Phase Diagram Analysis of High-Pressure/High-Temperature Polymorphs of Ammonia Borane.

Ammonia borane (NH3BH3) is a promising hydrogen-storage material because of its high hydrogen density. It is employed as a hydrogen source when synthesizing superconducting polyhydrides under high pressure. Additionally, NH3BH3 is a crystallographically interesting compound that features protonic hydrogen (Hδ+) and hydridic hydrogen (Hδ-), and it forms a dihydrogen bond, which explains its stable existence as a solid. Herein, X-ray diffraction experiments were performed at high pressures (HPs) and high temperatures (HTs) of up to 30 GPa and 300 °C, respectively, to investigate the HP/HT phase diagram of NH3BH3. A new HP/HT phase (HPHT2) was identified above 9 GPa and 150 °C. Crystal-structure analysis using the Rietveld method and stability verification using density functional theory calculations revealed that HPHT2 has a P21/n (Z = 4) structure, similar to that of a previously reported HP/HT phase (HPHT) that appears at a lower pressure. HPHT2 is denser than the HP phases that appear at room temperature (HP1 and HP2) at the same pressure (up to ∼17 GPa). In the phase diagram, the phase-boundary line between HPHT and HP1 is a downward convex curve. These unconventional phenomena in the density and phase boundary can be attributed to the influence of dihydrogen bonding on the crystal structure and phase diagram.

Influence of pressure-induced formation of dihydrogen bonds on lattice parameters, volume, and vibrational modes of ammonia borane.

The high-pressure phase of ammonia borane (NH3BH3) observed at ∼1.2 GPa has been reported to result in pressure-induced formation of dihydrogen bonds at ∼4 GPa. In this study, we performed high-pressure x-ray diffraction measurements on the high-pressure phase (up to ∼10.2 GPa) using a He hydrostatic pressure medium to examine the influence of the formation of dihydrogen bonds on the lattice parameters and unit cell volume of NH3BH3. We observed a unique behavior in the pressure dependence of lattice parameters close to the pressure at which the dihydrogen bond was formed. The lattice parameters demonstrated hysteresis curves under compression and decompression conditions but the unit cell volume did not. Moreover, the pressure dependence of the unit cell volume could not be expressed using a single Birch-Murnaghan equation within an acceptable margin of error, thus suggesting a change in bulk modulus under compression. These results are considered to have originated from the pressure-induced formation of dihydrogen bonds. Moreover, high-pressure Raman scattering measurements and a simulation using density functional theory calculations revealed the vibrational modes of the high-pressure phase of NH3BH3. The results demonstrated that librational modes were enhanced by forming dihydrogen bonds. Moreover, the intramolecular stretching modes of BN, BH, and NH monotonically shifted with pressure, while the symmetrical in-plane bending modes of BH3 and NH3 split irrespective of the formation of dihydrogen bonds.

Observation of Dihydrogen Bonds in High-Pressure Phases of Ammonia Borane by X-ray and Neutron Diffraction Measurements.

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 Å, 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 Å 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 Å. 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.

Titanium boride equation of state determined by in-situ X-ray diffraction.

The equation of state (EOS) of titanium boride, TiB2, was investigated by in situ X-ray diffraction in a diamond anvil cell and multianvil high-pressure apparatus. The pressure-volume-temperature (P-V-T) data were collected at up to 111 GPa and room temperature for the diamond-anvil cell experiments and at up to 15 GPa and 1300 K for the multianvil experiments. No phase transition was observed through the entire range of experimental conditions. The pressure-volume data at room temperature were fitted using a Vinet EOS to obtain the isothermal bulk modulus, BT0 = 256.7 GPa, and its pressure derivative, B' T0 = 3.83. When fitting a thermal EOS using the P-V-T data for the multianvil experiments, we find that [Formula: see text] = 0.095 (GPa/K) and α 0 = 2.49 × 10-5 K-1.

Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth's upper mantle.

Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth's upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 10(19.6) to 10(20.7) Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size-sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle.

Synthesis, Direct Formation under High Pressure, Structure, and Electronic Properties of LiNbO3-type Oxide PbZnO3.

