Low-pressure Phase Research Articles

Pressure-induced isostructural phase transition in α-Ni(OH)2 nanowires**Project supported by the National Key Research and Development Program of China (Grant No. 2018YFA0305900), the National Natural Science Foundation of China (Grant Nos. 11874172, 11374120, 11634004, and 51320105007), and the Fund from Jilin University for Science and Technology Innovative Research Team (Grant No. 2017TD-01).

High pressure structural phase transition of monoclinic paraotwayite type α-Ni(OH)2 nanowires with a diameter of 15 nm–20 nm and a length of several micrometers were studied by synchrotron x-ray diffraction (XRD) and Raman spectra. It is found that the α-Ni(OH)2 nanowires experience an isostructural phase transition associated with the amorphization of the H-sublattice of hydroxide in the interlayer spaces of the two-dimensional crystal structure at 6.3 GPa–9.3 GPa. We suggest that the isostructural phase transition can be attributed to the amorphization of the H-sublattice. The bulk moduli for the low pressure phase and the high pressure phase are 41.2 (4.2) GPa and 94.4 (5.6) GPa, respectively. Both the pressure-induced isostructural phase transition and the amorphization of the H-sublattice in the α-Ni(OH)2 nanowires are reversible upon decompression. Our results show that the foreign anions intercalated between the α-Ni(OH)2 layers play important roles in their structural phase transition.

Origin of the piezochromism in Cs2CuCl4 : Electron-phonon and crystal-structure correlations

By using optical absorption and Raman spectroscopy, we have investigated the effects of pressure in the structural, electronic, and vibrational properties of ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ in the 0--20 GPa range. Our study has provided a complete characterization of the electronic and vibrational structures of ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ in the low-pressure Pnma phase as a function of cell volume and the Cu-Cl bond length, ${R}_{\text{Cu-Cl}}$. We have focused on the electronic structure in the Pnma phase, which is mainly related to the tetrahedral ${\mathrm{CuCl}}_{4}^{2\ensuremath{-}}$ units distorted by the Jahn-Teller effect. The strong piezochromism of ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ is caused by the opposite shift of the charge-transfer band gap and the ${\mathrm{Cu}}^{2+}$ d-d crystal-field transitions with pressure. The high-pressure structure of ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ above 4.9 GPa yields structural transformations that we ascribe to a change of coordination around ${\mathrm{Cu}}^{2+}$. The high-pressure phase appears largely amorphized, therefore any structural information from x-ray diffraction is ruled out. Here we use electronic and vibrational probes to get inside the structural information.

Pressure-Induced Phase Transitions in Germanium Telluride: Raman Signatures of Anharmonicity and Oxidation.

Pressure-induced phase transitions in GeTe, a prototype phase change material, have been studied to date with diffraction which is not sensitive to anharmonicity-induced dynamical effects. GeTe is also prone to surface oxidation which may compromise surface sensitive measurements. These factors could be responsible for the lack of clarity about the phases and transitions intervening in the phase diagram of GeTe. We have used high-pressure Raman scattering and abinitio pseudopotential density functional calculations to unambiguously establish the high-pressure phase diagram and identify three phases up to 57GPa, a low-pressure rhombohedral phase, an intermediate pressure cubic phase, and a high-pressure orthorhombic phase. We detect substantial broadening and softening of Raman modes at low pressure and identify the transition regions and possible intermediate phases.

Structural phase transitions in malononitrile, CH2(CN)2: crystal structure of the δ phase by neutron powder diffraction, and ab initio calculations of the structures and phonons of the α and δ phases

The crystal structure of the low-temperature phase of crystalline malononitrile, CH2(CN)2 (stable phase below 260 K), has been determined using Rietveld refinement on neutron powder diffraction data. The phase has a slightly lower density than the other three low-pressure phases, and unlike those phases it has a polar structure. The transition from the to phase involves a major reconstruction of the structure, including establishing a network of hydrogen bonds. DFT simulations of the structure and phonon dispersion curves of both and phases give free energy curves consistent with the phase transitions. It is noted that the transition from the to phase at 260 K is facilitated by the entropy arising from the low-frequency phonons associated with the soft mode for the sequence of –– phase transitions.

