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Laser heating in a diamond anvil cell (DAC) is a common method for studying material behavior at high-pressure and high-temperature conditions. It has been previously proven that during continuous wave (CW) laser heating of a sample, carbon of the diamond anvils is mobilized, and its diffusion into the sample can lead to undesirable chemical reactions, which, if not detected, may cause misinterpretations of the results of the experiment. Minimizing the heating time with the use of a pulsed laser (PL) is thought to reduce the risk of possible carbon contamination of the sample; however, this has not been proven experimentally. Here, we report the results of our comparative study of the effect of pulsed and continuous wave (CW) laser heating on the mobilization of carbon and its chemical interaction with iron in a diamond anvil cell. Using X-ray absorption near edge structure spectroscopy, Synchrotron Mössbauer Source spectroscopy, and Synchrotron X-ray diffraction, we examined iron samples that were laser heated in DACs in various pressure transmitting media (neon, argon, and potassium chloride). According to our results, the use of the PL heating does not prevent the sample from carbon contamination. A reaction between carbon and iron happens within a few seconds even at moderate temperatures. We found that one analytical technique was generally insufficient to fully characterize the phase composition of the laser-heated samples. [ABSTRACT FROM AUTHOR]

Mg-carbonate is one of the main carbonates subducting from the Earth's surface into the mantle and possible reactions of this compound with the main mantle minerals determine the stability of carbonates into the Earth's mantle. Recently, we have theoretically shown that at pressures and temperatures of the Earth's lower mantle MgCO3 should react with MgO, producing Mg2CO4. Here, using diamond anvil cell technique, we have performed in situ investigation of the product of MgCO3 + MgO reaction at pressures around 50 GPa and temperatures above 2000 K. Investigation of Raman and X-ray diffraction patterns unambiguously indicates that reaction has been realized and as the reaction product we suggest the modification similar to M-g2CO4- . Using the transmission electron microscopy technique, we show the presence of the new Mg-orthocarbonate/carbonate phase in the sample extracted from high-pressure environment. [ABSTRACT FROM AUTHOR]

Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides, resulting in numerous compounds forming homologous series nFeOmFe2O3 and the appearance of FeO2. Here, based on the results of in situ single-crystal x-ray diffraction, Mössbauer spectroscopy, x-ray absorption spectroscopy, and density-functional theory+dynamical mean-field theory calculations, we demonstrate that iron in high-pressure cubic FeO2 and isostructural FeO2H0.5 is ferric (Fe3+), and oxygen has a formal valence less than 2. Reduction of oxygen valence from 2, common for oxides, down to 1.5 can be explained by a formation of a localized hole at oxygen sites. [ABSTRACT FROM AUTHOR]

High-energy-density materials (HEDMs) are crucial in various applications, from energy storage to defense technologies. Transition metal polynitrides are promising candidates for HEDMs. Using single-crystal synchrotron X-ray diffraction, we investigated the crystal structures of niobium nitride, specifically Nb 2 N 3 and NbN 2 , under high-pressure conditions of up to 86 GPa. At higher pressures, niobium polynitrides NbN 4 and NbN 5 were observed to be stable from 100 to 120 GPa, which feature low-order nitrogen bonding. The low-order bonded nitrogen in NbN 4 and NbN 5 forms multiple polynitrogen anions at megabar pressure ranges. In the Nb-N system, we observed an increasing coordination number of metal-nitrogen as pressure increased. These structures were supported by density functional theory (DFT) calculations and Raman spectroscopy.

