Nonhydrostatic Compression Research Articles

Structural and Electronic Phase Transitions in Three Stable Tin-Sulfur Metallic Chalcogenides under High Pressure.

As a representative homologous series, tin-bearing metallic chalcogenides (SnxSy) have sparked considerable attention because of their stoichiometric compositions and structural diversities. In this work, three stable compounds, SnS, Sn2S3, and SnS2, were screened from SnxSy and a comprehensive investigation on their structural and electrical transport properties was performed up to 60.1 GPa using a diamond anvil cell (DAC) under different hydrostatic environments. Upon nonhydrostatic compression, SnS underwent the Pnma-to-Cmcm transition accompanied by metallization at 7.6 GPa, followed by the Cmcm-to-Pm3̅m transformation at 17.8 GPa. For SnS2, the pressure-induced metallization and isostructural phase transition (IPT) occurred successively at 31.2 and 46.6 GPa, respectively. As an intermediate composition, Sn2S3 first experienced an IPT at 10.8 GPa, and then, the Pnma-to-Cmcm transition concomitantly with metallization occurred at 16.9 GPa, analogous to the high-pressure structural transformation routes of SnS and SnS2. The 0.6-5.4 GPa pressure hysteresis was detectable for the phase transitions of SnxSy under quasi-hydrostatic and hydrostatic conditions owing to the influence of deviatoric stress. In comprehensive consideration of our high-pressure Raman scattering and electrical conductivity results, the systematic construction of a pressure-phase state diagram on SnxSy not only unveils its composition-structure-property relation but also advances the in-depth exploration for other IVA-VIA metallic chalcogenides.

Constraints on the spin-state transition of siderite from laboratory-based Raman spectroscopy and electrical conductivity under high temperature and high pressure

The vibrational and electrical transport properties of natural siderite are systematically investigated by means of in-situ Raman spectroscopy and alternating current impedance spectroscopy under conditions of 0.6–55.6 GPa, 298–873 K and different hydrostatic environments using a diamond anvil cell (DAC). Upon non-hydrostatic compression, all of these observable characteristic variations of siderite including the appearance of three absolutely new Raman peaks (L’, v4′ and v1′), the disappearance of Raman peaks (T, L and v4) and the discontinuity in the pressure-dependent electrical conductivity can provide robust evidence of electronic spin transitions of Fe2+ from high-spin to mixed-spin to low-spin states at the respective pressures of 42.5 GPa and 48.5 GPa. As far as hydrostatic condition, the electronic spin states from high-spin to mixed-spin to low-spin states occurred at the higher pressures of 45.7 GPa and 50.4 GPa, respectively, which implied the highly sensitive hydrostaticity of electronic spin transition pressures. Upon decompression, the reverse electronic spin transitions from low-spin to mixed-spin to high-spin states were detected at the respective pressures of 47.2 GPa and 28.7 GPa under non-hydrostatic condition, and as well as at the pressures of 49.4 GPa and 25.1 GPa under hydrostatic condition, respectively. The huge pressure hysteresis of 13.8 GPa and 20.6 GPa for the electronic spin state transition was revealed under non-hydrostatic and hydrostatic environments, respectively. In order to explore the effect of temperature on the electronic spin transition, a series of electrical conductivity experiments on siderite were performed over the temperature range of 323–873 K under conditions of three typical pressures of 47.7, 49.8 and 51.6 GPa. Furthermore, the functional relationships between the temperature and pressure describing the high-spin to mixed-spin to low-spin transitions for siderite were successfully established: P1 (GPa) = 39.318 + 0.015 T (K) and P2 (GPa) = 41.277 + 0.018 T (K), respectively. In conclusion, our acquired phase diagram of the electronic spin transition on siderite is beneficial to deep insight into the electronic spin behavior for those of iron-bearing carbonate minerals under high-temperature and high-pressure conditions.