Structural and electrical properties of Ga–Te systems under high pressure**Project supported by the National Key R&D Program of China (Grant Nos. 2018YFA0305900 and 2016YFB0201204), the National Natural Science Foundation of China (Grant Nos. 51632002, 51572108, 11574109, 91745203, and 11634004), Program for Changjiang Scholars and Innovative Research Team in University, China (Grant No. IRT_15R23), and National Fund for Fostering Talents of

First-principles evolutionary calculation was performed to search for all probable stable Ga–Te compounds at extreme pressure. In addition to the well-known structures of P63/mmc and Fm-3m GaTe and I4/m Ga2Te5, several new structures were uncovered at high pressure, namely, orthorhombic I4/mmm GaTe2 and monoclinic C2/m GaTe3, and all the Ga–Te structures stabilize up to a maximum pressure of 80 GPa. The calculation of the electronic energy band indicated that the high-pressure phases of the Ga–Te system are metallic, whereas the low-pressure phases are semiconductors. The electronic localization functions (ELFs) of the Ga–Te system were also calculated to explore the bond characteristics. The results showed that a covalent bond is formed at low pressure, however, this bond disappears at high pressure, and an ionic bond is formed at extreme pressure.

Pressure-Induced Hexagonal to Monoclinic Phase Transition of Partially Hydrated CePO4.

We present a study of the pressure dependence of the structure of partially hydrated hexagonal CePO4 up to 21 GPa using synchrotron powder X-ray diffraction. At a pressure of 10 GPa, a second-order structural phase transition is observed, associated with a novel polymorph. The previously unknown high-pressure phase has a monoclinic structure with a similar atomic arrangement as the low-pressure phase, but with reduced symmetry, belonging to space group C2. Group-subgroup relations hold for the space symmetry groups of both structures. There is no detectable volume discontinuity at the phase transition. Here we provide structural information on the new phase and determine the axial compressibility and bulk modulus for both phases. They are found to have an anisotropic behavior and to be much more compressible than the denser monazite-like polymorph of CePO4. In addition, the isothermal compressibility tensor for the high-pressure structure is reported at 10 GPa and the direction of maximum compressibility described. Finally, the possible role of water and the pressure medium in the high-pressure behavior is discussed. The results are compared with those from other rare-earth orthophosphates.

First-principles prediction of carbon monoxide nanotube bundles in low pressure phase

The crystal structure of carbon monoxide has been studied for more than half a century. The internal structures of low-pressure carbon monoxide crystals have been investigated by means of infrared analysis and Raman analysis, and the internal structure of carbon monoxide has also been studied through computational analysis. Previous studies showed that carbon monoxide can produce different phase transitions at different pressures, and thus forming new polymers with new physical properties such as electrical, optical and mechanical properties. In this paper, from first-principles calculations, we propose six nanotube structures made of carbon monoxide, named Tube-3–Tube-8. The nanotubes are packed into the nanotube bundles, and carbon monoxide nanotube bundle structures that are similar to carbon nanotube bundles are constructed by first-principles calculation. We study the structural, energy and electronic properties of the nanotubes and nanotube bundles. In order to evaluate the relative stability of the predicted nanotubes, we calculate the cohesive energy and phonon spectrum, and we also carry out the molecular dynamics analysis. The results show that there are three nanotubes (Tube-4–Tube-6) that are relatively stable, of which Tube-5 nanotube is the most stable phase. We attribute the stability of Tube-5 to sp<sup>3</sup>-hybridized C atoms being nearest to the hybridized atoms of diamond. Then we investigate nanotube bundles from the three stable nanotubes, and accordingly name them Bundles-4–Bundles-6. We calculate the enthalpy function under pressure and compare it with the enthalpy function of several known carbon monoxide molecular crystal and chain crystal, which are the most stable structures according to the current studies. More pleasingly, we find that these nanotube bundles are more stable than these carbon monoxide molecular crystal and chain crystal at low pressure. In addition, by calculating the energy bands of Tube-4–Tube-6, we can deduce that these nanotube bundles (Bundles-4– Bundles-6) are all wide band gap semiconductors, which are entirely different from molecular and chain crystals that are metals. We expect that the discovery of nanotube bundle structures will increase the diversity of carbon monoxide crystal under low pressure, and provide a new understanding of exploring the internal structure of carbon monoxide crystal.