Inorganic ternary metal-C-N compounds with covalently bonded C-N anions encompass important classes of solids such as cyanides and carbodiimides, well known at ambient conditions and composed of [CN] - and [CN 2 ] 2- anions, as well as the high-pressure formed guanidinates featuring [CN 3 ] 5- anion. At still higher pressures, carbon is expected to be 4-fold coordinated by nitrogen atoms, but hitherto, such CN 4 -built anions are missing. In this study, four polycarbonitride compounds (LaCN 3 , TbCN 3 , CeCN 5 , and TbCN 5 ) are synthesized in laser-heated diamond anvil cells at pressures between 90 and 111 GPa. Synchrotron single-crystal X-ray diffraction (SCXRD) reveals that their crystal structures are built of a previously unobserved anionic single-bonded carbon-nitrogen three-dimensional (3D) framework consisting of CN 4 tetrahedra connected via di- or oligo-nitrogen linkers. A crystal-chemical analysis demonstrates that these polycarbonitride compounds have similarities to lanthanide silicon phosphides. Decompression experiments reveal the existence of LaCN 3 and CeCN 5 compounds over a very large pressure range. Density functional theory (DFT) supports these discoveries and provides further insight into the stability and physical properties of the synthesized compounds.

In this study, we conduct extensive high-pressure experiments to investigate phase stability in the cobalt-nitrogen system. Through a combination of synthesis in a laser-heated diamond anvil cell, first-principles calculations, Raman spectroscopy, and single-crystal X-ray diffraction, we establish the stability fields of known high-pressure phases, hexagonal NiAs-type CoN, and marcasite-type CoN 2 within the pressure range of 50-90 GPa. We synthesize and characterize previously unknown nitrides, Co 3 N 2 , Pnma-CoN and two polynitrides, CoN 3 and CoN 5 , within the pressure range of 90-120 GPa. Both polynitrides exhibit novel types of polymeric nitrogen chains and networks. CoN 3 feature branched-type nitrogen trimers (N 3 ) and CoN 5 show π-bonded nitrogen chain. As the nitrogen content in the cobalt nitride increases, the CoN 6 polyhedral frameworks transit from face-sharing (in CoN) to edge-sharing (in CoN 2 and CoN 3 ), and finally to isolated (in CoN 5 ). Our study provides insights into the intricate interplay between structure evolution, bonding arrangements, and high-pressure synthesis in polynitrides, expanding the knowledge for the development of advanced energy materials., (© 2024 Wiley-VCH GmbH.)

We have performed in situ X-ray diffraction measurements of cubic silicon carbide (SiC) with a zinc-blende crystal structure (B3) at high pressures and temperatures using multi-anvil apparatus. The ambient volume inferred from the compression curves is smaller than that of the starting material. Using the 3 rd -order Birch-Murnaghan equation of state and the Mie-Grüneisen-Debye model, we have determined the thermoelastic parameters of the B3-SiC to be K 0 =228±3 GPa, K 0 ',=4.4±0.4, q=0.27±0.37, where K 0 , K 0 ' and q are the isothermal bulk modulus, its pressure derivative and logarithmic volume dependence of the Grüneisen parameter, respectively. Using the 3 rd -order Birch-Murnaghan EOS with the thermal expansion coefficient, the thermoelastic parameters have been found as K 0 =221±3 GPa, K 0 ',=5.2±0.4, α 0 =0.90±0.02 ⋅ 10 -5  ⋅ K -1 , where α 0 is the thermal expansion coefficient at room pressure and temperature. We have determined that paired B3-SiC - MgO calibrants can be used to estimate pressure and temperature simultaneously in ultrahigh-pressure experiments up to 60 GPa., (© 2024 The Authors. ChemPhysChem published by Wiley-VCH GmbH.)

Metal carbides are known to contain small carbon units similar to those found in the molecules of methane, acetylene, and allene. However, for numerous binary systems ab initio calculations predict the formation of unusual metal carbides with exotic polycarbon units, [C 6 ] rings, and graphitic carbon sheets at high pressure (HP). Here we report the synthesis and structural characterization of a HP-CaC 2 polymorph and a Ca 3 C 7 compound featuring deprotonated polyacene-like and para-poly(indenoindene)-like nanoribbons, respectively. We also demonstrate that carbides with infinite chains of fused [C 6 ] rings can exist even at conditions of deep planetary interiors ( ~ 140 GPa and ~3300 K). Hydrolysis of high-pressure carbides may provide a possible abiotic route to polycyclic aromatic hydrocarbons in Universe., (© 2024. The Author(s).)

