High-pressure Crystal Structure Research Articles

High-pressure synthesis, crystal structure and magnetic properties of Ba3CuOs2O9

High-pressure synthesis, crystal structure and magnetic properties of Ba₃CuOs₂O₉

High-pressure synthesis and crystal structure of a novel intermetallic compound Mn(Al,Ge)5

A novel intermetallic compound of the Mn–Al–Ge system, Mn(Al,Ge)5, was synthesized under high pressure and high temperature (5 GPa and 600 °C). The crystal structure was tetragonal with a space group of I4/mcm (no. 140), which was isostructural with PdGa5 or FeAl3Si2. The crystallographic parameters were refined by Rietveld analysis from powder X-ray diffraction data, yielding a = 0.624026 (5) nm, c = 0.981858 (10) nm, and Z = 4. The structure could be considered a defect-Cr5B3 (D8l) structure. The hysteresis loop and magnetization–temperature curve revealed the synthesized Mn(Al,Ge)5 to be paramagnetic: the long Mn–Mn distance led to a structure without magnetic ordering.

Bulk moduli and high pressure crystal structure of U3Si2

Bulk moduli are important parameters to assess the mechanical performance of materials including nuclear fuels. However, little experimental data exist for U3Si2, a potential accident-tolerant nuclear fuel, whose bulk modulus has only been measured by resonant ultrasonic spectroscopy (RUS). In addition, the knowledge for high-pressure structural behavior and phase equilibrium of U3Si2 is largely lacking. Here we studied pressure dependence of the crystal structure of U3Si2 using high-energy synchrotron X-ray diffraction coupled with Rietveld analysis. The pressurization was achieved using a diamond anvil cell (DAC) which provides quasi-hydrostatic pressures up to 37.6 GPa. Crystal structural variation and equations of state of U3Si2 were obtained, and its bulk modulus, a- and c-axial moduli were derived to be 107.11 ± 5.65 GPa, 82.87 ± 4.78 GPa and 194.52 ± 12.03 GPa, respectively. The determined elastic parameters are compared with those obtained by RUS and nanoindentation.

High Pressure Crystal Structure and Electrical Properties of a Single Component Molecular Crystal [Ni(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate).

Single-component molecular conductors form an important class of materials showing exotic quantum phenomena, owing to the range of behavior they exhibit under physical stimuli. We report the effect of high pressure on the electrical properties and crystal structure of the single-component crystal [Ni(dddt)2] (where dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate). The system is isoelectronic and isostructural with [Pd(dddt)2], which is the first example of a single-component molecular crystal that exhibits nodal line semimetallic behavior under high pressure. Systematic high pressure four-probe electrical resistivity measurements were performed up to 21.6 GPa, using a Diamond Anvil Cell (DAC), and high pressure single crystal synchrotron X-ray diffraction was performed up to 11.2 GPa. We found that [Ni(dddt)2] initially exhibits a decrease of resistivity upon increasing pressure but, unlike [Pd(dddt)2], it shows pressure-independent semiconductivity above 9.5 GPa. This correlates with decreasing changes in the unit cell parameters and intermolecular interactions, most notably the π-π stacking distance within chains of [Ni(dddt)2] molecules. Using first-principles density functional theory (DFT) calculations, based on the experimentally-determined crystal structures, we confirm that the band gap decreases with increasing pressure. Thus, we have been able to rationalize the electrical behavior of [Ni(dddt)2] in the pressure-dependent regime, and suggest possible explanations for its pressure-independent behavior at higher pressures.