New high-pressure phases of Fe7N3 and Fe7C3 stable at Earth's core conditions: evidences for carbon-nitrogen isomorphism in Fe-compounds.

We carried out ab initio calculations on the crystal structure prediction and determination of P–T diagrams within the quasi-harmonic approximation for Fe7N3 and Fe7C3. Two new isostructural phases Fe7N3-C2/m and Fe7C3-C2/m which are dynamically and thermodynamically stable under the Earth's core conditions were predicted. The Fe7C3-C2/m phase stabilizes preferentially to the known h-Fe7C3 at 253–344 GPa in the temperature range of 0–5000 K, and the Fe7N3-C2/m stabilizes preferentially relative to the β-Fe7N3 – at ∼305 GPa over the entire temperature range. This indicate that carbon and nitrogen can mutually coexist and replace each other in the Earth's and other planetary cores similarly to low pressure phases of the same compounds.

High-Pressure Synthesis, Crystal Chemistry, and Ionic Conductivity of a Structural Polymorph of Li3BP2O8.

A new structural polymorph of Li3BP2O8 has been successfully synthesized via a solid-state reaction between Li3PO4 and BPO4 at 4 GPa and 600 °C. The high-pressure phase of Li3BP2O8 (HP-Li3BP2O8) was found to crystallize in monoclinic symmetry with the cell parameters of a = 8.57010(4) Å, b = 11.11812(5) Å, c = 5.55380(3) Å, and β = 97.7269(3)° [space group P21/ a (No. 14)]. HP-Li3BP2O8 has a new crystal structure that has not been reported so far. The total ionic conductivities measured for the polycrystalline sample by alternating-currrent impedance were 3.4 × 10-7 and 2.1 × 10-6 S/cm at 399 and 456 K, respectively. The lithium ionic conductivity of HP-Li3BP2O8 was higher than that of the low-pressure phase Li3BP2O8 in the temperature range of 375-456 K. This is caused by the difference in the dimensions of the lithium arrangements between LP- and HP-Li3BP2O8.

Low Pressure Phase Transformations During High-Speed, High-Temperature Scratching of Silicon

Higher temperature assisted processing of silicon, such as heat-assisted diamond turning, is often being considered to improve surface integrity. At higher temperatures and under mechanical loading and unloading, caused by a moving tool, silicon deforms plastically often in association with occurrence of phase transformations. This paper investigates such phase transformations in rotational scratching of single crystal (100) p-type silicon with a conical diamond tool under various furnace-controlled temperatures ranging from room temperature (RT) to 500 °C and at scratching speeds comparable to that used in the diamond turning process (1 m/s). Phase transformation study, using Raman spectroscopy, at various crystal orientations, shows differences in phases formed at various temperatures when compared to that reported in indentation. The tendency to form phases is compared between scratched and diamond turned surfaces at RT, and also with that reported at low scratching speeds in the literature. Analytical indenting-based pressure calculations show that at higher temperatures, phase transformations can happen in silicon at significantly lower pressures. Analysis of depths of the scratched groove indicates that at temperatures beyond a certain threshold, plastic deformation and significant elastic recovery may be causing shallow grooves. Abrasive wear coefficients are thus seen to decrease with the increase in temperatures. This study is expected to help tune heat-assisted diamond turning conditions to improve surface formation.