The reaction between PrO 2 and SiO 2 was investigated at various pressure points up to 29 GPa in a diamond anvil cell using laser heating and in situ single-crystal structure analysis. The pressure points at 5 and 10 GPa produced Pr 2 III (Si 2 O 7 ), whereas Pr 4 III Si 3 O 12 and Pr 2 IV (O 2 )O 3 were obtained at 15 GPa. Pr 4 III Si 3 O 12 can be interpreted as a high-pressure modification of the still unknown orthosilicate Pr 4 III (SiO 4 ) 3 . Pr IV Si 3 O 8 and Pr 2 IV Si 7 O 18 that contain praseodymium in its rare + IV oxidation state were identified at 29 GPa. After the pressure was released from the reaction chamber, the Pr(IV) silicates could be recovered, indicating that they are metastable at ambient pressure. Density functional theory calculations of the electronic structure corroborate the oxidation state of praseodymium in both Pr IV Si 3 O 8 and Pr 2 IV Si 7 O 18 . Both silicates are the first structurally characterized representatives of Pr 4+ -containing salts with oxoanions. All three silicates contain condensed networks of [SiO 6 ] octahedra which is unprecedented in the rich chemistry of lanthanoid silicates.

The yttrium-hydrogen system has gained attention because of near-ambient temperature superconductivity reports in yttrium hydrides at high pressures. We conducted a study using synchrotron single-crystal x-ray diffraction (SCXRD) at 87 to 171 GPa, resulting in the discovery of known (two YH 3 phases) and five previously unknown yttrium hydrides. These were synthesized in diamond anvil cells by laser heating yttrium with hydrogen-rich precursors-ammonia borane or paraffin oil. The arrangements of yttrium atoms in the crystal structures of new phases were determined on the basis of SCXRD, and the hydrogen content estimations based on empirical relations and ab initio calculations revealed the following compounds: Y 3 H 11 , Y 2 H 9 , Y 4 H 23 , Y 13 H 75 , and Y 4 H 25 . The study also uncovered a carbide (YC 2 ) and two yttrium allotropes. Complex phase diversity, variable hydrogen content in yttrium hydrides, and their metallic nature, as revealed by ab initio calculations, underline the challenges in identifying superconducting phases and understanding electronic transitions in high-pressure synthesized materials.

Carbon nitrides featuring three-dimensional frameworks of CN 4 tetrahedra are one of the great aspirations of materials science, expected to have a hardness greater than or comparable to diamond. After more than three decades of efforts to synthesize them, no unambiguous evidence of their existence has been delivered. Here, the high-pressure high-temperature synthesis of three carbon-nitrogen compounds, tI14-C 3 N 4 , hP126-C 3 N 4 , and tI24-CN 2 , in laser-heated diamond anvil cells, is reported. Their structures are solved and refined using synchrotron single-crystal X-ray diffraction. Physical properties investigations show that these strongly covalently bonded materials, ultra-incompressible and superhard, also possess high energy density, piezoelectric, and photoluminescence properties. The novel carbon nitrides are unique among high-pressure materials, as being produced above 100 GPa they are recoverable in air at ambient conditions., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

