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The field of polyhalogen chemistry, specifically polyhalogenanions(polyhalides), is rapidly evolving. Here, we present the synthesisof three sodium halides with unpredicted chemical compositions andstructures (tP10-Na2Cl3, hP18-Na4Cl5, and hP18-Na4Br5), a series of isostructural cubic cP8-AX(3) halides (NaCl3, KCl3, NaBr3, and KBr3), and a trigonal potassiumchloride (hP24-KCl3). The high-pressuresyntheses were realized at 41-80 GPa in diamond anvil cellslaser-heated at about 2000 K. Single-crystal synchrotron X-ray diffraction(XRD) provided the first accurate structural data for the symmetrictrichloride Cl-3 (-) anion in hP24-KCl3 and revealed the existence of two different typesof infinite linear polyhalogen chains, [Cl]( infinity ) ( n-) and [Br]( infinity ) ( n-), in the structures of cP8-AX(3) compounds and in hP18-Na4Cl5 and hP18-Na4Br5. In Na4Cl5 and Na4Br5, we found unusually short, likely pressure-stabilized, contactsbetween sodium cations. Ab initio calculations support the analysisof structures, bonding, and properties of the studied halogenides., Funding Agencies|China Scholarship Council (CSC); Federal Ministry of Education and Research, Germany (BMBF) [05K19WC1]; Deutsche Forschungsgemeinschaft (DFG) [DU 954-11/1, DU 393-9/2, DU 393-13/1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [MR/V025724/1]; UKRI Future Leaders Fellowship [2019-05600]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar grant) [2022-06725]; Swedish Research Council (VR) [BY112/2-1]; Swedish Research Council

A series of isostructural Ln(3)O(2)(CN3) (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)(CN3) solids are composed of the hitherto unknown CN35- guanidinate anion-deprotonated guanidine. Changes in unit cell volumes and compressibility of Ln(3)O(2)(CN3) (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)(CN3) compounds are recoverable to ambient conditions. The stabilization of the CN35- guanidinate anion at ambient conditions provides new opportunities in inorganic and organic synthetic chemistry., Funding Agencies|National Science Foundation; DOE Office of Science by Argonne National Laboratory; UKRI Future Leaders Fellowship; Deutsche Forschungsgemeinschaft (DFG) [DE-AC02-06CH11357, MR/V025724/1, 2009 00971]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University; Projekt DEAL; [DU 954-11/1]; [DU 393-9/2]; [DU 393-13/1]; [DU 945/15-1]; [EAR- 1634415]

A high-pressure (HP) yttrium allotrope, hP3-Y (space group P6/mmm), was synthesized in a multi-anvil press at 20 GPa and 2000 K which is recoverable to ambient conditions. Its relative stability and electronic properties were investigated using density functional theory calculations. A hP3-Y derivative hydride, hP3-YHx, with a variable hydrogen content (x = 2.8, 3, 2.4), was synthesized in diamond anvil cells by the direct reaction of yttrium with paraffin oil, hydrogen gas, and ammonia borane upon laser heating to similar to 3000 K at 51, 45 and 38 GPa, respectively. Room-temperature decompression leads to gradual reduction and eventually the complete loss of hydrogen at ambient conditions. Isostructural hP3-NdHx and hP3-GdHx hydrides were synthesized from Nd and Gd metals and paraffin oil, suggesting that the hP3-Y structure type may be common for rare-earth elements. Our results expand the list of allotropes of trivalent lanthanides and their hydrides and suggest that they should be considered in the context of studies of HP behavior and properties of this broad class of materials., Funding Agencies|UKRI Future Leaders Fellowship [MR/V025724/1]; Federal Ministry of Education and Research, Germany [05K19WC1]; Deutsche Forschungsgemeinschaft (DFG) [STE1105/13-2]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [DU 954-11/1]; DFG [DU 393-9/2, DU 393-13/1]; [2009 00971]

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 Fe0.79Si0.07B0.14 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 Fe0.79Si0.07B0.14 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 materials importance for a great number of delicate experiments conducted under extreme conditions. They indicate that the application of Fe0.79Si0.07B0.14 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., Funding Agencies|2019 Helmoltz-OCPC-Program [20191006]; Federal Ministry of Education and Research, Germany (BMBF) [05K13WC3, 05K19WC1]; Deutsche Forschungsgemeinschaft (DFG projects) [DU 954-11/1, DU 393-9/2, DU 393-13/1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Slovak Science Grant Agency [VEGA 1/0406/20]

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., Funding Agencies|Federal Ministry of Education and Research, Germany (BMBF) [05K19WC1]; Deutsche Forschungsgemeinschaft (DFG) [DU 954-11/1, DU 393-9/2, DU 393-13/1, DU 393-13/2]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [200900971]

Charged molecular species, such as [N-2](x-), [O-2](x-), [C-2](x-), and [S-2](x-), follow the paradigm of carrying integer values of electrons. Here, the Na-3(N-2)(4), Ca-3(N-2)(4), Sr-3(N-2)(4), and Ba(N-2)(3) compounds were produced and characterized <70 GPa and evidenced to be composed of paradigm-breaking [N-2](x-) dimers with noninteger charges of -0.75, -1.5, -1.5, and -0.67, respectively. The anion-driven metallicity of the compounds is proposed as the physical mechanism enabling the noninteger electron count of the [N-2](x-) dimers. The properties of these dimers and the compounds bearing them are demonstrated to depend on their noninteger charge, paving the way to materials with electron-tunable features., Funding Agencies|Alexander von Humboldt FoundationAlexander von Humboldt Foundation; Federal Ministry of Education and Research, GermanyFederal Ministry of Education & Research (BMBF) [05K19WC1]; Deutsche Forschungsgemeinschaft (DFG)German Research Foundation (DFG) [DU 954-11/1, DU 393-9/2, DU 393-13/1]; DFGGerman Research Foundation (DFG)European Commission [DFG FOR2125, WI1232]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; BIOVIA

Magnetite, Fe3O4, 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 similar to 80 GPa and 5000 K in diamond anvil cells, which reveal two previously unknown Fe3O4 polymorphs, gamma-Fe3O4 with the orthorhombic Yb3S4-type structure and delta-Fe3O4 with the modified Th3P4-type structure. The latter has never been predicted for iron compounds. The decomposition of Fe3O4 at HPHT conditions was found to result in the formation of exotic phases, Fe5O7 and Fe25O32, with complex structures. Crystal-chemical analysis of iron complex Crystal-chemical analysis oxides suggests the high-spin to low-spin crossover in octahedrally coordinated Fe3+ 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., Funding Agencies|Federal Ministry of Education and Research, Germany (BMBF)Federal Ministry of Education & Research (BMBF) [05K19WC1]; Deutsche Forschungsgemeinschaft (DFG)German Research Foundation (DFG) [DU 954-11/1, DU 393-9/2, FOR 2440, DU 393-13/1, DU 393-13/2]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [200900971]