High-pressure synthesis, crystal structure, and magnetic properties of hexagonal Ba3CuOs2O9

A new polymorph of the triple perovskite Ba3CuOs2O9, which usually exists in the orthorhombic phase, was synthesized under high-pressure and high-temperature conditions at 6 GPa and 1100 °C. Under the synthetic condition, Ba3CuOs2O9 crystallizes into a hexagonal structure (P63/mmc) with a = 5.75178(1) Å and c = 14.1832(1) Å, and undergoes a 1.36% increment in density, compared to that of the orthorhombic phase. Although Ba3CuOs2O9 maintains its 6 H perovskite-type structure, the distribution of Cu and Os atoms are dramatically altered; (Cu)4a(Os,Os)8f transits to (Os)2a(Cu,Os)4f ordering over the corner- and face-sharing sites, respectively. The hexagonal Ba3CuOs2O9 exhibits a ferrimagnetic transition at 290 K, which is in stark contrast to the antiferromagnetic transition at 47 K exhibited by the orthorhombic Ba3CuOs2O9. The enhanced transition temperature is most likely due to the strongly antiferromagnetic Os5+–O–Os5+ bonds and the moderately antiferromagnetic Os5+–O–Cu2+ bonds, the angles of which are both approximately 180°. The 290 K ferrimagnetic transition temperature is the highest reported for triple-perovskite osmium oxides. Besides, the coercive field is greater than 70 kOe at 5 K, which is remarkable among the coercive fields of magnetic oxides.

Novel superconducting structures of BH2 under high pressure.

The crystal structures of boron hydrides in a pressure range of 50-400 GPa were studied using the genetic algorithm (GA) method combined with first-principles density functional theory calculations. BH4 and BH5 are predicted to be thermodynamically unstable. Two new BH2 structures with Cmcm and C2/c space group symmetries, respectively, were predicted, in which the B atoms tend to form two-dimensional sheets. The calculated band structures showed that in the pressure range of 50-150 GPa, the Cmcm-BH2 phase has very small gaps, while the C2/c-BH2 phase at 200-400 GPa is metallic. The superconductivity of the C2/c-BH2 structure was also investigated, and electron-phonon coupling calculations revealed that the estimated Tc values of C2/c-BH2 are about 28.18-37.31 K at 250 GPa.

Gold(i) sulfide: unusual bonding and an unexpected computational challenge in a simple solid.

We report the experimental high-pressure crystal structure and equation of state of gold(i) sulfide (Au2S) determined using diamond-anvil cell synchrotron X-ray diffraction. Our data shows that Au2S has a simple cubic structure with six atoms in the unit cell (four Au in linear, and two S in tetrahedral, coordination), no internal degrees of freedom, and relatively low bulk modulus. Despite its structural simplicity, Au2S displays very unusual chemical bonding. The very similar and relatively high electronegativities of Au and S rule out any significant metallic or ionic character. Using a simple valence bond (Lewis) model, we argue that the Au2S crystal possesses two different types of covalent bonds: dative and shared. These bonds are distributed in such a way that each Au atom engages in one bond of each kind. The multiple arrangements in space of dative and shared bonds are degenerate, and the multiplicity of configurations imparts the system with multireference character, which is highly unusual for an extended solid. The other striking feature of this system is that common computational (DFT) methods fail quite spectacularly to describe it, with 20% and 400% errors in the equilibrium volume and bulk modulus, respectively. We explain this by the poor treatment of static correlation in common density-functional approximations. The fact that the solid is structurally very simple, yet presents unique chemical bonding and is unmodelable using current DFT methods, makes it an interesting case study and a computational challenge.

High pressure crystal structure of nitroethane.

Ambient temperature static high pressure compression of liquid nitroethane has been performed using the diamond anvil technique. The first transition to a crystalline powder around 4.2 GPa did not return reliable indexing solutions and could be the result of a mixture of phases. Subsequent cycling of the pressure apparatus with the same sample triggered a solid-solid transition around 4.9-4.3 GPa on the downstroke. Indexing returned a monoclinic structure with spacegroup P21 or P21/m which is consistent with the lowest enthalpy solution predicted from density functional theory (DFT) calculations performed in this study, namely, P21. Attempts to reproduce the first anomalous mixture of phases with other samples were not successful; rather, the numerically preferred monoclinic structure manifests itself after a liquid-solid transition around 4.3-3.6 GPa, a value consistent with the results of a previous study. The transition typically occurs more readily on the downstroke. Incomplete Debye rings resulting from preferred orientation complicates any Rietveld refinement, though the DFT simulations predict 2 molecules per unit cell. Unit cell volumes during the upstroke were within 3% of that predicted by DFT calculations. This departure increases to about 9% below that from DFT predictions during the downstroke suggesting the presence of residual stresses due to non-hydrostatic conditions. Finally, a sudden relief in d-spacing values around 3.7-4.3 GPa during the downstroke is thought to be due to the influence of intermolecular hydrogen bonding.