A series of isostructural Ln 3 O 2 (CN 3 ) (Ln=La, Eu, Gd, Tb, Ho, Yb) oxoguanidinates was synthesized under high-pressure (25-54 GPa) high-temperature (2000-3000 K) conditions in laser-heated diamond anvil cells. The crystal structure of this novel class of compounds was determined via synchrotron single-crystal X-ray diffraction (SCXRD) as well as corroborated by X-ray absorption near edge structure (XANES) measurements and density functional theory (DFT) calculations. The Ln 3 O 2 (CN 3 ) solids are composed of the hitherto unknown CN 3 5- guanidinate anion-deprotonated guanidine. Changes in unit cell volumes and compressibility of Ln 3 O 2 (CN 3 ) (Ln=La, Eu, Gd, Tb, Ho, Yb) compounds are found to be dictated by the lanthanide contraction phenomenon. Decompression experiments show that Ln 3 O 2 (CN 3 ) compounds are recoverable to ambient conditions. The stabilization of the CN 3 5- guanidinate anion at ambient conditions provides new opportunities in inorganic and organic synthetic chemistry., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The stabilization of nitrogen-rich phases presents a significant chemical challenge due to the inherent stability of the dinitrogen molecule. This stabilization can be achieved by utilizing strong covalent bonds in complex anions with carbon, such as cyanide CN - and NCN 2- carbodiimide, while more nitrogen-rich carbonitrides are hitherto unknown. Following a rational chemical design approach, we synthesized antimony guanidinate SbCN 3 at pressures of 32-38 GPa using various synthetic routes in laser-heated diamond anvil cells. SbCN 3 , which is isostructural to calcite CaCO 3 , can be recovered under ambient conditions. Its structure contains the previously elusive guanidinate anion [CN 3 ] 5- , marking a fundamental milestone in carbonitride chemistry. The crystal structure of SbCN 3 was solved and refined from synchrotron single-crystal X-ray diffraction data and was fully corroborated by theoretical calculations, which also predict that SbCN 3 has a direct band gap with the value of 2.20 eV. This study opens a straightforward route to the entire new family of inorganic nitridocarbonates., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The inorganic pyrocarbonate salt Ba[C 2 O 5 ] contains twisted pyrocarbonate anions ([C 2 O 5 ] 2- ), an atomic arrangement previously not observed in other pyrocarbonates. This unexpected additional structural degree of freedom points towards an enlarged chemical variability in this novel group of compounds. Ba[C 2 O 5 ] was synthesized in a laser-heated diamond anvil cell at 30(2) GPa by heating a mixture of Ba[CO 3 ] + CO 2 to ≈ 1500(200) K. Its crystal structure was solved from single crystal synchrotron X-ray diffraction data and confirmed by density functional theory-based calculations. The two planar [CO 3 ] 2- -groups of the [C 2 O 5 ] 2- -anion are strongly twisted around the bridging oxygen atom. Ba[C 2 O 5 ] has been observed in the pressure range of 5-30 GPa, where its symmetry is P 6/ m with Z = 12.

The allotropy of solid molecular nitrogen is the consequence of a complex interplay between fundamental intermolecular as well as intramolecular interactions. Understanding the underlying physical mechanisms hinges on knowledge of the crystal structures of these molecular phases. That is especially true for ζ-N 2 , key to shed light on nitrogen's polymerization. Here, we perform single-crystal X-ray diffraction on laser-heated N 2 samples at 54, 63, 70 and 86 GPa and solve and refine the hitherto unknown structure of ζ-N 2 . In its monoclinic unit cell (space group C2/c), 16 N 2 molecules are arranged in a configuration similar to that of ε-N 2 . The structure model provides an explanation for the previously identified Raman and infrared lattice and vibrational modes of ζ-N 2 . Density functional theory calculations give an insight into the gradual delocalization of electronic density from intramolecular bonds to intermolecular space and suggest a possible pathway towards nitrogen's polymerization., (© 2023. Springer Nature Limited.)