Pressure-induced ferroelectric and electronic transitions in two-dimensional ferroelectric semiconductor of NbOCl2 up to 41.7 GPa

NbOCl2, a representative van der Waals ferroelectric (FE) semiconductor, has become the research frontier due to its peculiar appeal in both fundamental research studies and potential applications. In the present work, the high-pressure structural, vibrational, and electrical transport properties of NbOCl2 under different hydrostatic environments were systematically investigated over a wide pressure range of 1.7–41.7 GPa using a diamond anvil cell coupled with in situ Raman spectroscopy, electrical conductivity, and high-resolution transmission electron microscopy (HRTEM) observations. Upon non-hydrostatic compression, NbOCl2 underwent a FE-to-antiferroelectric phase transition at 3.4 GPa, followed by a semiconductor-to-metal transformation at 15.7 GPa. Under hydrostatic compression, the FE transformation and metallization of NbOCl2 were postponed by ∼2.0 and ∼4.0 GPa due to the effect of helium pressure-transmitting medium. Upon decompression, the phase transition was demonstrated to be reversible under different hydrostatic environments, which was well corroborated by HRTEM analyses. In addition, the linear relations between electrical current and sinusoidal voltage with the nonlinearity factors of ∼1.0 reflect the Ohmic response of NbOCl2 before and after the FE transition. Our findings on NbOCl2 provide a guideline for exploring other layered FE materials under high pressure and establishing a design paradigm for new generations of FE-based devices.

Comparison of hydrostatic and non-hydrostatic compression of glassy carbon to 80 GPa

Understanding new mechanisms for phase transformation in carbon is of considerable interest. This study investigates on the compression conditions required to create recoverable diamond during room-temperature high-pressure compression of glassy carbon. Under non-hydrostatic compression conditions when shear is present, glassy carbon transforms into an oriented graphitic structure at ∼45 GPa, and then forms mixed diamond and lonsdaleite nanocrystals when the pressure is higher than ∼80 GPa. In contrast, during hydrostatic compression no significant changes in the microstructure was observed, highlighting glassy carbon’s resilience under compression. Molecular dynamics modelling supports the proposed model that shear drives the phase transition mechanism and causes a temperature spike that drives crystallisation. Our work demonstrates that shear is key to high-pressure diamond formation in the absence of heating.

Time-domain Brillouin scattering for evaluation of materials interface inclination: Application to photoacoustic imaging of crystal destruction upon non-hydrostatic compression

Time-domain Brillouin scattering (TDBS) is a developing technique for imaging/evaluation of materials, currently used in material science and biology. Three-dimensional imaging and characterization of polycrystalline materials has been recently reported, demonstrating evaluation of inclined material boundaries. Here, the TDBS technique is applied to monitor the destruction of a lithium niobate single crystal upon non-hydrostatic compression in a diamond anvil cell. The 3D TDBS experiments reveal, among others, modifications of the single crystal plate with initially plane-parallel surfaces, caused by non-hydrostatic compression, the laterally inhomogeneous variations of the plate thickness and relative inclination of opposite surfaces. Our experimental observations, supported by theoretical interpretation, indicate that TDBS enables the evaluation of materials interface orientation/inclination locally, from single point measurements, avoiding interface profilometry. A variety of observations reported in this paper paves the way to further expansion of the TDBS imaging use to analyze fascinating processes/phenomena occurring when materials are subjected to destruction.

The preferential orientation of silicon phases formed during non-hydrostatic compression

The preferential orientation of silicon phases formed during non-hydrostatic compression

High-pressure and high-temperature sintering of pure cubic silicon carbide: A study on stress–strain and densification

Silicon carbide (SiC) is a high-performance structural ceramic material with excellent comprehensive properties, and is unmatched by metals and other structural materials. In this paper, raw SiC powder with an average grain size of 5 μm was sintered by an isothermal-compression process at 5.0 GPa and 1500 °C; the maximum hardness of the sintered samples was 31.3 GPa. Subsequently, scanning electron microscopy was used to observe the microscopic morphology of the recovered SiC samples treated in a temperature and extended pressure range of 0–1500 °C and 0–16.0 GPa, respectively. Defects and plastic deformation in the SiC grains were further analyzed by transmission electron microscopy. Further, high-pressure in situ synchrotron radiation x-ray diffraction was used to study the intergranular stress distribution and yield strength under non-hydrostatic compression. This study provides a new viewpoint for the sintering of pure phase micron-sized SiC particles.