High-pressure protein crystal structure analysis of Escherichia coli dihydrofolate reductase complexed with folate and NADP.

A high-pressure crystallographic study was conducted on Escherichia coli dihydrofolate reductase (ecDHFR) complexed with folate and NADP+ in crystal forms containing both the open and closed conformations of the M20 loop under high-pressure conditions of up to 800 MPa. At pressures between 270 and 500 MPa the crystal form containing the open conformation exhibited a phase transition from P21 to C2. Several structural changes in ecDHFR were observed at high pressure that were also accompanied by structural changes in the NADP+ cofactor and the hydration structure. In the crystal form with the closed conformation the M20 loop moved as the pressure changed, with accompanying conformational changes around the active site, including NADP+ and folate. These movements were consistent with the suggested hypothesis that movement of the M20 loop was necessary for ecDHFR to catalyze the reaction. In the crystal form with the open conformation the nicotinamide ring of the NADP+ cofactor undergoes a large flip as an intermediate step in the reaction, despite being in a crystalline state. Furthermore, observation of the water molecules between Arg57 and folate elucidated an early step in the substrate-binding pathway. These results demonstrate the possibility of using high-pressure protein crystallography as a method to capture high-energy substates or transient structures related to the protein reaction cycle.

Evolution of Interatomic and Intermolecular Interactions and Polymorphism of Melamine at High Pressure

Melamine (C3H6N6; 1,3,5-triazine-2,4,6-triamine) is an aromatic substituted s-triazine, with carbon and nitrogen atoms forming the ring body, and amino groups bonded to each carbon. Melamine is widely used to produce laminate products, adhesives, and flame retardants, but is also similar chemically and structurally to many energetic materials, including TATB (2,4,6-triamino-1,3,5- trinitrobenzene) and RDX (1,3,5-trinitroperhydro-1,3,5-triazine). Additionally, melamine may be a precursor in the synthesis of superhard carbon-nitrides, such as β-C3N4. In the crystalline state melamine forms corrugated sheets of individual molecules, which are stacked on top of one another, and linked by intra- and inter-plane N-H hydrogen bonds. Several previous high-pressure X-ray diffraction and Raman spectroscopy studies have claimed that melamine undergoes two or more phase transformations below 25 GPa. Our results show no indication of previously reported low pressure polymorphism up to approximately 30 GPa. High-pressure crystal structure refinements demonstrate that the individual molecular units of melamine are remarkably rigid, and their geometry changes very little despite volume decrease by almost a factor of two at 30 GPa and major re-arrangements of the intermolecular interactions, as seen through the Hirshfeld surface analysis. A symmetry change from monoclinic to triclinic, indicated by both dramatic changes in diffraction pattern, as well as discontinuities in the vibration mode behavior, was observed above approximately 36 GPa in helium and 30 GPa in neon pressure media. Examination of the hydrogen bonding behavior in melamine’s structure will allow its improved utilization as a chemical feedstock and analog for related energetic compounds.

High-Pressure Synthesis, Crystal Structure, and Semimetallic Properties of HgPbO3.