Chemical reactions between dysprosium and carbon were studied in laser-heated diamond anvil cells at pressures of 19, 55, and 58 GPa and temperatures of ∼2500 K. In situ single-crystal synchrotron X-ray diffraction analysis of the reaction products revealed the formation of novel dysprosium carbides, Dy 4 C 3 and Dy 3 C 2 , and dysprosium sesquicarbide Dy 2 C 3 previously known only at ambient conditions. The structure of Dy 4 C 3 was found to be closely related to that of dysprosium sesquicarbide Dy 2 C 3 with the Pu 2 C 3 -type structure. Ab initio calculations reproduce well crystal structures of all synthesized phases and predict their compressional behavior in agreement with our experimental data. Our work gives evidence that high-pressure synthesis conditions enrich the chemistry of rare earth metal carbides., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Akbar, Aslandukova, Aslandukov, Yin, Trybel, Khandarkhaeva, Fedotenko, Laniel, Bykov, Bykova, Dubrovinskaia and Dubrovinsky.)

The field of polyhalogen chemistry, specifically polyhalogen anions (polyhalides), is rapidly evolving. Here, we present the synthesis of three sodium halides with unpredicted chemical compositions and structures ( tP 10-Na 2 Cl 3 , hP 18-Na 4 Cl 5 , and hP 18-Na 4 Br 5 ), a series of isostructural cubic cP 8-AX 3 halides (NaCl 3 , KCl 3 , NaBr 3 , and KBr 3 ), and a trigonal potassium chloride ( hP 24-KCl 3 ). The high-pressure syntheses were realized at 41-80 GPa in diamond anvil cells laser-heated at about 2000 K. Single-crystal synchrotron X-ray diffraction (XRD) provided the first accurate structural data for the symmetric trichloride Cl 3 - anion in hP 24-KCl 3 and revealed the existence of two different types of infinite linear polyhalogen chains, [Cl] ∞ n - and [Br] ∞ n - , in the structures of cP 8-AX 3 compounds and in hP 18-Na 4 Cl 5 and hP 18-Na 4 Br 5 . In Na 4 Cl 5 and Na 4 Br 5 , we found unusually short, likely pressure-stabilized, contacts between sodium cations. Ab initio calculations support the analysis of structures, bonding, and properties of the studied halogenides., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

The recent high-pressure synthesis of pentazolates and the subsequent stabilization of the aromatic [N 5 ] - anion at atmospheric pressure have had an immense impact on nitrogen chemistry. Other aromatic nitrogen species have also been actively sought, including the hexaazabenzene N 6 ring. Although a variety of configurations and geometries have been proposed based on ab initio calculations, one that stands out as a likely candidate is the aromatic hexazine anion [N 6 ] 4- . Here we present the synthesis of this species, realized in the high-pressure potassium nitrogen compound K 9 N 56 formed at high pressures (46 and 61 GPa) and high temperature (estimated to be above 2,000 K) by direct reaction between nitrogen and KN 3 in a laser-heated diamond anvil cell. The complex structure of K 9 N 56 -composed of 520 atoms per unit cell-was solved based on synchrotron single-crystal X-ray diffraction and corroborated by density functional theory calculations. The observed hexazine anion [N 6 ] 4- is planar and proposed to be aromatic., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

Ferropericlase (Mg,Fe)O is the second most abundant mineral in Earth's lower mantle and a common inclusion found in subcratonic diamonds. Pyrolitic mantle has Mg# (100 × Mg/(Mg+Fe)) ~89. However, ferropericlase inclusions in diamonds show a broad range of Mg# between 12 and 93. Here we use Synchrotron Mössbauer Source (SMS) spectroscopy and single-crystal X-ray diffraction to determine the iron oxidation state and structure of two magnesiowüstite and three ferropericlase inclusions in diamonds from São Luiz, Brazil. Inclusion Mg#s vary between 16.1 and 84.5. Ferropericlase inclusions contain no ferric iron within the detection limit of SMS, while both magnesiowüstite inclusions show the presence of monocrystalline magnesioferrite ((Mg,Fe)Fe 3+ 2 O 4 ) with an estimated 47-53 wt% Fe 2 O 3 . We argue that the wide range of Fe concentrations observed in (Mg,Fe)O inclusions in diamonds and the appearance of magnesioferrite result from oxidation of ferropericlase triggered by the introduction of subducted material into sublithospheric mantle., (© 2022. The Author(s).)

