Metastable High-pressure Phase Research Articles

Pressure-induced shape and color changes and mechanical-stimulation-driven reverse transition in a one-dimensional hybrid halide

Dynamic crystals with directional deformations in response to external stimuli through molecular reconfiguration, are observed predominantly in certain organic crystals and metal complexes. Low-dimensional hybrid halides, resemble these materials due to the presence of strong hydrogen bonds between bulky organic moieties and inorganic units, whereas their dynamic behavior remains largely unexplored. Here we show that a one-dimensional hybrid halide (MV)BiBr5 (MV = methylviologen) undergoes an isosymmetric phase transition at hydrostatic pressure of 0.20 GPa, accompanied by a remarkable length expansion of 20–30% and red to dark yellow color change. Unexpectedly, the backward transition can be fully reversed by mechanical stimulation rather than decompression. In the high-pressure phase, the coexistence of strong Bi3+ lone pair stereochemical activity and large reorientations of the planar MV2+ cations, together with the newly formed CH···Br hydrogen interactions, are the structural features that facilitate microscopic changes and stabilize the metastable high-pressure phase at ambient conditions.

Slightly Hydrogen-Ordered State of Ice IV Evidenced by In Situ Neutron Diffraction.

Ice IV is a metastable high-pressure phase of ice in which the water molecules exhibit orientational disorder. Although orientational ordering is commonly observed for other ice phases, it has not been reported for ice IV. We conducted in situ powder neutron diffraction experiments for DCl-doped D2O ice IV to investigate its hydrogen ordering. We found abrupt changes in the temperature derivative of unit-cell volume, dV/dT, at ∼120 K, and revealed a slightly ordered structure at low temperatures based on the Rietveld method. The occupancy of the D1 site deviates from 0.5 in particular; it increased when samples were cooled at higher pressures and reached 0.174(14) at 2.38 GPa, 58 K. Our results evidence the presence of a low-symmetry hydrogen-ordered state corresponding to ice IV. It seems, however, difficult to experimentally access the completely ordered phase corresponding to ice IV by slow cooling at high pressure.

Recent <i>In Situ</i> Experimental and Theoretical Advances in Severe Plastic Deformations, Strain-Induced Phase Transformations, and Microstructure Evolution under High Pressure

Severe plastic deformations (SPD) under high pressure, mostly by high-pressure torsion, are employed for producing nanostructured materials and stable or metastable high-pressure phases. However, they were studied postmortem after pressure release. Here, we review recent in situ experimental and theoretical studies of coupled SPD, strain-induced phase transformations (PTs), and microstructure evolution under high pressure obtained under compression in diamond anvil cell or compression and torsion in rotational diamond anvil cell. The utilization of x-ray diffraction with synchrotron radiation allows one to determine the radial distribution of volume fraction of phases, pressure, dislocation density, and crystallite size in each phase and find the main laws of their evolution and interaction. Coupling with the finite element simulations of the sample behavior allows the determination of fields of all components of the stress and plastic strain tensors and volume fraction of high-pressure phase and provides a better understanding of ways to control occurring processes. Atomistic, nanoscale and scale-free phase-field simulations allow elucidation of the main physical mechanisms of the plastic strain-induced drastic reduction in phase transformation pressure (by one to two orders of magnitude), the appearance of new phases, and strain-controlled PT kinetics in comparison with hydrostatic loading. Combining in situ experiments with multiscale theory potentially leads to the formulation of methods to control strain-induced PT and microstructure evolution and designing economic synthetic paths for the defect-induced synthesis of desired high-pressure phases, nanostructures, and nanocomposites.

Characterization of metastable high pressure phase transition positions and its influence on the behavior of microbial destruction

Effect of perturbation factors on phase transition metastable positions of whole milk (4% fat content) and their influence on microbial destruction characteristics of non-pathogenic Escherichia coli inoculated in milk subjected to high pressure low temperature treatment were evaluated using a specially developed high pressure (HP) cooling system. Initially, the phase transition data of milk transitioning through the metastable phases were obtained and fitted successfully using Simon-like models as done in previous studies and polynomial formulas with R2 of 0.997 & 0.996 for ice I, and 0.989 & 0.989 for ice III, respectively. The phase transition position of milk was explored with 5% and 10% sodium chloride solution as perturbation sources, respectively. Results showed that the 5% sodium chloride solution can reduce the transition pressure of milk by 43 MPa and increase the transition temperature by 4.1 °C, so that the milk can achieve phase transition at lower pressure and higher temperature. Phase transition microbial destruction was characterized by discontinuity, mutation and segmentation when the phase transition pressure interval 250– 300 MPa was carefully refined. The inactivation amount of E. coli before the phase transition (250 MPa) was 1.11 log and the phase transition process itself brought an additional 1.26 log destruction of E. coli population in milk. Industrial relevanceHigh pressure low temperature (HPLT) phase change kinetics were employed to enhance microbial destruction. HPLT was established based on a self-cooling unit positioned inside conventional HP chamber offering opportunities for scale up and commercialization. The effectiveness of HPLT phase transition for Escherichia coli destruction was demonstrated. The related research in metastable state provides a reference point for commercial application of high-pressure-low-temperature technology for microbial destruction and quality enhancement.

