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The density of the Earth’s core is several percent lower than that of iron-nickel alloy under conditions of pressure and temperature equivalent to the Earth’s core. Hydrogen is one of the most promising constituents accounting for the density deficit, but hydrogen occupation sites and density decrease of iron-nickel alloy caused by hydrogenation have never been investigated. In this study, the phase relation and crystal structure of Fe0.9Ni0.1Hx(Dx) at high pressures and temperatures up to 12 GPa and 1000 K were clarified by in situ X-ray diffraction and neutron diffraction measurements. Under the P-T conditions of the present study, no deuterium atoms occupied tetragonal (T) sites of face-centered cubic (fcc) Fe0.9Ni0.1Dx, although the T-site occupation was previously reported for fcc FeHx(Dx). The deuterium-induced volume expansion per deuterium vD was determined to be 2.45(4) and 3.31(6) Å3 for fcc and hcp Fe0.9Ni0.1Dx, respectively. These vD values are significantly larger than the corresponding values for FeDx. The vD value for fcc Fe0.9Ni0.1Dx slightly increases with increasing temperature. This study suggests that only 10% of nickel in iron drastically changes the behaviors of hydrogen in metal. Assuming that vD is constant regardless of pressure, the maximum hydrogen content in the Earth’s inner core is estimated to be one to two times the amount of hydrogen in the oceans.

The creation of a high-pressure rhombohedral phase leads to a colossal barocaloric effect in KPF6.

The structures of cold-compressed basaltic glass were investigated at pressures up to 18 GPa using in situ X-ray and neutron diffraction techniques to understand the physicochemical properties of deep magmas. On compression, basaltic glass changes its compression behavior: the mean O-O coordination number (CNOO) starts to rise while maintaining the mean O-O distance (rOO) above about 2–4 GPa, and then CNOO stops increasing, and rOO begins to shrink along with the increase in the mean coordination number of Al (CNAlO) above ~9 GPa. The change around 9 GPa is interpreted by the change in contraction mechanism from bending tetrahedral networks of glass to increasing oxygen packing ratio via the increase in CNAlO. The analysis of the oxygen packing fraction (ηO) under high pressure reveals that ηO exceeds the value for dense random packing, suggesting that the oxygen-packing hypothesis recently proposed cannot account for pressure-induced structural transformations of silica and silicate glasses. The rise of the CNOO at 2–4 GPa reflects the elastic softening of fourfold-coordinated silicate glass, which may be the origin of anomalies of elastic moduli in basaltic glass at ~2 GPa previously reported by Liu and Lin (2014). The widths of both the first sharp diffraction peak and the principal peak show contrastive compression behaviors between modified silicate and silica glasses. This result suggests that modified silicate glasses represent different pressure evolutions in the intermediate- and extended-range order structures from those of silica glass, likely due to the presence of modifier cations and the resultant formations of smaller rings and cavity volume.

地球の核の主要成分であるFeには軽金属が固溶していると考えられている。本研究ではそのモデル物質としてFe0.95Si0.05合金の水素化物について、水素吸蔵サイトと水素誘起格子体積膨張量を中性子その場観察により評価した。14.7 GPaでの測定とリートベルト解析の結果、水素は八面体サイトのみを占有し、水素誘起体積膨張量は2.33Å^3と算出された。SiがFeに加わることによって水素誘起体積膨張量はほとんど変化しないことが明らかになった。密度一定の条件では水素とSiの量は反相関の関係にあると結論づけた。