The crystal structure of HgPbO3 was studied using single-crystal X-ray diffraction and powder synchrotron X-ray diffraction. The structure was well characterized as a centrosymmetric model with a space group of R-3 m [hexagonal setting: a = 5.74413(6) Å and c = 7.25464(8) Å] rather than as a noncentrosymmetric model as was expected. It was found that Pb4+ is octahedrally coordinated by six oxygen atoms as usual, while Hg2+ is coordinated by three oxygen atoms in a planar manner, this being a very rare coordination of Hg in a solid-state material. The magnetic and electronic transport properties were investigated in terms of the magnetic susceptibility, magnetization, Hall coefficient, and specific heat capacity of polycrystalline HgPbO3. Although HgPbO3 has a carrier concentration (=7.3-8.5 × 1020 cm-3) that is equal to that of metallic oxides, the very weak temperature dependence of the electrical resistivity (residual-resistivity ratio ∼1.5), the significant diamagnetism (= -1.02 × 10-4 emu mol-1 at 300 K) that is in the same order of that of Bi powder and the remarkably small Sommerfeld coefficient [=1.6(1) × 10-3 J mol-1 K-2] implied that it is semimetallic in nature. HgPbO3 does not have a cage structure; nevertheless, at temperatures below approximately 50 K, it clearly exhibits phonon excitation of an anharmonic vibrational mode that is as significant as those of RbOs2O6. The mechanism of the anharmonic mode of the HgPbO3 has yet to be identified, however.

Benzoquinone-Bridged Heterocyclic Zwitterions as Building Blocks for Molecular Semiconductors and Metals.

In pursuit of closed-shell building blocks for single-component organic semiconductors and metals, we have prepared benzoquino-bis-1,2,3-thiaselenazole QS, a heterocyclic selenium-based zwitterion with a small gap (λmax = 729 nm) between its highest occupied and lowest unoccupied molecular orbitals. In the solid state, QS exists in two crystalline phases and one nanocrystalline phase. The structures of the crystalline phases (space groups R3 c and P21/ c) have been determined by high-resolution powder X-ray diffraction methods at ambient and elevated pressures (0-15 GPa), and their crystal packing patterns have been compared with that of the related all-sulfur zwitterion benzoquino-bis-1,2,3-dithiazole QT (space group Cmc21). Structural differences between the S- and Se-based materials are interpreted in terms of local intermolecular S/Se···N'/O' secondary bonding interactions, the strength of which varies with the nature of the chalcogen (S vs Se). While the perfectly two-dimensional "brick-wall" packing pattern associated with the Cmc21 phase of QT is not found for QS, all three phases of QS are nonetheless small band gap semiconductors, with σRT ranging from 10-5 S cm-1 for the P21/ c phase to 10-3 S cm-1 for the R3 c phase. The bandwidths of the valence and conduction bands increase with applied pressure, leading to an increase in conductivity and a decrease in thermal activation energy Eact. For the R3 c phase, band gap closure to yield an organic molecular metal with a σRT of ∼102 S cm-1 occurs at 6 GPa. Band gaps estimated from density functional theory band structure calculations on the ambient- and high-pressure crystal structures of QT and QS correlate well with those obtained experimentally.

The influence of channel anion identity on the high-pressure crystal structure, compressibility, and stability of apatite

The material properties of the common phosphate mineral apatite are influenced by the identity of the channel anion, which is usually F−, Cl−, or (OH)−. Density functional theory calculations have been used to determine the effect of channel anion identity on the compressibility and structure of apatite. Hydroxyapatite and fluorapatite are found to have similar zero pressure bulk moduli, of 79.2 and 82.1 GPa, respectively, while chlorapatite is considerably more compressible, with K0 = 55.0 GPa. While the space groups of hydroxyapatite and fluorapatite do not change between 0 and 25 GPa, symmetrization of the Cl− site in chlorapatite at ~ 7.5 GPa causes the space group to change from P21/b to P63/m. Examination of the valence electron density distribution in chlorapatite reveals that this symmetry change is associated with a change in the coordination of the Cl− anion from threefold to sixfold coordinated by Ca. We also calculate the pressure at which apatite decomposes to form tuite, a calcium orthophosphate mineral, and find that the transition pressure is sensitive to the identity of the channel anion, being lowest for fluorapatite (13.8 GPa) and highest for chlorapatite (26.9 GPa). Calculations are also performed within the DFT-D2 framework to investigate the influence of dispersion forces on the compressibility of apatite minerals.