The lanthanum-hydrogen system has attracted significant attention following the report of superconductivity in LaH 10 at near-ambient temperatures and high pressures. Phases other than LaH 10 are suspected to be synthesized based on both powder X-ray diffraction and resistivity data, although they have not yet been identified. Here, we present the results of our single-crystal X-ray diffraction studies on this system, supported by density functional theory calculations, which reveal an unexpected chemical and structural diversity of lanthanum hydrides synthesized in the range of 50 to 180 GPa. Seven lanthanum hydrides were produced, LaH 3 , LaH ~4 , LaH 4+δ , La 4 H 23 , LaH 6+δ , LaH 9+δ , and LaH 10+δ , and the atomic coordinates of lanthanum in their structures determined. The regularities in rare-earth element hydrides unveiled here provide clues to guide the search for other synthesizable hydrides and candidate high-temperature superconductors. The hydrogen content variability in lanthanum hydrides and the samples' phase heterogeneity underline the challenges related to assessing potentially superconducting phases and the nature of electronic transitions in high-pressure hydrides., (© 2022. The Author(s).)

Invited for the cover of this issue are Dominique Laniel (University of Edinburgh), Florian Trybel (University of Linköping), and their colleagues. The image depicts a bridge built of the newly discovered δ-P 3 N 5 solid with the structure featuring PN 6 units, a previously missing connection between the carbon group elements nitrides and chalcogens nitrides. Read the full text of the article at 10.1002/chem.202201998., (© 2022 Wiley-VCH GmbH.)

A gasket is an important constituent of a diamond anvil cell (DAC) assembly, responsible for the sample chamber stability at extreme conditions for X-ray diffraction studies. In this work, we studied the performance of gaskets made of metallic glass Fe 0.79 Si 0.07 B 0.14 in a number of high-pressure X-ray diffraction (XRD) experiments in DACs equipped with conventional and toroidal-shape diamond anvils. The experiments were conducted in either axial or radial geometry with X-ray beams of micrometre to sub-micrometre size. We report that Fe 0.79 Si 0.07 B 0.14 metallic glass gaskets offer a stable sample environment under compression exceeding 1 Mbar in all XRD experiments described here, even in those involving small-molecule gases (e.g. Ne, H 2 ) used as pressure-transmitting media or in those with laser heating in a DAC. Our results emphasize the material's importance for a great number of delicate experiments conducted under extreme conditions. They indicate that the application of Fe 0.79 Si 0.07 B 0.14 metallic glass gaskets in XRD experiments for both axial and radial geometries substantially improves various aspects of megabar experiments and, in particular, the signal-to-noise ratio in comparison to that with conventional gaskets made of Re, W, steel or other crystalline metals., (open access.)

Two novel yttrium nitrides, YN 6 and Y 2 N 11 , were synthesized by direct reaction between yttrium and nitrogen at 100 GPa and 3000 K in a laser-heated diamond anvil cell. High-pressure synchrotron single-crystal X-ray diffraction revealed that the crystal structures of YN 6 and Y 2 N 11 feature a unique organization of nitrogen atoms-a previously unknown anionic N 18 macrocycle and a polynitrogen double helix, respectively. Density functional theory calculations, confirming the dynamical stability of the YN 6 and Y 2 N 11 compounds, show an anion-driven metallicity, explaining the unusual bond orders in the polynitrogen units. As the charge state of the polynitrogen double helix in Y 2 N 11 is different from that previously found in Hf 2 N 11 and because N 18 macrocycles have never been predicted or observed, their discovery significantly extends the chemistry of polynitrides., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Theoretical modelling predicts very unusual structures and properties of materials at extreme pressure and temperature conditions 1,2 . Hitherto, their synthesis and investigation above 200 gigapascals have been hindered both by the technical complexity of ultrahigh-pressure experiments and by the absence of relevant in situ methods of materials analysis. Here we report on a methodology developed to enable experiments at static compression in the terapascal regime with laser heating. We apply this method to realize pressures of about 600 and 900 gigapascals in a laser-heated double-stage diamond anvil cell 3 , producing a rhenium-nitrogen alloy and achieving the synthesis of rhenium nitride Re 7 N 3 -which, as our theoretical analysis shows, is only stable under extreme compression. Full chemical and structural characterization of the materials, realized using synchrotron single-crystal X-ray diffraction on microcrystals in situ, demonstrates the capabilities of the methodology to extend high-pressure crystallography to the terapascal regime., (© 2022. The Author(s).)