Magnetic and structure transitions of Mn3–xFexO4 solid solutions under extreme conditions are clarified by neutron time-of-flight scattering diffraction and X-ray Mössbauer measurement. The ferrimagnetic-to-paramagnetic transition temperature (100 °C) of Mn2FeO4 spinel is different from the tetragonal-to-cubic structure transition temperature (180 °C). The structure transition temperature decreases with increasing pressure. The transition is not coupled with the magnetic transition. Synchrotron X-ray Mössbauer experiments have revealed the pressure effects on the distribution of Fe2+ and Fe3+ at the tetrahedral and octahedral sites in the spinel structure. Ferrimagnetic MnFe2O4 and Mn2FeO4 spinels show sextet spectral features with hyperfine structure elicited by internal magnetic fields. Cubic MnFe2O4 spinel and tetragonal Mn2FeO4 transform to high-pressure orthorhombic postspinel phase above pressures of 18.4 GPa and 14.0 GPa, respectively. The transition pressure decreases with increasing Mn content. The postspinel phase has a paramagnetic property. Mn2O10 dimers of two octahedra are linked via common edge in three dimentional direction. The occupancy of Fe2+ in the tatrahedral site is decreased with increasig pressure, indicating more oredered structure. Consequently, the inverse parameter of the spinel structure is increased with increasing pressure. The magnetic structure refinements clarify the paramagnetic and ferrimagnetic structure of MnFe2O4 and Mn2FeO4 spinel as a function of pressure. The magnetic moment is ordered between A and B sites with the anti-parallel distribution along the b axis. The nuclear tetragonal structure (aN, aN, cN) has the ferrimagnetic structure but the orthorhombic magnetic structure has the ferrimagnetic structure with the lattice constants (aM, bM, cM). The magnetic moment is ordered between A and B sites with the anti-parallel distribution along the bM axis.

International audience; Ice polymorphs show extraordinary structural diversity depending on pressure and temperature. The behavior of hydrogen-bond disorder not only is a key ingredient for their structural diversity but also controls their physical properties. However, it has been a challenge to determine the details of the disordered structure in ice polymorphs under pressure, because of the limited observable reciprocal space and inaccuracies related to high-pressure techniques. Here, we present an elucidation of the disordered structure of ice VII, the dominant high-pressure form of water, at 2.2 GPa and 298 K, from both single-crystal and powder neutron-diffraction techniques. We reveal the three-dimensional atomic distributions from the maximum entropy method and unexpectedly find a ring-like distribution of hydrogen in contrast to the commonly accepted discrete sites. In addition, total scattering analysis at 274 K clarified the difference in the intermolecular structure from ice VIII, the ordered counterpart of ice VII, despite an identical molecular geometry. Our complementary structure analyses robustly demonstrate the unique disordered structure of ice VII. Furthermore, these findings are related to proton dynamics, which drastically vary with pressure, and will contribute to an understanding of the structural origin of anomalous physical properties of ice VII under pressures.

High-pressure X-ray and neutron diffraction analyses of an ambient-pressure phase (AP) and two high-pressure phases (HP1 and HP2) of ammonia borane (i.e., NH3BH3 and ND3BD3) were conducted to investigate the relationship between their crystal structures and dihydrogen bonds. It was confirmed that the hydrogen atoms in AP formed dihydrogen bonds between adjacent molecules, and the H-H distance between the hydrogen atoms forming this interaction was shorter than 2.4 A, which was nearly 2 times larger than the van der Waals radius of hydrogen. In the case of half of the hydrogen bonds, a phase transition from AP to the first high-pressure phase (HP1) at ∼1.2 GPa resulted in an increase in the H-H distances, which suggested that the dihydrogen bonds were broken. However, when HP1 was further pressurized to ∼4 GPa, all of the H-H distances became shorter than 2.4 A again, which implied the occurrence of pressure-induced re-formation of the dihydrogen bonds. It was speculated that the re-formation was consistent with a second-order phase transition suggested in previous studies by Raman spectroscopy and X-ray diffraction measurement. Furthermore, at ∼11 GPa, HP1 transformed to the second high-pressure phase (HP2), and its structure was determined to be P21 (Z = 2). In this phase transition, the inclination of the molecule axis became larger, and the number of types of dihydrogen bonds increased from 6 to 11. At 18.9 GPa, which was close to the upper pressure limit of HP2, the shortest dihydrogen bond decreased to ∼1.65 A. Additionally, the X-ray diffraction results suggested another phase transition to the third high-pressure phase (HP3) at ∼20 GPa. The outcomes of this study confirmed experimentally for the first time that the structural change under pressure causes the breakage and re-formation of the dihydrogen bonds of NH3BH3.