Effect of stress state on graphitization behavior of diamond under high pressure and high temperature

Pure diamond powder and mixed powder of diamond and sodium chloride were treated at high pressures (5 GPa and 10 GPa) and high temperatures (1000–2700 °C), and then the graphitization behavior of diamond powder under non-hydrostatic (pure diamond powder) and quasi-hydrostatic (mixed powder of diamond and sodium chloride) compression was quantitatively studied by analyzing the X-ray diffraction peaks' area of treated samples. It is found that the graphitization under non-hydrostatic compression is much more easily triggered by heating due to the heterogeneous stress distribution within the diamond grains. Under the quasi-hydrostatic compression of 5 GPa, the starting graphitization temperature is about 1600 °C, which is 300 °C higher than that under non-hydrostatic compression. Under a pressure of 10 GPa, graphitization could be observed after pure diamond powder was treated at 1400 °C, but no graphitization occurred in the mixed powder even after a high temperature treatment at 2300 °C. Optimizing the stress distribution among diamond grains by the mixing of grain-size can significantly reduce the graphitization at high temperature even under non-hydrostatic compression.

Simulation of Stress tests using a Poroelastic model to estimate the Permeability behavior of bedford limestone samples

Por su relevancia en Geociencias, el efecto de los esfuerzos sobre las propiedades de flujo de fluido en rocas ha sido estudiado experimental y teóricamente por varias décadas. Los trabajos se han orientado principalmente a rocas tipo arenisca, y relativamente poco, a pesar de su relevancia, en rocas carbonatadas. Por otro lado, la descripción del fenómeno es matemática y numéricamente complejo, ya que involucra el acoplamiento no lineal del flujo de fluido con la deformación de la roca. En este trabajo se busca incursionar en esa parte poco explorada de pruebas experimentales en carbonato y su simulación numérica.En él se presentan pruebas experimentales realizadas en núcleos de caliza Bedford, y se analiza el efecto de los esfuerzos de confinamiento y de la presión del fluido (presión de poro) sobre la permeabilidad a través de dos tipos de pruebas, conocidas como hidrostáticas (bajos esfuerzos de corte) y no hidrostáticas (altos esfuerzos de corte). En este trabajo se presenta un modelo matemático para describir el fenómeno, considerando un fluido monofásico, una respuesta elástica de la roca, y su implementación numérica en elementos finitos.El acoplamiento del flujo de fluido con los esfuerzos se modela utilizando relaciones algebraicas de porosidad y permeabilidad en función del esfuerzo. La relación permeabilidad-esfuerzo utilizada en este trabajo depende linealmente de la deformación volumétrica y la presión de poro. Los resultados experimentales muestran que en general la permeabilidad se incrementa con la presión del fluido y se reduce con el esfuerzo de confinamiento. La reducción con el esfuerzo de confinamiento en el rango de esfuerzos de confinamiento analizado es relativamente pequeña, pero es notoriamente más importante en las pruebas no-hidrostáticas (8%) que en las hidrostáticas (2%).El ajuste del modelo a los resultados experimentales, específicamente a la caída de presión en el núcleo en función del tiempo, se realiza mediante la variación de los parámetros de la relación permeabilidad-esfuerzo. El valor de estos parámetros de ajuste difiere del valor reportado para areniscas, lo cual puede ser indicativo de la diferencia en la estructura porosa y las propiedades mecánicas de las rocas.

Phase Transitions in Amorphous Germanium under Non-Hydrostatic Compression

As the pioneer semiconductor in transistor, germanium (Ge) has been widely applied in information technology for over half a century. Although many phase transitions in Ge have been reported, the complicated phenomena of the phase structures in amorphous Ge under extreme conditions are still not fully investigated. Here, we report the different routes of phase transition in amorphous Ge under different compression conditions utilizing diamond anvil cell (DAC) combined with synchrotron-based X-ray diffraction (XRD) and Raman spectroscopy techniques. Upon non-hydrostatic compression of amorphous Ge, we observed that shear stress facilitates a reversible pressure-induced phase transformation, in contrast to the pressure-quenchable structure under a hydrostatic compression. These findings afford better understanding of the structural behaviors of Ge under extreme conditions, which contributes to more potential applications in the semiconductor field.

A new high-pressure structure of SiO2 directly converted from α-quartz under nonhydrostatic compression

High-pressure behavior of SiO2 is one of the prototypical subjects in several research areas including condensed matter physics, inorganic chemistry, mineralogy, materials science, and crystallography. Therefore, numerous studies have been performed on the structure evolution of SiO2 under pressure. Here, we show a new structure directly converted from α-quartz under uniaxial compression. Our ab initio calculations elucidate a simple transition pathway from α-quartz to the Fe2P-type phase, and an intermediate state with the Li2ZrF6-type structure appears in this structure conversion. Some interesting properties are found on this intermediate state. (1) The Li2ZrF6-type phase is metastable probably due to a volumetric unbalance between the Li and Zr sites but becomes more energetically stable than α-quartz over ∼12 GPa. (2) It is vibrationally stable at 0 GPa, suggesting that this phase can be recovered down to ambient condition once synthesized. (3) The crystal structures of Li2ZrF6-type SiO2 and phase D, one of dense magnesium hydrous silicates, are found identical, suggesting the stabilization of their solid solution under high-P, T condition.