The success of high-pressure research relies on the inventive design of pressure-generating instruments and materials used for their construction. In this study, the anvils of conical frustum or disk shapes with flat or modified culet profiles (toroidal or beveled) were prepared by milling an Ia-type diamond plate made of a (100)-oriented single crystal using the focused ion beam. Raman spectroscopy and synchrotron x-ray diffraction were applied to evaluate the efficiency of the anvils for pressure multiplication in different modes of operation: as single indenters forced against the primary anvil in diamond anvil cells (DACs) or as pairs of anvils forced together in double-stage DACs (dsDACs). All types of secondary anvils performed well up to about 250 GPa. The pressure multiplication factor of single indenters appeared to be insignificantly dependent on the shape of the anvils and their culets' profiles. The enhanced pressure multiplication factor found for pairs of toroidally shaped secondary anvils makes this design very promising for ultrahigh-pressure experiments in dsDACs.

The high-pressure behavior of simple molecular systems, devoid of strong intermolecular interactions, provides a unique avenue toward a fundamental understanding of matter. Tetrahalides of the carbon group elements (group 14), lacking all intermolecular interactions but van der Waals, are among the most elementary of molecular compounds. Here, we report the investigation of CF 4 up to 46.5 GPa-the highest pressure up to which any tetrahalides of group 14 elements have been studied so far-by a combination of single-crystal x-ray diffraction (SC-XRDp), Raman spectroscopy, and ab initio calculations. These measurements reveal a pressure-induced reentrant phase transition (phase II →2.8GPa phase III →∼20GPa phase II R ) at room temperature and the formation of a previously unknown CF 4 cubic polymorph, named phase IV, after the laser heating of CF 4 at 46.5 GPa. In this work, the structures of phases II R , III, and IV were solved and the atomic coordinates were refined on the basis of SC-XRDp. A comparison of tetrahalides of group 14 elements underlines that reducing the intermolecular halogen-halogen distances leads to a structural rearrangement from close packing of the tetrahedral molecules to close packing of the halogen atoms.

Magnetite, Fe 3 O 4 , is the oldest known magnetic mineral and archetypal mixed-valence oxide. Despite its recognized role in deep Earth processes, the behavior of magnetite at extreme high-pressure high-temperature (HPHT) conditions remains insufficiently studied. Here, we report on single-crystal synchrotron X-ray diffraction experiments up to ∼80 GPa and 5000 K in diamond anvil cells, which reveal two previously unknown Fe 3 O 4 polymorphs, γ-Fe 3 O 4 with the orthorhombic Yb 3 S 4 -type structure and δ-Fe 3 O 4 with the modified Th 3 P 4 -type structure. The latter has never been predicted for iron compounds. The decomposition of Fe 3 O 4 at HPHT conditions was found to result in the formation of exotic phases, Fe 5 O 7 and Fe 25 O 32 , with complex structures. Crystal-chemical analysis of iron oxides suggests the high-spin to low-spin crossover in octahedrally coordinated Fe 3+ in the pressure interval between 43 and 51 GPa. Our experiments demonstrate that HPHT conditions promote the formation of ferric-rich Fe-O compounds, thus arguing for the possible involvement of magnetite in the deep oxygen cycle.