We applied Kawai-type multi-anvil assemblies (MA6-8) for time-of-flight neutron-diffraction experiments to achieve high-pressures and high-temperatures simultaneously. To achieve sufficient signal ...

Neutron powder diffraction profiles were collected for iron deuteride (FeDx) while the temperature decreased from 1023 to 300 K for a pressure range of 4–6 gigapascal (GPa). The ε′ deuteride with a double hexagonal close-packed (dhcp) structure, which coexisted with other stable or metastable deutrides at each temperature and pressure condition, formed solid solutions with a composition of FeD0.68(1) at 673 K and 6.1 GPa and FeD0.74(1) at 603 K and 4.8 GPa. Upon stepwise cooling to 300 K, the D-content x increased to a stoichiometric value of 1.0 to form monodeuteride FeD1.0. In the dhcp FeD1.0 at 300 K and 4.2 GPa, dissolved D atoms fully occupied the octahedral interstitial sites, slightly displaced from the octahedral centers in the dhcp metal lattice, and the dhcp sequence of close-packed Fe planes contained hcp-stacking faults at 12%. Magnetic moments with 2.11 ± 0.06 μB/Fe-atom aligned ferromagnetically in parallel on the Fe planes.

Water freezes below 0 °C at ambient pressure ordinarily to ice Ih, with hexagonal stacking sequence. Under certain conditions, ice with a cubic stacking sequence can also be formed, but ideal ice Ic without stacking-disorder has never been formed until recently. Here we demonstrate a route to obtain ice Ic without stacking-disorder by degassing hydrogen from the high-pressure form of hydrogen hydrate, C2, which has a host framework isostructural with ice Ic. The stacking-disorder free ice Ic is formed from C2 via an intermediate amorphous or nano-crystalline form under decompression, unlike the direct transformations occurring in ice XVI from neon hydrate, or ice XVII from hydrogen hydrate. The obtained ice Ic shows remarkable thermal stability, until the phase transition to ice Ih at 250 K, originating from the lack of dislocations. This discovery of ideal ice Ic will promote understanding of the role of stacking-disorder on the physical properties of ice as a counter end-member of ice Ih., Metastable cubic ice has been identified in several conditions relevant to geo and astrochemistry, but was always characterized by stacking disorder. Here the authors synthesize a hydrogen hydrate and degas hydrogen, obtaining pure non-defected cubic ice, observed by X-ray and neutron diffraction.

Substituted polyacetylene is expected to improve the chemical stability, physical properties, and combine new functions to the polyacetylene backbones, but its diversity is very limited. Here, by applying external pressure on solid acetylenedicarboxylic acid, we report the first crystalline poly-dicarboxylacetylene with every carbon on the

The pressure-induced phase transition and polymerization of nitrogen-rich molecules are widely focused on due to their extreme importance for the development of green high-energy-density materials. Here, we present a study of the phase-transition behaviour and chemical reaction of 1H-tetrazole up to 100 GPa using in situ Raman, IR, X-ray diffraction, neutron diffraction techniques and theoretical calculations. A phase transition above 2.6 GPa was identified and the high-pressure structure was determined with one molecule in a unit cell instead of two molecules as reported before. The 1H-tetrazole polymerized reversibly below 100 GPa, probably through carbon–nitrogen bonding instead of nitrogen–nitrogen bonding. Our studies update the structure model of the high-pressure phase of 1H-tetrazole, and present the possible intermolecular bonding route for the first time, which gives new insights to understand the phase transition and chemical reaction of nitrogen-rich compounds, and is of benefit for designing new high-energy-density materials.

