges of molybdenum. These results point out that the ferromagnetic behavior originates in the Ce atoms with $4{f}^{1}$ electronic configuration lying within the Al planes. The evolution with pressure of the mixed valence of Ce atoms in the Mo-C planes determined via Ce ${L}_{3}$ x-ray-absorption spectra along with the magnetoresistance measurements across the ferromagnetic transition unambiguously reveal a Kondo behavior. Angle-resolved photoemission spectroscopy and density functional theory confirm a certain degree of Ce $4f$ electron delocalization. More generally, conduction electrons are not restricted to lie in the MoC planes but are also delocalized in the Al planes.

International audience; The liquid-liquid transition (LLT), in which a single-component liquid transforms into another one via a first-order phase transition, is an intriguing phenomenon that has changed our perception of the liquid state. LLTs have been predicted from computer simulations of water 1,2 , silicon 3 , carbon dioxide 4 , carbon 5 , hydrogen 6 and nitrogen 7. Experimental evidence has been found mostly in supercooled (that is, metastable) liquids such as Y 2 O 3-Al 2 O 3 mixtures 8 , water 9 and other molecular liquids 10-12. However, the LLT in supercooled liquids often occurs simultaneously with crystallization, making it difficult to separate the two phenomena 13. A liquid-liquid critical point (LLCP), similar to the gas-liquid critical point, has been predicted at the end of the LLT line that separates the low-and high-density liquids in some cases, but has not yet been experimentally observed for any materials. This putative LLCP has been invoked to explain the thermodynamic anomalies of water 1. Here we report combined in situ density, X-ray diffraction and Raman scattering measurements that provide direct evidence for a first-order LLT and an LLCP in sulfur. The transformation manifests itself as a sharp density jump between the low-and high-density liquids and by distinct features in the pair distribution function. We observe a non-monotonic variation of the density jump with increasing temperature: it first increases and then decreases when moving away from the critical point. This behaviour is linked to the competing effects of density and entropy in driving the transition. The existence of a first-order LLT and a critical point in sulfur could provide insight into the anomalous behaviour of important liquids such as water.

We present a study on the phase stability of dense carbon dioxide (${\mathrm{CO}}_{2}$) at extreme pressure-temperature conditions, up to 6200 K within the pressure range $37\ifmmode\pm\else\textpm\fi{}9$ to $106\ifmmode\pm\else\textpm\fi{}17\text{ }\text{ }\mathrm{GPa}$. The investigations of high-pressure high-temperature in situ x-ray diffraction patterns recorded from laser-heated ${\mathrm{CO}}_{2}$, as densified in diamond-anvil cells, consistently reproduced the exclusive formation of polymeric tetragonal ${\mathrm{CO}}_{2}$-V at any condition achieved in repetitive laser-heating cycles. Using well-considered experimental arrangements, which prevent reactions with metal components of the pressure cells, annealing through laser heating was extended individually up to approximately 40 min per cycle in order to keep track of upcoming instabilities and changes with time. The results clearly exclude any decomposition of ${\mathrm{CO}}_{2}$-V into the elements as previously suggested. Alterations of the Bragg peak distribution on Debye-Scherrer rings indicate grain coarsening at temperatures $g4000\text{ }\text{ }\mathrm{K}$, giving a glimpse of the possible extension of the stability of the polymeric solid phase.

The phase diagram and melting curve of water ice is investigated up to 45 GPa and 1600 K by synchrotron x-ray diffraction in the resistively and laser heated diamond anvil cell. Our melting data evidence a triple point at 14.6 GPa, 850 K. The latter is shown to be related to a first-order solid transition from the dynamically disordered form of ice VII, denoted ice ${\mathrm{VII}}^{\ensuremath{'}}$, toward a high-temperature phase with the same bcc oxygen lattice but larger volume and higher entropy. Our experiments are compared to ab initio molecular dynamics simulations, enabling us to identify the high-temperature bcc phase with the predicted superionic ice ${\mathrm{VII}}^{\ensuremath{'}\ensuremath{'}}$ phase [J.-A. Hernandez and R. Caracas, Phys. Rev. Lett. 117, 135503 (2016).].