A novel LiNbO3-type (LN-type) lead zinc oxide, PbZnO3, was successfully synthesized under high pressure and temperature. Rietveld structure refinement using synchrotron powder X-ray diffraction (XRD) data demonstrated that LN-type PbZnO3 crystallized into a trigonal structure with a polar space group (R3c). The bond valence sum estimated from the interatomic distances indicated that the sample possesses a Pb(4+)Zn(2+)O3 valence state. Polarization could evolve as a result of the repulsion between constituent cations because PbZnO3 does not contain a stereochemical 6s(2) cation or a Jahn-Teller active d(0) cation. Distortion of ZnO6 octahedra resulting from cation shift is comparable with that of d(0) TiO6 in ZnTiO3 and MnTiO3 with LN-type oxides, which leads to stabilization of the polar structure. PbZnO3 exhibited metallic behavior and temperature-independent diamagnetic character. In situ XRD measurement revealed that the formation of LN-type PbZnO3 occurred directly without the formation of a perovskite phase, which is unusual among LN-type materials obtained by high-pressure synthesis.

Stability of the Liquid State of Imidazolium-Based Ionic Liquids under High Pressure at Room Temperature.

To understand the stability of the liquid phase of ionic liquids under high pressure, we investigated the phase behavior of a series of 1-alkyl-3-methylimidazolium tetrafluoroborate ([Cnmim][BF4]) homologues with different alkyl chain lengths for 2 ≤ n ≤ 8 up to ∼7 GPa at room temperature. The ionic liquids exhibited complicated phase behavior, which was likely due to the conformational flexibility in the alkyl chain. The present results reveal that [Cnmim][BF4] falls into superpressed state around 2-3 GPa range upon compression with an implication of multiple phase or structural transitions to ∼7 GPa. Remarkably, a characteristic nanostructural organization in ionic liquids largely diminishes at the superpressed state. The behaviors of imidazolium-based ionic liquids can be classified into, at least, three patterns: (1) pressure-induced crystallization, (2) superpressurization upon compression, and (3) decompression-induced crystallization from the superpressurized glass. Interestingly, the high-pressure phase behavior was relevant to the glass transition behavior at low temperatures and ambient pressure. As n increases, the glass transition pressure (pg) decreases (from 2.8 GPa to ∼2 GPa), and the glass transition temperature increases. The results indicate that the p-T range of the liquid phase is regulated by the alkyl chain length of [Cnmim][BF4] homologues.

Postperovskite phase transition of ZnGeO3: comparative crystal chemistry of postperovskite phase transition from germanate perovskites.

The postperovskite phase of ZnGeO3 was confirmed by laser heating experiments of the perovskite phase under 110-130 GPa at high temperature. Ab initio calculations indicated that the phase transition occurs at 133 GPa at 0 K. This postperovskite transition pressure is significantly higher than those reported for other germanates, such as MnGeO3 and MgGeO3. The comparative crystal chemistry of the perovskite-to-postperovskite transition suggests that a relatively elongated b-axis in the low-pressure range resulted in the delay in the transition to the postperovskite phase. Similar to most GdFeO3-type perovskites that transform to the CaIrO3-type postperovskite phase, ZnGeO3 perovskite eventually transformed to the CaIrO3-type postperovskite phase at a critical rotational angle of the GeO6 octahedron. The formation of the postperovskite structure at a very low critical rotational angle for MnGeO3 suggests that relatively large divalent cations likely break down the corner-sharing GeO6 frameworks without a large rotation of GeO6 to form the postperovskite phase.

Discovery of the last remaining binary platinum-group pernitride RuN2.

The last remaining marcasite-type RuN2 was successfully synthesized by direct chemical reaction between ruthenium and molecular nitrogen above the pressure of 32 GPa. For the first time, we found that Ru 4d is weakly hybridized with N 2p in the structure by using transmission electron microscopy equipped with electron-energy-loss spectroscopy. Our finding give important knowledge about the platinum-group pernitride with respect to the chemical bonding between platinum-group element and nitrogen.

Perovskite-to-postperovskite transitions in NaNiF3 and NaCoF3 and disproportionation of NaCoF3 postperovskite under high pressure and high temperature.