A high-pressure phase of magnesium chloride hexahydrate (MgCl2·6H2O-II) and its deuterated counterpart (MgCl2·6D2O-II) have been identified for the first time byin-situsingle-crystal X-ray and powder neutron diffraction. The crystal structure was analyzed by the Rietveld method for the neutron diffraction pattern based on the initial structure determined by single-crystal X-ray diffraction. This high-pressure phase has a similar framework to that in the known ambient-pressure phase, but exhibits some structural changes with symmetry reduction caused by a subtle modification in the hydrogen-bond network around the Mg(H2O)6octahedra. These structural features reflect the strain in the high-pressure phases of MgCl2hydrates.

Monoclinic ZrO2 baddeleyite exhibits anomalous softenings of the bulk modulus and atom vibrations with compression. The pressure evolution of the structure is investigated using neutron powder diffraction combined with ab initio calculations. The results show that the anomalous pressure response of the bulk modulus is related not to the change in the bonding characters but to the deformation of an oxygen sublattice, especially one of the layers made of oxygen atoms in the crystallographic a* plane. The layer consists of two parallelograms; one is rotated with little distortion and the other is distorted with increasing pressure. The deformation of this layer lengthens one of the Zr—O distances, resulting in the softening of some atom vibrational modes.

We have developed a technique for neutron diffraction experiments at pressures up to 40 GPa using a Paris-Edinburgh press at the PLANET beamline in J-PARC. To increase the maximum accessible pressure, the diameter of the dimple for sample chamber at the top of the sintered diamond anvils is sequentially reduced from 4.0 mm to 1.0 mm. As a result, the maximum pressure increased and finally reached 40 GPa. By combining this technique with the beam optics which defines the gauge volume, diffraction patterns sufficient for full-structure refinements are obtainable at such pressures.

Pressure-induced polymerization (PIP) of aromatics is a novel method for constructing sp3 -carbon frameworks, and nanothreads with diamond-like structures were synthesized by compressing benzene and its derivatives. Here by compressing a benzene-hexafluorobenzene cocrystal (CHCF), H-F-substituted graphane with a layered structure in the PIP product was identified. Based on the crystal structure determined from the in situ neutron diffraction and the intermediate products identified by gas chromatography-mass spectrum, we found that at 20 GPa CHCF forms tilted columns with benzene and hexafluorobenzene stacked alternatively, and leads to a [4+2] polymer, which then transforms to short-range ordered H-F-substituted graphane. The reaction process involves [4+2] Diels-Alder, retro-Diels-Alder, and 1-1' coupling reactions, and the former is the key reaction in the PIP. These studies confirm the elemental reactions of PIP of CHCF for the first time, and provide insight into the PIP of aromatics.

Hydrogen (H) is considered to be one of the candidates for light elements in the Earth’s core, but the amount and timing of delivery have been unknown. We investigated the effects of sulfur (S), another candidate element in the core, on deuteration of iron (Fe) in iron–silicate–water system up to 6–12 GPa, ~ 1200 K using in situ neutron diffraction measurements. The sample initially contained saturated water (D2O) as Mg(OD)2 in the ideal composition (Fe–MgSiO3–D2O) of the primitive Earth. In the existence of water and sulfur, phase transitions of Fe, dehydration of Mg(OD)2, and formation of iron sulfide (FeS) and silicates occurred with increasing temperature. The deuterium (D) solubility (x) in iron deuterides (FeDx) increased with temperature and pressure, resulting in a maximum of x = 0.33(4) for the hydrous sample without S at 11.2 GPa and 1067 K. FeS was hardly deuterated until Fe deuteration had completed. The lower D concentrations in the S-containing system do not exceed the miscibility gap (x

A Correction to this paper has been published: https://doi.org/10.1038/s41467-021-22085-4