Equal compressibility structural phase transition of molybdenum at high pressure

We have studied the high-pressure compression behavior of molybdenum up to 60 GPa by synchrotron radial x-ray diffraction (RXRD) in a diamond anvil cell (DAC). It is found that all diffraction peaks of molybdenum undergo a split at around 27 GPa, and we believe that a phase transition from a body-centered cubic structure to a rhombohedral structure at room pressure has occurred. The slope of pressure–volume curve shows continuity before and after this phase transition, when fitting the pressure–volume curves of the body-centered cubic structure at low pressure and the rhombohedral structure at high pressure. A bulk modulus of 261.3 (2.7) GPa and a first-order derivative of the bulk modulus of 4.15 (0.14) are obtained by using the nonhydrostatic compression data at the angle ψ = 54.7° between the diffracting plane normal and stress axis.

Strength, deformation, and equation of state of tungsten carbide to 66 GPa

Strength, texture, and equation of state of hexagonal tungsten monocarbide (WC) have been determined under quasi-hydrostatic and non-hydrostatic compression to 66 GPa using angle-dispersive X-ray diffraction in the diamond anvil cell. Quasi-hydrostatic compression in a Ne pressure medium demonstrates that nanocrystalline WC is slightly less incompressible than bulk-scale WC, with respective bulk moduli of K0 = 377 ± 7 and 397 ± 7 GPa and pressure derivatives K0’ = 3.8 ± 0.3 and 3.7 ± 0.3. This decrease in incompressibility with grain size is similar to behavior observed in other ceramics. Under nonhydrostatic compression, WC supports a mean differential stress of ∼12-15 GPa at plastic yielding, which occurs at ∼30 GPa. Strength in WC is anisotropic, with the (001) plane supporting 29-42% higher stress than stresses calculated from mean strain. Simulations using an Elasto-ViscoPlastic Self-Consistent model indicate that strength inferred from lattice strain theory may be overestimated due to effects of plastic deformation. Plastic deformation generates a texture maximum near 〈2¯110〉 in the compression orientation, initially through prismatic slip on the {101¯0}〈1¯21¯0〉 and {101¯0}〈0001〉 slip systems, followed by activation of pyramidal slip on {101¯1}〈2¯113〉 at ∼40-50 GPa.

Shear strength measurements and hydrostatic compression of rhenium diboride under high pressures

Shear strength measurements have been carried out on rhenium diboride, ReB2, to a pressure of 74 GPa using a Radial X-ray Diffraction (R-XRD) technique in a diamond anvil cell using platinum as an internal x-ray pressure standard. The R-XRD result has provided a unique insight into the deformation of hexagonal crystal lattice under non-hydrostatic compression and variation of shear strength with increasing pressure. From R-XRD data, we have estimated hydrostatic component of compression to determine an equation of state of rhenium diboride yielding a bulk modulus of K0 = 366 ± 25 GPa with a pressure derivative K0′ = 4.3 ± 0.5 in good agreement with hydrostatic density functional theory calculations. The average lower bound of shear strength (τ) from various diffraction planes was then calculated using the measured interplanar d-spacing (dm) and hydrostatic component of d-spacing (dp) to be shown to approach 6.7 ± 0.4 GPa at 70 GPa. Our results show that the anisotropic compression effects observed in ReB2 under hydrostatic compression are correlated to electronic structure changes under compression as predicted by theoretical calculations.