Electron-phonon interaction is of central importance for the electrical and heat transport properties of metals, and is directly responsible for charge-density-waves or (conventional) superconducting instabilities. The direct observation of phonon dispersion anomalies across electronic phase transitions can provide insightful information regarding the mechanisms underlying their formation. Here, we review the current status of phonon dispersion studies in superconductors under hydrostatic and uniaxial pressure. Advances in the instrumentation of high resolution inelastic X-ray scattering beamlines and pressure generating devices allow these measurements to be performed routinely at synchrotron beamlines worldwide., 8 pages, 6 figures. Invited review submitted to Superconductor Science and Technology, Focus issue on Hydride & High-Pressure Superconductors. References in figure caption fixed. Hyperlinks added

International audience; Properties of liquid silicates under high pressure and high temperature conditions are critical for modeling the dynamics and solidification mechanisms of the magma ocean in the early Earth, as well as for constraining entrainment of melts in the mantle and in the present-day core-mantle boundary. Here, we present in situ structural measurements by X-ray diffraction of selected amorphous silicates compressed statically in diamond anvil cells (up to 157 GPa at room temperature) or dynamically by laser-generated shock compression (up to 130 GPa and 6000 K along the MgSiO3 glass Hugoniot). The X-ray diffraction patterns of silicate glasses and liquids reveal similar characteristics over a wide pressure and temperature range. Beyond the increase in Si coordination observed at 20 GPa, we find no evidence for major structural changes occurring in the silicate melts studied up to pressures and temperatures exceeding Earth's core mantle boundary conditions. This result is supported by molecular dynamics calculations. Our findings reinforce the widely-used assumption that the silicate glasses studies are appropriate structural analogs for understanding the atomic arrangement of silicate liquids at these high pressures.

Single crystals of solid chlorine (Cl2) were synthesized at room temperature and high pressure (HP, P=1.45 GPa) in a diamond anvil cell (DAC). At these conditions Cl2 adapts the same structure as its corresponding low-temperature (LT) ambient pressure modification (TCmce spacegroup) with Cl2 molecules forming monolayers parallel to the bc plane and this structure is preserved up to at least 64 GPa. The pressure of 1.45 GPa is to be considered as a solidification point of liquid Cl2 at room temperature.

Iron oxides are among the major constituents of the deep Earth’s interior. Among them, the epsilon phase of Fe2O3 is one of the less studied polymorphs and there is a lack of information about its structural, electronic and magnetic transformations at extreme conditions. Here we report the precise determination of its equation of state and a deep analysis of the evolution of the polyhedral units under compression, thanks to the agreement between our experiments and ab-initio simulations. Our results indicate that this material, with remarkable magnetic properties, is stable at pressures up to 27 GPa. Above 27 GPa, a volume collapse has been observed and ascribed to a change of the local environment of the tetrahedrally coordinated iron towards an octahedral coordination, finding evidence for a different iron oxide polymorph., Iron oxides exist in a variety of polymorphs at different pressure and temperature conditions, displaying important magnetic properties, and are major constituents of the Earth’s interior. Here the authors investigate the structural and electronic changes in the uncommon epsilon phase under compression to deep Mantle pressures.

In this article we present a direct comparison between reflective (Schwarzschild mirrors) and refractive (achromatic doublets) optics commonly used in spectroradiometric temperature measurements in...

The high pressure chemistry of transition metals and nitrogen was recently discovered to be richer than previously thought, due to the synthesis of several transition metal pernitrides. Here, we explore the pressure-temperature domain of iron with an excess of nitrogen up to 91 GPa and 2200 K. Above 72 GPa and 2200 K, the iron pernitride FeN2 is produced in a laser-heated diamond anvil cell. This iron-nitrogen compound is the first with a N/Fe ratio greater than 1. The FeN2 samples were characterized from the maximum observed pressure down to ambient conditions by powder X-ray diffraction and Raman spectroscopy measurements. The crystal structure of FeN2 is resolved to be a Pnnm marcasite structure, analogously to other transition metal pernitrides. On the basis of the lattice’s axial ratios and the recorded N-N vibrational modes of FeN2, a bond order of 1.5 for the nitrogen dimer is suggested. The bulk modulus of the iron pernitride is determined to be of K0 = 344(13) GPa, corresponding to an astounding ...

Rb0.5Cs2.5C60 is a member of the family of face-centred-cubic (fcc)–structured alkali fullerides, A3C60 (A = alkali metal) with a highly expanded lattice size. At ambient temperature and pressure, it is a Mott–Jahn–Teller insulator. However, upon cooling it exhibits first a crossover to an anomalous metallic state (Jahn–Teller metal) in the vicinity of 90 K followed next by a transition to a bulk superconductor with a transition temperature, Tc of 29.4 K. Here we study its structural and electronic response to the application of pressure. Synchrotron X-ray powder diffraction at ambient temperature shows that the same Mott-insulator–metal electronic state crossover can be induced through pressure application under isothermal conditions, as evidenced by a distinct broad symmetry-preserving compressibility anomaly, which sets in at ∼0.4 GPa and extends to ∼0.75 GPa. Complementary magnetic susceptibility measurements also reveal that the crossover temperature, T′ tunable at ambient pressure through adjusting the dopant ratio to vary the unit cell volume, can be controlled through pressure application and be mapped onto the same global electronic phase diagram. The observed electronic response, as a function of both physical and ‘chemical’ pressure, is thus of the same electronic origin, namely the control of the bandwidth, W via outer wave function overlap of the constituent fulleride ions.