Solid-state topochemical polymerization (SSTP) is a promising method to construct functional crystalline polymeric materials, but in contrast to various reactions that happen in solution, only very limited types of SSTP reactions are reported. Diels-Alder (DA) and dehydro-DA (DDA) reactions are textbook reactions for preparing six-membered rings in solution but are scarcely seen in solid-state synthesis. Here, using multiple cutting-edge techniques, we demonstrate that the solid 1,4-diphenylbutadiyne (DPB) undergoes a DDA reaction under 10-20 GPa with the phenyl as the dienophile. The crystal structure at the critical pressure shows that this reaction is "distance-selected". The distance of 3.2 A between the phenyl and the phenylethynyl facilitates the DDA reaction, while the distances for other DDA and 1,4-addition reactions are too large to allow the bonding. The obtained products are crystalline armchair graphitic nanoribbons, and hence our studies open a new route to construct the crystalline carbon materials with atomic-scale control.

Ice exhibits extraordinary structural variety in its polymorphic structures. The existence of a new form of diversity in ice polymorphism has recently been debated in both experimental and theoretical studies, questioning whether hydrogen-disordered ice can transform into multiple hydrogen-ordered phases, contrary to the known one-to-one correspondence between disordered ice and its ordered phase. Here, we report a high-pressure phase, ice XIX, which is a second hydrogen-partially-ordered phase of ice VI. We demonstrate that disordered ice undergoes different manners of hydrogen ordering, which are thermodynamically controlled by pressure in the case of ice VI. Such multiplicity can appear in all disordered ice, and it widely provides a research approach to deepen our knowledge, for example of the crucial issues of ice: the centrosymmetry of hydrogen-ordered configurations and potentially induced (anti-)ferroelectricity. Ultimately, this research opens up the possibility of completing the phase diagram of ice., Water ice exhibits several hydrogen-ordered and disordered phases and it’s unclear if a disordered phase can transform into only one ordered phase. Here, the authors identify a partially hydrogen-ordered phase at high pressure, ice XIX, as the second hydrogen-ordered phase of ice VI beside ice XV.

International audience; Developments of nano-polycrystalline diamond anvil cells for neutron diffraction experiments A new high-pressure cell for neutron diffraction experiments using nano-polycrystalline anvils is presented. The cell design, off-line pressure generation tests and a gas-loading procedure for this cell are described. The performance is illustrated by powder neutron diffraction patterns of ice VII to ~82 GPa. We also demonstrate the feasibility of single crystal neutron diffraction experiments of Fe3O4 at ambient conditions using this cell and discuss the current limitation and future developments.

More than 20 crystalline and amorphous phases have been reported for ice so far. This extraordinary polymorphism of ice arises from the geometric flexibility of hydrogen bonds and hydrogen ordering, and makes ice a unique presence with its universality in the wide fields of material and earth and planetary science. A prominent unsolved question concerning the diversity is whether a hydrogen-disordered phase of ice transforms into only one hydrogen-ordered phase, as inferred from the current phase diagram of ice, although its possible hydrogen configurations have close energies. Recent experiments on a high-pressure hydrogen-disordered phase, ice VI, revealed an unknown hydrogen-ordered form ($\beta$-XV) besides the known ordered phase, ice XV, which would be a counterexample of the question. However, due to lack of experimental evidence, it has not been clarified whether $\beta$-XV is a distinct crystalline phase. Herein we report a second hydrogen-ordered phase for ice VI, ice XIX, unambiguously demonstrated by neutron diffraction measurements. The phase boundary between ice VI and ice XIX shows that ice VI contracts upon the hydrogen ordering, which thermodynamically stabilizes ice XIX in higher-pressure region than ice XV because of its smaller volume than ice XV. The pressure-driven phase competition between hydrogen-ordered phases, also theoretically suggested in other ice polymorphs, can induce hydrogen ordering of ice in different manners. Thus, this study demonstrates a hitherto undiscovered polymorphism of ice.