High-pressure study of dravite tourmaline: Insights into the accommodating nature of the tourmaline structure

Author(s): O'Bannon, E; Beavers, CM; Kunz, M; Williams, Q | Abstract: The high-pressure behavior of dravite tourmaline [Na(Mg3)Al6(Si6O18)(BO3)3(OH)3(OH)] has been studied using luminescence spectroscopy and synchrotron-based single-crystal diffraction up to ~65 and ~24 GPa, respectively. Two emission bands associated with Cr3+/V2+ substitution are constant in energy up to ~9.0 GPa, and they shift to longer wavelength at higher pressures, suggesting that a change in compressional mechanism could occur at this pressure. Single-crystal diffraction data show subtle changes in ring ditrigonality occur near 9.0 GPa, which could cause the observed change in luminescence. Near 15 GPa, a splitting of one of the emission bands is observed, suggesting that a phase transition occurs at this pressure and that two unique octahedral sites are present in the high-pressure phase. Hysteresis is not observed on decompression, which indicates that this is a second-order transition, and the high-pressure structure appears to be metastable up to ~65 GPa. Single-crystal diffraction measurements show that a phase transition from rhombohedral R3m to rhombohedral R3 occurs at pressures near 15.4 GPa. The high-pressure phase is characterized by a distorted Si6O18 ring (e.g., the Si-Si-Si angles deviate from 120°), and the Si, Al, O6, O7, and O8 sites of the low-pressure phase split, implying that the high-pressure phase of tourmaline is a higher entropy phase. The large X-site exerts the primary control on compressibility, and the substitution of larger cations into this site will likely lower the pressure at which this transition occurs. Dravite tourmaline shows anisotropic compression with the c-axis being more compressible than the a-axis. The pressure and volume data up to ~15.4 GPa were fit with second- and third-order Birch-Murnaghan equations of state. We obtain a bulk modulus, K0 = 109.6(3.2) GPa, and a pressure derivative, K0′ c K'0 = 4.6(8) GPa, and with the pressure derivative set to 4, a bulk modulus of 112.0(1.0) GPa is derived. Moreover, our high-pressure results show that massive overbonding of the X and Y sites can be accommodated by the tourmaline structure. This unexpected result may explain the extraordinary structural tolerance with respect to chemical substitution on the X, Y, and Z sites.

Dynamics and superconductivity in compressed lanthanum superhydride

Recent computational studies have predicted that rare-earth superhydrides are promising high-temperature superconductors. Of these phases having very high hydrogen content ($\mathrm{X}{\mathrm{H}}_{n},ng6$, where X is the rare-earth atom) a cubic phase of lanthanum hydride, recently synthesized at 170 GPa and identified as $\mathrm{La}{{\mathrm{H}}_{10}}_{\ifmmode\pm\else\textpm\fi{}x}$, is in good agreement with theoretical predictions. The experiments found that the stability of the phase extended to lower pressure and a distorted form was found on decompression. Here we examine the nuclear quantum effects and anharmonic dynamics on $\mathrm{La}{\mathrm{H}}_{10}$, including the behavior of the hydrogen sublattice in comparison with the predicted atomic metallic hydrogen at higher pressure. We also examine the vibrational dynamics and electronic properties of a distorted lower pressure phase of $\mathrm{La}{\mathrm{H}}_{10}$ and find that superconducting ${T}_{c}$ decreases relative to cubic but remains relatively high (i.e., 229--245 K).

Origin of extremely large magnetoresistance in the candidate type-II Weyl semimetal MoTe2\u2212x

The recent observation of extremely large magnetoresistance (MR) in the transition-metal dichalcogenide MoTe2 has attracted considerable interest due to its potential technological applications as well as its relationship with novel electronic states predicted for a candidate type-II Weyl semimetal. In order to understand the origin of the MR, the electronic structure of MoTe2−x (x = 0.08) is systematically tuned by application of pressure and probed via its Hall and longitudinal conductivities. With increasing pressure, a monoclinic-to-orthorhombic (1 T′ to Td) structural phase transition temperature (T*) gradually decreases from 210 K at 1 bar to 58 K at 1.1 GPa, and there is no anomaly associated with the phase transition at 1.4 GPa, indicating that a T = 0 K quantum phase transition occurs at a critical pressure (Pc) between 1.1 and 1.4 GPa. The large MR observed at 1 bar is suppressed with increasing pressure and is almost saturated at 100% for P > Pc. The dependence on magnetic field of the Hall and longitudinal conductivities of MoTe2−x shows that a pair of electron and hole bands are important in the low-pressure Td phase, while another pair of electron and hole bands are additionally required in the high-pressure 1 T′ phase. The MR peaks at a characteristic hole-to-electron concentration ratio (nc) and is sharply suppressed when the ratio deviates from nc within the Td phase. These results establish the comprehensive temperature-pressure phase diagram of MoTe2−x and underscore that its MR originates from balanced electron-hole carrier concentrations.