Nanocrystals with metastable high-pressure phases under ambient conditions.

The ambient metastability of the rock-salt phase in well-defined model systems comprising nanospheres or nanorods of cadmium selenide, cadmium sulfide, or both was investigated as a function of composition, initial crystal phase, particle structure, shape, surface functionalization, and ordering level of their assemblies. Our experiments show that these nanocrystal systems exhibit ligand-tailorable reversibility in the rock salt-to-zinc blende solid-phase transformation. Interparticle sintering was used to engineer kinetic barriers in the phase transformation to produce ambient-pressure metastable rock-salt structures in a controllable manner. Interconnected nanocrystal networks were identified as an essential structure that hosted metastable high-energy phases at ambient conditions. These findings suggest general rules for transformation-barrier engineering that are useful in the rational design of next-generation materials.

The New High-Pressure Phases of Nitrogen-Rich Ag-N Compounds.

The high-pressure phase diagram of Ag–N compounds is enriched by proposing three stable high-pressure phases (P4/mmm-AgN2, P1-AgN7 and P-1-AgN7) and two metastable high-pressure phases (P-1-AgN4 and P-1-AgN8). The novel N7 rings and N20 rings are firstly found in the folded layer structure of P-1-AgN7. The electronic structure properties of predicted five structures are studied by the calculations of the band structure and DOS. The analyses of ELF and Bader charge show that the strong N–N covalent bond interaction and the weak Ag–N ionic bond interaction constitute the stable mechanism of Ag–N compounds. The charge transfer between the Ag and N atoms plays an important role for the structural stability. Moreover, the P-1-AgN7 and P-1-AgN8 with the high-energy density and excellent detonation properties are potential candidates for new high-energy density species.

Tungsten Oxide/Zirconia As an Efficient Polysulfide Mediator for High-Performance Lithium−Sulfur Batteries

In this study, an effective strategy is proposed to enhance the limitations associated with the use of a Li2S cathode in lithium−sulfur (Li−S) batteries. Tungsten oxide/zirconia (WO3/ZrO2) with a ratio of 3:7 is prepared and integrated into a 2D compacted Li2S−graphene matrix through pelletization. The newly developed composite material has a great potential as cathode material for Li−S batteries. In the composite cathode, presence of a metastable high-pressure Li2S phase enable the direct conversion of Li2S to S8 during the first activation process, the high electrical conductivity of graphene, and simultaneously the integrated tungsten oxide/zirconia particles act as a polysulfide mediator to obstruct the outflow of soluble polysulfide into the electrolyte solution. Hence, the composite cathode shows high interfacial stability and thus substantially extends the lifetime of the Li−S battery over 200 cycles at a high Li2S loading of 10 mg cm−2. The practical Li−S full battery fabricated by combining the composite cathode and graphite anode provides outstanding cycle retention over 1500 cycles. References Hwang, J.-Y.; Shin, S.; Yoon, C. S.; Sun, Y.-K. ACS Energy Lett., 2019, 4, 2787−2795. Lee, S.-K.; Lee, Y. J.; Sun, Y.-K. J. Power Sources, 2016, 323, 174−188. Agostini, M.; Hassoun, J.; Liu, J.; Jeong, M.; Nara, H.; Momma, T.; Osaka, T.; Sun, Y.-K.; Scrosati, B. ACS Appl. Mater. Interfaces, 2014, 6, 10924−10928.

New Cadmium-Nitrogen Compounds at High Pressures.