High-Pressure Formation of Cobalt Polyhydrides: A First-Principle Study.

The high-pressure phase diagram, crystal structures, and electronic properties of cobalt hydrides are systematically investigated in the pressure range of 1 atm to 300 GPa by first-principle calculations. Except for the experimentally found CoH, two new cobalt polyhydrides CoH2 and CoH3 are discovered at 10 and 30 GPa, respectively. The crystal structure of CoH2 is determined to have cubic symmetry with the space group Fm3̅m and then transforms into the I4/mmm phase above 42 GPa. In addition, CoH3 with Pm3̅m is stable between 30 and 300 GPa, which can be used as a potential hydrogen storage material with a high volumetric hydrogen density of 425 g H2/L. All the cobalt polyhydrides exhibit metallic and ionic characteristics at high pressure. Furthermore, application of the Allen-Dynes-modified McMillan equation estimated no superconductivity for cobalt polyhydrides.

Covalency Competition in the Quadruple Perovskite CdCu3Fe4O12.

Cadmium ions (Cd2+) are similar to calcium ions (Ca2+) in size, whereas the Cd2+ ions tend to form covalent bonds with the neighboring anions because of the high electronegativity. The covalent Cd-O bonds affect other metal-oxygen bonds, inducing drastic changes in crystal structures and electronic states. Herein, we demonstrate high-pressure synthesis, crystal structure, and properties of a new quadruple perovskite CdCu3Fe4O12. This compound exhibits an electronic phase transition accompanying a charge disproportionation of Fe ions without charge ordering below ∼200 K, unlike charge-disproportionation transition with rock-salt-type charge ordering for CaCu3Fe4O12. First-principle calculations and Mössbauer spectroscopy display that covalent Cd-O bonds effectively suppress the Fe-O bond covalency, resulting in an electronic state different from that of CaCu3Fe4O12. This finding proposes covalency competition among constituent metal ions dominating electronic states of complex metal oxides.

High-Pressure Synthesis, Crystal Structure, and Magnetic and Transport Properties of a Six-Layered SrRhO3.

A SrRhO3 polytype with six-layered (6M) structure was synthesized under high pressure and high temperature. The crystal structure was obtained by refining X-ray powder diffraction with the monoclinic space group C2/c with lattice parameters a = 5.5650(1) Å, b = 9.5967(2) Å, c = 14.0224(4) Å, and β = 92.846(2)°. It is isostructural with SrIrO3 synthesized under ambient pressure and consists of dimers of the face-shared Rh(2)O6 octahedra connected by their vertices to the corner-shared Rh(1)O6 octahedra along the c axis with a stacking of SrO3 layers in the sequence of CCHCCH, where C and H denote cubic and hexagonal closed packing, respectively. With increasing pressure, the 6M SrRhO3 transforms to an orthorhombic perovskite (Pv) phase, having a = 5.5673(1) Å, b = 5.5399(2) Å, c = 7.8550(2) Å in the space group Pbnm. A pressure-temperature phase diagram shows that the 6M-Pv phase boundary moves to lower temperatures with increasing pressure. Both the 6M and the Pv phases of SrRhO3 were characterized by magnetic susceptibility, resistivity, and thermopower; they are all metals with an enhanced and temperature-dependent magnetic susceptibility; no long-range magnetic order has been found. The polytype structures are normally found in ABO3 oxides with the geometric tolerance factor t > 1. SrRhO3 represents another example (in addition to SrIrO3) where the polytype 6M structure can be stabilized with a t < 1.