Co-extraction of vanadium and chromium from high chromium containing vanadium slag by low-pressure liquid phase oxidation method

High chromium containing vanadium slag is refractory, and therefore cannot be processed efficiently by traditional roasting method. In this study, a low pressure leaching process using NaOH solution to co-extract both vanadium and chromium has been proposed. The effects of NaOH concentration, leaching temperature, oxygen pressure, and liquid-to-solid mass ratio on the vanadium and chromium leaching rate were discussed in detail. The results suggested that the NaOH concentration and leaching temperature were more influential factors for the extraction of vanadium and chromium. Under the optimal reaction conditions (temperature 200 °C, liquid-to-solid mass ratio of 6:1, NaOH mass concentration of 50%, agitation speed 700 rpm, pressure of 1 MPa, and reaction time 180 min), the recoveries of vanadium and chromium could reach up to 98.3% and 93.3%, respectively. The kinetics study indicated that the dissolution process of vanadium slag followed the shrinking core model, and the extraction of vanadium and chromium were both governed by the internal diffusion control. Furthermore, the apparent activation energies for the oxidation of vanadium and chromium were calculated to be 27.58 and 36.74 kJ/mol, respectively. The result from this study provides insights on effectively co-extracting V and Cr from high Cr-containing vanadium slag while minimizing the environmental contamination.

ROSA/LSTF test and RELAP5 code analyses on PWR hot leg small-break LOCA with accident management measure based on core exit temperature and PKL counterpart test

An experiment was performed for the OECD/NEA ROSA-2 Project using the large scale test facility (LSTF), which simulated a hot leg small-break loss-of-coolant accident with steam generator (SG) secondary-side depressurization as an accident management measure based on core exit temperature in a pressurized water reactor (PWR). This experiment was conducted under two conditions of high-pressure to meet the PWR pressure condition and of low-pressure to meet the Primӓrkreislӓufe Versuchsanlage (PKL) condition. Core uncovery took place by core boil-off with no reflux coolant from the SGs in the LSTF test. The increase rate of the cladding surface temperatures from top to center of the core relative to the core exit temperature increased according to the linear heat rate in the LSTF test. Some discrepancies appeared between the LSTF low-pressure phase and PKL test results for the core exit temperature increase due to differences in low-temperature structures around the core exit. The RELAP5/MOD3.3 code indicated a remaining problem in the prediction of the core exit temperature due to pseudo coolant mixing. Results of uncertainty analysis for the LSTF low-pressure phase test clarified influences of the combination of the multiple uncertain parameters on peak cladding temperature within the defined uncertain ranges.

DFT study of [BH4]− rotation in pressure-driven phase transition of MBH4

The pressure-induced phase transitions of MBH4 (M = Na, K) have been studied using first-principles calculations. The optimized results indicate that their high-pressure phases are space group P42/nmc, which have the same symmetry as the C-substructure of their low-pressure phases. The result of [BH4]− rotational barrier shows that rotational barrier will increase with the increase of external pressure. The rotational energy barrier of KBH4 is larger than that of NaBH4 at same pressure due to larger radius of potassium atom, which can explain why potassium borohydride has a lower phase transition pressure. The theoretical rotation barrier obtained with Tkatchenko-Scheffler correction is higher and is more sensitive to change under different pressures than that without vdW correction.