A systematic high-pressure study of the CdNx (x = 2, 3, 4, 5, and 6) system is performed by using the first-principles calculation method in combination with the particle swarm optimization algorithm. We proposed four stable high-pressure phases (P4mbm-CdN2, Cmmm-CdN4, I4̅2d-CdN4, and C2/c-CdN5) and one metastable high-pressure phase (C2/m-CdN6), for which the structural frames are composed of a diatomic quasi-molecule N2, standard armchair N-chain, S-type bent armchair N-chain, zigzag-antizigzag N-chain, and N14 network structure. Among them, the novel zigzag-antizigzag N-chain and N14 network structure are reported for the first time. More importantly, Cmmm-CdN4 and C2/m-CdN6 possess high stability under ambient conditions, which may be quenched to ambient conditions once they are synthesized at high-pressure conditions. The high decomposition energy barrier (1.14 eV) results in a high decomposition temperature (2500 K) of Cmmm-CdN4, while a low decomposition energy barrier (0.19 eV) results in a mild decomposition temperature (500 K) of C2/m-CdN6. The high energy density and outstanding explosive performance make Cmmm-CdN4, I4̅2d-CdN4, C2/c-CdN5, and C2/m-CdN6 potential high-energy materials. The electronic structure analyses show that these predicted high-pressure structures are all metallic phases, and the N-N and Cd-N bonds are the strong covalent and ionic bond interactions, respectively. The charge transfer from the Cd atom plays an important role in the stability of the proposed structures.

Exotic silicon phases synthesized through ultrashort laser-induced microexplosion: Characterization with Raman microspectroscopy

Confined microexplosions induced in silicon by powerful ultrashort laser pulses can lead to new Si phases. Some of these have not previously been observed via near-equilibrium compression of silicon. In this study, confocal Raman micro-spectroscopy and Raman imaging of arrays of microexplosions have been conducted to search for Raman signatures of these novel allotropes of silicon. A microexplosion is generated at the interface between a thick silicon dioxide confinement layer and underlying silicon. It is characterised by a void at the interface above a region of compressed silicon. Raman data show a rich assembly of silicon phases within the modified silicon. Residual stresses up to 4.5 GPa in the modifications have been determined from the shift in the main diamond-cubic Si Raman peak. The computed Raman spectra for a number of Si allotropes show reasonable agreement with the experimental spectra. Two structurally similar tetragonal phases of silicon (the rhombohedral r8 and the body-centred bc8) phases as well as recently identified bt8-Si are all highly likely to be contained in Raman spectra from many laser-modified sites. Although the st12-Si phase, previously observed in our electron diffraction studies of the highly compressively stressed laser-modified regions, was not reliably identified from Raman data, we suggest this could be due to the possible difference in residual stress level in the sites analysed by the electron diffraction and Raman spectra. Several other unidentified Raman peaks were observed, suggesting the presence of other unknown silicon phases. All of these silicon phases are expected to have attractive semiconducting properties including narrow band gap that open up novel applications.

Stabilizing the metastable superhard material wurtzite boron nitride by three-dimensional networks of planar defects

Wurtzite boron nitride (w-BN) is a metastable superhard material that is a high-pressure polymorph of BN. Clarifying how the metastable high-pressure material can be stabilized at atmospheric pressure is a challenging issue of fundamental scientific importance and promising technological value. Here, we fabricate millimeter-size w-BN bulk crystals via the hexagonal-to-wurtzite phase transformation at high pressure and high temperature. By combining transmission electron microscopy and ab initio molecular dynamics simulations, we reveal a stabilization mechanism for w-BN, i.e., the metastable high-pressure phase can be stabilized by 3D networks of planar defects which are constructed by a high density of intersecting (0001) stacking faults and {10[Formula: see text]0} inversion domain boundaries. The 3D networks of planar defects segment the w-BN bulk crystal into numerous nanometer-size prismatic domains with the reverse crystallographic polarities. Our findings unambiguously demonstrate the retarding effect of crystal defects on the phase transformations of metastable materials, which is in contrast to the common knowledge that the crystal defects in materials will facilitate the occurrence of phase transformations.

MnBi2: A Metastable High-Pressure Phase in the Mn–Bi System

MnBi₂: A Metastable High-Pressure Phase in the Mn–Bi System

Metastable silica high pressure polymorphs as structural proxies of deep Earth silicate melts

Modelling of processes involving deep Earth liquids requires information on their structures and compression mechanisms. However, knowledge of the local structures of silicates and silica (SiO2) melts at deep mantle conditions and of their densification mechanisms is still limited. Here we report the synthesis and characterization of metastable high-pressure silica phases, coesite-IV and coesite-V, using in situ single-crystal X-ray diffraction and ab initio simulations. Their crystal structures are drastically different from any previously considered models, but explain well features of pair-distribution functions of highly densified silica glass and molten basalt at high pressure. Built of four, five-, and six-coordinated silicon, coesite-IV and coesite-V contain SiO6 octahedra, which, at odds with 3rd Pauling’s rule, are connected through common faces. Our results suggest that possible silicate liquids in Earth’s lower mantle may have complex structures making them more compressible than previously supposed.