Confirmation of the Structural Phase Transitions in XeF2 under High Pressure

Unique structures and properties will be introduced upon compression. To figure out the high pressure crystal structure of XeF2, in situ synchrotron X-ray diffraction, Raman spectra, UV–vis absorption spectra, and theoretical calculations have been performed up to 86 GPa. The structural dispute between reported experimental and theoretical results is settled by this study. The experimental and theoretical Raman spectra results indicated that the ambient structure of XeF2 (I4/mmm) transformed into a Immm structure at 28 GPa. Then this Immm structure transformed into Pnma structure at 59 GPa. The Rietveld refinement of the XRD results was in accordance with our Raman study. The optical absorption spectra revealed a reduction in the band gap as pressure increases. The reduction in the band gap decreases to 1.83 eV at 82 GPa while the color of the sample is getting dark. Our results provide a new insight into the high pressure behavior of noble gas compounds with the example of XeF2.

Crystal structure and transporting properties of Bi2S3 under high pressure: Experimental and theoretical studies

The high-pressure crystal structure and transporting properties of bismuthinite (Bi2S3) have been studied with a combination of experimental and theoretical methods. The present experimental results show that the structure of orthorhombic Bi2S3 stably exists in the experimental pressure range up to around 55 GPa. Upon compression, the X-Ray diffraction (XRD) patterns are dominated by broad Bragg features and seemingly pressure-induced structural amorphization or disorder occurs. However, by analyzing the XRD data obtained during the decompression cycle we suggest that the broad Bragg diffraction peaks under high pressure could be due to the nonhydrostatic conditions or crystal structural defects. To construct the correlation between the structural variation and the physical properties of Bi2S3, the transporting properties of Bi2S3 were investigated under high pressure. With increasing pressure, the resistance value decreases sharply below 5 GPa and then decreases gently as the pressure further increasing. There is an obvious inflection point at about 5 GPa in the relationships of log (R) versus pressure. We speculate that the observed electrical variation is resulted from the pressure-induced second-order isostructural phase transition according to the reported literature. Temperature dependence of the resistance indicated that a pressure-induced semiconductor → metal transition in Bi2S3 happens at around 20 GPa. The present high pressure resistance measurement indicated that high pressure could help to improve the thermoelectric properties.

High-pressure crystal structures of TaAs from first-principles calculations

Abstract In this work, we systematically studied the phase transition of TaAs under high pressures and reported three high-pressure structures P-6m2 (hexagonal, stable at 13–32 GPa), P21/c (monoclinic, stable at 32–103 GPa) and Pm-3m (cubic, stable above 103 GPa), by using particle swarm optimization in combination with first principles electronic structure methodology. All predicted structures are dynamically stable, since there is no imaginary mode to be found in the whole Brillouin zone. At high pressures, the TaAs was found to become superconductor with the superconducting critical temperature of ~1 K at about 100 GPa.

High-Pressure Crystal Structure, Lattice Vibrations, and Band Structure of BiSbO4.

The high-pressure crystal structure, lattice-vibrations, and electronic band structure of BiSbO4 were studied by ab initio simulations. We also performed Raman spectroscopy, infrared spectroscopy, and diffuse-reflectance measurements, as well as synchrotron powder X-ray diffraction. High-pressure X-ray diffraction measurements show that the crystal structure of BiSbO4 remains stable up to at least 70 GPa, unlike other known MTO4-type ternary oxides. These experiments also give information on the pressure dependence of the unit-cell parameters. Calculations properly describe the crystal structure of BiSbO4 and the changes induced by pressure on it. They also predict a possible high-pressure phase. A room-temperature pressure-volume equation of state is determined, and the effect of pressure on the coordination polyhedron of Bi and Sb is discussed. Raman- and infrared-active phonons were measured and calculated. In particular, calculations provide assignments for all the vibrational modes as well as their pressure dependence. In addition, the band structure and electronic density of states under pressure were also calculated. The calculations combined with the optical measurements allow us to conclude that BiSbO4 is an indirect-gap semiconductor, with an electronic band gap of 2.9(1) eV. Finally, the isothermal compressibility tensor for BiSbO4 is given at 1.8 GPa. The experimental (theoretical) data revealed that the direction of maximum compressibility is in the (0 1 0) plane at ∼33° (38°) to the c-axis and 47° (42°) to the a-axis. The reliability of the reported results is supported by the consistency between experiments and calculations.