Continuous roll-to-roll fabrication of organic photovoltaic cells via interconnected high-vacuum and low-pressure organic vapor phase deposition systems

We demonstrate continuous roll-to-roll (R2R) fabrication of single junction and tandem organic photovoltaic (OPV) cells on flexible plastic substrates employing a system that integrates organic deposition by high vacuum thermal evaporation (VTE) and low pressure organic vapor phase deposition (OVPD). By moving the substrate from chamber to chamber and then depositing films on stationary substrates, we achieve power conversion efficiencies of PCE = 8.6 ± 0.3% and 8.9 ± 0.2% for the single junction and tandem cells, respectively. Single junction OPVs are also fabricated on a continuously translating substrate at 0.3 cm/s, to achieve PCE = 8.5 ± 0.2%. Thin films grown on translating substrates by OVPD show <3% thickness non-uniformity and 0.66 nm root mean square surface roughness, similar to that obtained by VTE. Our results suggest that R2R film deposition comprising multiple vapor deposition technologies is a promising method for rapid speed and continuous manufacturing of high quality, small molecular weight organic electronic materials.

From semilocal density functionals to random phase approximation renormalized perturbation theory: A methodological assessment of structural phase transitions

The structural phase transitions of different materials, including metal to metal, metal to semiconductor, and semiconductor to semiconductor transitions, were explored using methods based on the random phase approximation. Transition pressures for Si, Ge, SiC, GaAs, ${\mathrm{SiO}}_{2}$, Pb, C, and BN from their stable low-pressure phases to certain high-pressure phases were computed with several semilocal density functionals and from the adiabatic connection fluctuation-dissipation formulation of density functional theory at zero temperature. In addition to the random phase approximation (RPA), three approximate beyond-RPA methods were also investigated to determine the impact of exchange-correlation kernel corrections. Results at finite temperature were obtained with the inclusion of zero-point energy contributions from the phonon spectra. We find that including temperature effects is most important for systems with nearly degenerate phases such as for boron nitride and carbon. In combination with thermal corrections, the kernel-corrected correlation methods deliver high accuracy compared to experimental data and can serve as a useful benchmark method in place of more expensive correlated calculations.

Pressure-induced structural and semiconductor-semiconductor transitions in Co0.5Mg0.5Cr2O4

The effect of pressure on the structural, vibrational, and electronic properties of Mg-doped Cr bearing spinel $\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{M}{\mathrm{g}}_{0.5}\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ was studied up to 55 GPa at room-temperature using x-ray diffraction, Raman spectroscopy, electrical transport measurements, and ab initio calculations. We found that the ambient-pressure phase is cubic (spinel-type, $Fd\overline{3}m$) and underwent a pressure-induced structural transition to a tetragonal phase (space group $I\overline{4}m2$) above 28 GPa. The ab initio calculation confirmed this first-order phase transition. The resistivity of the sample decreased at low pressures with the existence of a low-pressure (LP) phase and started to increase with the emergence of a high-pressure (HP) phase. The temperature dependent resistivity experiments at different pressures illustrated the wide band gap semiconducting nature of both the LP and HP phases with different activation energies, suggesting a semiconductor-semiconductor transition at HP. No evidence of chemical decomposition or a semiconductor-metal transition was observed in our studies.

Molecular simulation of capillary phase transitions in flexible porous materials.

We used flat-histogram sampling Monte Carlo to study capillary phase transitions in deformable adsorbent materials. Specifically, we considered a pure adsorbate fluid below its bulk critical temperature within a slit pore of variable pore width. The instantaneous pore width is dictated by a number of factors, such as adsorbate loading, reservoir pressure, fluid-wall interaction, and bare adsorbent properties. In the slit pores studied here, the bare adsorbent free energy was assumed to be biparabolic, consisting of two preferential pore configurations, namely, the narrow pore and the large pore configurations. Four distinct phases could be found in the adsorption isotherms. We found a low-pressure phase transition, driven primarily by capillary condensation/evaporation and accompanied by adsorbent deformation in response. The deformation can be a relatively small contraction/expansion as seen in elastic materials, or a large-scale structural transformation of the adsorbent. We also found a high-pressure transition driven by excluded volume effects, which tends to expand the material and thus results in a large-scale structural transformation of the adsorbent. The adsorption isotherms and osmotic free energies can be rationalized by considering the relative free energy differences between the basins of the bare adsorbent free energy.