Synthesis of Hexagonal Diamond under Shock Loading Using Single Stage Powder Gun

Hexagonal diamond is considered to be a metastable high-pressure phase of carbon. In previous ab-initio studies, elastic constants of hexagonal diamond were suggested to be higher than those of cubic diamond, which is the stiffest known material. However, the elastic constants of hexagonal diamond have not been investigated experimentally because the size of hexagonal diamond single crystal ever synthesized is not more than 0.5 μm. In this study, we synthesized hexagonal diamond with the size of more than 50 μm in order to measure the elastic constants accurately. Kish graphite powder was used as a starting material and a green compact of kish graphite and copper powder was fabricated as a target. The target was placed into a container made of stainless steel and shock-compressed by a copper projectile accelerated using a single stage powder gun with the velocity of approximately 800 m/s. The estimated shock pressure and temperature were 22 GPa and 1200 °C, respectively. After the shock compression, copper component of the target was dissolved in nitric acid for 24 hours and only carbon component was recovered. A clear (100) peak of hexagonal diamond was observed by a XRD result of the recovered carbon. No peak of cubic diamond was observed. Opaque particles including hexagonal diamond with the size of approximately 100 μm were separated from the graphite and SiO2 contaminations using heavy-liquid separation method with sodium polytungstate aqueous solution.

Coexistence of site- and bond-centered electron localization in the high-pressure phase ofLuFe2O4

Magnetic-electronic hyperfine interaction parameters of spectral components are obtained from in situ $^{57}\mathrm{Fe}$ M\ossbauer spectroscopy pressure studies of the mixed-valence $\mathrm{LuF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ multiferroic, up to $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and on recovered high-pressure phase samples. Temperature-dependent M\ossbauer spectra of the low-pressure phase show that $\mathrm{F}{\mathrm{e}}^{2+}$ and $\mathrm{F}{\mathrm{e}}^{3+}$ sites are discernible, consistent with known site-centered charge order in the triangular (frustrated) Fe sublattice network. Magnetic spectra of the high-pressure phase, stabilized in a rectangular Fe sublattice network at $Pg8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, exhibit fingerprints of iron in an intermediate valence state only. Temperature-dependent resistivity pressure studies evidence thermally activated small polaron motion in the high-pressure phase. These experimental signatures, complemented by ab initio calculations of electronic structure, are considered evidence of asymmetric dimer formation ${\mathrm{Fe}}^{(2+\mathrm{\ensuremath{\Delta}}+)}\ensuremath{\Leftrightarrow}{\mathrm{Fe}}^{(3\ensuremath{-}\mathrm{\ensuremath{\Delta}})+}$, where the minority-spin electron deconfinement coefficient is $\mathrm{\ensuremath{\Delta}}=0.3\ensuremath{-}0.4$. Bragg satellites discerned in electron diffraction patterns of the metastable high-pressure phase possibly stem from this admixture of site- and bond-centered localization (intermediate-state charge order) in a magnetic background. This breaks inversion symmetry and potentially renders $\mathrm{LuF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ in its high-pressure phase as a new charge order instigated (electronic) ferroelectric.

Femtosecond laser-induced confined microexplosion: tool for creation high-pressure phases

ABSTRACTNew material phases formed under non-equilibrium conditions at pressures above 100 GPa and temperatures exceeding 104K, the conditions of the warm dense matter (WDM), have become accessible using micro-explosions triggered by ultra-short sub-1 ps pulses tightly focused into micro-volume with cross sections comparable with the wavelength of light. Laser-induced micro-explosions convert a material in a focal volume into a non-equilibrium disordered plasma state confined inside the bulk of pristine crystal. Ultra-high quenching rates overcome kinetic barriers to the formation of new metastable high pressure phases, which are preserved in the surrounding pristine crystal for following recovery and exploitation. Direct laser writing was used to pattern large areas by closely packed arrays of the microexplosion modified sites for structural characterisation of the minute volumes of nano-materials with transmission electron microscopy, diffraction and synchrotron X-ray diffraction. The method of ultrafast-laser induced confined microexplosion is demonstrated for modification and creation of new phases in case of bcc-Al inside sapphire, valence change of Fe-ions in olivine, formation of new tetragonal bt8 and st12 phases of silicon, Ge and O separation in GeO2 glass and molecular oxygen formation inside voids at the site of microexplosion inside glasses.