High-pressure structural phase transitions in NaNiF(3) and NaCoF(3) were investigated by conducting in situ synchrotron powder X-ray diffraction experiments using a diamond anvil cell. The perovskite phases (GdFeO(3) type) started to transform into postperovskite phases (CaIrO(3) type) at about 11-14 GPa, even at room temperature. The transition pressure is much lower than those of oxide perovskites. The anisotropic compression behavior led to heavily tilted octahedra that triggered the transition. Unlike oxide postperovskites, fluoropostperovskites remained after decompression to 1 atm. The postperovskite phase in NaCoF(3) broke down into a mixture of unknown phases after laser heating above 26 GPa, and the phases changed into amorphous ones when the pressure was released. High-pressure and high-temperature experiments using a multianvil apparatus were also conducted to elucidate the phase relations in NaCoF(3). Elemental analysis of the recovered amorphous samples indicated that the NaCoF(3) postperovskite disproportionated into two phases. This kind of disproportionation was not evident in NaNiF(3) even after laser heating at 54 GPa. In contrast to the single postpostperovskite phase reported in NaMgF(3), such a postpostperovskite phase was not found in the present compounds.

B1-to-B2 structural transitions in rock salt intergrowth structures.

The rock salt (B1) structure of binary oxides or chalcogenides transforms to the CsCl (B2) structure under high pressure, with critical pressures P(s) depending on the cation to anion size ratio (R(c)/R(a)). We investigated structural changes of A(2)MO(3) (A = Sr, Ca; M = Cu, Pd) comprising alternate 7-fold B1 AO blocks and corner-shared MO(2) square-planar chains under pressure. All of the examined compounds exhibit a structural transition at P(s) = 29-41 GPa involving a change in the A-site geometry to an 8-fold B2 coordination. This observation demonstrates, together with the high pressure study on the structurally related Sr(3)Fe(2)O(5), that the B1-to-B2 transition generally occurs in these intergrowth structures. An empirical relation of P(s) and the R(c)/R(a) ratio for the binary system holds well for the intergrowth structure also, which means that P(s) is predominantly determined by the rock salt blocks. However, a large deviation from the relation is found in LaSrNiO(3.4), where oxygen atoms partially occupy the apical site of the MO(4) square plane. We predict furthermore the occurrence of the same structural transition for Ruddlesden-Popper-type layered perovskite oxides (AO)(AMO(3))(n), under higher pressures. For investigating the effect on the physical properties, an electrical resistivity of Sr(2)CuO(3) is studied.

Compression behaviors of binary skutterudite CoP3 in noble gases up to 40 GPa at room temperature.

The binary skutterudite CoP(3) has a large void at the body-centered site of each cubic unit cell and is, therefore, called a nonfilled skutterudite. We investigated its room-temperature compression behavior up to 40.4 GPa in helium and argon using a diamond-anvil cell. High-pressure in situ X-ray diffraction and Raman scattering measurements found no phase transition and a stable cubic structure up to the maximum pressure in both media. A fitting of the present pressure-volume data to the third-order Birch-Murnaghan equation of state yields a zero-pressure bulk modulus K(0) of 147(3) GPa [pressure derivative K(0)' of 4.4(2)] and 171(5) GPa [where K(0)' = 4.2(4)] in helium and argon, respectively. The Grüneisen parameter was determined to be 1.4 from the Raman scattering measurements. Thus, CoP(3) is stiffer than other binary skutterudites and could therefore be used as a host cage to accommodate large atoms under high pressure without structural collapse.

Pressure-induced structural, magnetic, and transport transitions in the two-legged ladder Sr3Fe2O5.

The layered compound SrFeO(2) with an FeO(4) square-planar motif exhibits an unprecedented pressure-induced spin state transition (S = 2 to 1), together with an insulator-to-metal (I-M) and an antiferromagnetic-to-ferromagnetic (AFM-FM) transition. In this work, we have studied the pressure effect on the structural, magnetic, and transport properties of the structurally related two-legged spin ladder Sr(3)Fe(2)O(5). When pressure was applied, this material first exhibited a structural transition from Immm to Ammm at P(s) = 30 ± 2 GPa. This transition involves a phase shift of the ladder blocks from (1/2,1/2,1/2) to (0,1/2,1/2), by which a rock-salt type SrO block with a 7-fold coordination around Sr changes into a CsCl-type block with 8-fold coordination, allowing a significant reduction of volume. However, the S = 2 antiferromagnetic state stays the same. Next, a spin state transition from S = 2 to S = 1, along with an AFM-FM transition, was observed at P(c) = 34 ± 2 GPa, similar to that of SrFeO(2). A sign of an I-M transition was also observed at pressure around P(c). These results suggest a generality of the spin state transition in square planar coordinated S = 2 irons of n-legged ladder series Sr(n+1)Fe(n)O(2n+1) (n = 1, 2, 3, ...). It appears that the structural transition independently occurs without respect to other transitions. The necessary conditions for a structural transition of this type and possible candidate materials are discussed.