Pressure-induced phase transitions in coesite

High-pressure behavior of coesite was studied on single crystals using diamond-anvil cells with neon as the pressure-transmitting medium by means of in situ Raman spectroscopy up to pressures of ~ 51 GPa. The experimental observations were complemented with theoretical computations of the Raman spectra under similar pressure conditions. We find that coesite undergoes two phase transitions and does not become amorphous at least up to ~ 51 GPa. The first phase transition (coesite I to coesite II) is reversible and occurs around 23 GPa. The second transition (coesite II to coesite III) at about 35 GPa is also reversible but involves a large hysteresis. Samples recovered from the highest pressure achieved, ~ 51 GPa, show Raman spectra of the initial coesite. The ab initio calculations gave insight into the initiation mechanism of the first phase transition, implying, from the analysis of unstable phonon modes, that it is probably a displacive phase transition due to shearing of the four-membered rings of SiO4 tetrahedra upon compression. The transition to the lowest-symmetry phase, coesite III, is possibly a first-order phase transition that leads to a very distinct structure. None of the metastable high-pressure phases of coesite has been previously studied and it was widely accepted that coesite undergoes pressure-induced amorphization at significantly lower pressures (30 GPa). The study of the high-pressure behavior of coesite is important to better constrain the metastable phase diagram of silica. Further crystallographic investigations are necessary for characterizing the structures of these metastable coesite forms. Crystalline or amorphous metastable phases derived from coesite under high-pressure conditions are of particular interest because they can be used as potential tracers of peak transient pressures (stress) reached in processes such as impacts or faulting.

Low-temperature thermal expansion of rock-salt ZnO

Lattice parameter of metastable high-pressure phase of zinc oxide, rock-salt ZnO, was measured in the 10–300K temperature range using synchrotron X-ray powder diffraction. No phase transition was observed down to 10K. The lattice parameter of rock-salt ZnO was found to increase from 4.266Å in the 10–80K range up to 4.2752(3)Å at 298K, while the volume thermal expansion coefficient increases from slight negative values below 40K up to 4.77×10-5K-1 at 298K.

Effects of substrates on the crystal structure, texturing and optical properties of AlN coatings deposited by inverted cylindrical magnetron sputtering

AlN films of thicknesses 670–780 nm were deposited on (1 1 1) silicon wafer, (0 0 · 1) sapphire, float glass and fused silica substrates by reactive mid-frequency (41 kHz) ac sputtering of dual hollow Al targets in a mixture of Ar and N2 gases. Nanostructured films of hexagonal wurtizite phase of AlN were prepared and characterized by UV-visible spectroscopy and X-ray diffraction. The novel geometry of dual target inverted cylindrical magnetrons produces very dense plasma and ion bombardment of the growing film, and facilitates the formation of highly crystalline AlN coatings without any deliberate substrate heating. Substrates showed significant influence on the crystal structure of AlN films; sapphire forms hexagonal AlN phase with very strong texturing toward (0 0 · 2) crystallographic orientation, silicon wafer grows small amount of metastable high-pressure cubic phase of AlN along with the predominant, thermodynamically stable hexagonal phase while the amorphous glass substrates form hexagonal AlN phase with a random orientation of the crystallites.

Femtosecond laser-driven shock synthesis of hexagonal diamond from highly oriented pyrolytic graphite

We synthesized hexagonal diamond directly from highly oriented pyrolytic graphite (HOPG) using a femtosecond laser pulse without catalyst. A femtosecond laser pulse with wavelength of 800 nm, pulse width of 130 fs, the intensity of 2×1015 W/cm2 was irradiated onto the HOPG surface in air. Crystalline structures of the fs laser-affected region in the HOPG were analyzed using grazing-incidence XRD method. We found that the hexagonal diamond which is the metastable high-pressure phase of carbon appeared in the HOPG which was irradiated by the femtosecond laser normal to the basal plane. We suggest that the femtosecond laser-driven shock wave induces the graphite – hexagonal diamond transformation and that the hexagonal diamond is synthesized due to the rapidly cooling in the shock heated region.

Metastable high-pressure single-bonded phases of nitrogen predicted via genetic algorithm

Metastable high-pressure single-bonded phases of nitrogen predicted via genetic algorithm