High-pressure in situ structure measurement of low-Z noncrystalline materials with a diamond-anvil cell by an x-ray diffraction method.

We have developed techniques for high-pressure in situ structure measurement of low-Z noncrystalline materials with a diamond-anvil cell (DAC) by an x-ray diffraction method. Since the interaction between low-Z materials and x rays is small and the sample thickness in a DAC is also small, the incoherent scattering from the anvils overwhelms the coherent scattering from the sample at a high-Q range. By using a cubic boron nitride gasket to increase the sample thickness and the energy-dispersive x-ray diffraction method with a slit system to narrow the region from which detected x rays are scattered, we can reduce unfavorable effects of the incoherent scattering from the anvils and correct them accurately. We have successfully measured the structure factor of SiO(2) glass in a DAC over a relatively wide range of Q under high pressure.

In situ observation of the formation of Si clathrate Ba8Si46 at high pressures and high temperatures.

X-ray diffraction measurements at high pressures and high temperatures revealed that Si clathrate Ba 8Si 46 is formed by a solid-phase reaction of an 8:30 molar mixture of SrSi 2-phase BaSi 2 and Si after BaSi 2 undergoes the BaSi 2-to-EuGe 2 and the EuGe 2-to-SrSi 2 transitions. The volume reduction during the formation of Ba 8Si 46 is the largest, 7.6%, among the observed transitions. On the other hand, an 8:30 molar mixture of SrSi 2-phase SrSi 2 and Si does not result in the formation of Sr 8Si 46 at high pressures and high temperatures; only SrSi 2 transforms from the SrSi 2 phase into the alpha-ThSi 2 phase, and Si remains in the diamond phase.

Two-dimensional diffraction study of ice crystallisation in polymer gels.

Ice crystallisation in crosslinked dextran (Sephadex) gels was studied by the method of two-dimensional X-ray diffraction (XRD) in combination with the simultaneous measurement of differential scanning calorimetry (DSC). With a Sephadex G25 gel where an exotherm due to ice crystallisation is observed in the DSC rewarming trace, it was indicated by the XRD pattern that small ice crystals less than approximately 10 microns in diameter are readily formed during freezing, and that the endothermic trend prior to the exotherm is not due to the glass transition but due to the melting of the small ice crystals. Moreover, the diffraction pattern observed with frozen Sephadex gels depended on the density of crosslink indicating that ice crystals of different size and dimension are formed in the gels.

Metastable garnet in oceanic crust at the top of the lower mantle.

As oceanic tectonic plates descend into the Earth's lower mantle, garnet (in the basaltic crust) and silicate spinel (in the underlying peridotite layer) each decompose to form silicate perovskite-the 'post-garnet' and 'post-spinel' transformations, respectively. Recent phase equilibrium studies have shown that the post-garnet transformation occurs in the shallow lower mantle in a cold slab, rather than at approximately 800 km depth as earlier studies indicated, with the implication that the subducted basaltic crust is unlikely to become buoyant enough to delaminate as it enters the lower mantle. But here we report results of a kinetic study of the post-garnet transformation, obtained from in situ X-ray observations using sintered diamond anvils, which show that the kinetics of the post-garnet transformation are significantly slower than for the post-spinel transformation. Although metastable spinel quickly breaks down at a temperature of 1,000 K, we estimate that metastable garnet should survive of the order of 10 Myr even at 1,600 K. Accordingly, the expectation of where the subducted oceanic crust would be buoyant spans a much wider depth range at the top of the lower mantle, when transformation kinetics are taken into account.