Phase transformations, microstructural refinement and defect evolution mechanisms in Al-Si alloys under non-hydrostatic diamond anvil cell compression

Non-hydrostatic compression of materials using a diamond anvil cell (DAC) can transform the equilibrium microstructure of an alloy to novel, potentially metastable states. In this study, in situ synchrotron X-ray diffraction (XRD) during compression up to 24 GPa and subsequent ex-situ, high resolution analytical electron microscopy (AEM) after decompression of an Al-Si alloy provided insight into the crystallographic changes during the compression as well as microstructural refinement, and defect structures caused by such a high pressure compression-decompression process. Pressure resolved in-situ synchrotron XRD was used to detail the phase transformation pathway of the eutectic Si phase in Al-Si alloy, from Si-I → Si-XI → Si-V during compression, and a final transformation predominantly to Si-III after decompression. Using scanning and transmission electron microscopy (S/TEM), site specific analysis of the alloy immediately underneath the anvil contact surface demonstrated a highly complex microstructure. A narrow region of thick amorphous Al oxide interspersed with nanocrystalline grains was found at the top surface. Underneath this Al oxide, while the majority of the eutectic Si was transformed into highly-deformed, polycrystalline (PC) Si-III, a complex intermediate layer was discovered at the interface between Al and Si, comprised of a small fraction of Al nanocrystals and a majority of nanocrystalline Si-I. This combination of pressure resolved in-situ synchrotron XRD coupled with subsequent high resolution, electron microscopy resolved the phase transformation as well as non-equilibrium microstructures in a metallic alloy induced by a non-hydrostatic high pressure compression followed by decompression.

Synthesis and Phase Stability of the High-Entropy Carbide (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)C under Extreme Conditions.

As a novel ultrahigh temperature ceramic, the stability of a high-entropy transition metal carbide under extreme conditions is of great concern to its application. Despite the intense research, the available high-pressure experimental results are few so far. Here, we synthesized the nanocrystalline (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)C by a high-pressure solid-state reaction successfully. Meanwhile, synchrotron radiation X-ray diffraction experiments were carried out to explore the phase stability and mechanical response under high pressure. The single cubic B1 phase structure of the high-entropy carbide is retained under extreme hydrostatic pressure. An abnormal cubic-to-cubic phase transition was observed unexpectedly under nonhydrostatic compression. This result reflects the effect of the severe lattice distortion of the initial B1 phase high-entropy carbide and the shear strain caused by deviatoric stress under high nonhydrostatic pressure. The physical mechanism about electronic/magnetic characteristics behind findings is an interesting issue for future studies.

Unraveling microstrain-promoted structural evolution and thermally driven phase transition in c−Sc2O3 nanocrystals at high pressure

Here, we report an irreversible cubic-to-monoclinic structural transition in cubic $c\text{\ensuremath{-}}{\mathrm{Sc}}_{2}{\mathrm{O}}_{3}$ nanocrystals which occur at pressures above $\ensuremath{\sim}8.9\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ upon nonhydrostatic compression in association with a pronounced volume collapse. This phase-transition--induced anomaly is further confirmed by our experimental Raman spectroscopy measurements and theoretical predictions. After annealing, however, this high-pressure monoclinic $m\text{\ensuremath{-}}{\mathrm{Sc}}_{2}{\mathrm{O}}_{3}$ phase undergoes a reversible back-transformation to the cubic counterpart at $\ensuremath{\sim}1123\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and 9.0 GPa. Our observed transition pressure of $\ensuremath{\sim}8.9\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ for the cubic-to-monoclinic structural evolution is significantly lower than that from the previously diamond-anvil-cell--based hydrostatic x-ray experiments because of the existence of internal microscopic stress and/or high-stress concentration in the specimen caused by grain-to-grain contacts upon nonhydrostatic compression, which promoted the cubic-to-monoclinic structural transition. Moreover, we have reported new thermoelastic properties of $c\text{\ensuremath{-}}{\mathrm{Sc}}_{2}{\mathrm{O}}_{3}$ nanocrystals at simultaneous high-pressure and high-temperature conditions. These findings/results may have significant implications for the design of phase-switching devices and for the exploration of the structural relationship among sesquioxides for their uses in extreme environments.

Stability and Lattice Dynamics of Ruddlesden–Popper Tetragonal Sr2TiO4

We report a combined experimental and theoretical lattice dynamics study of the Ruddlesden–Popper layered compound Sr2TiO4. From inelastic neutron scattering experiments, we derive the generalized phonon density of states of Sr2TiO4. We also report its heat capacity, thermal expansion, and thermodynamic Gruneisen parameters using the calculated bulk modulus and find a large value of about 2. Using Raman scattering experiments under pressure, we discuss a potential structural distortion of the tetragonal structure above 11 GPa, which could be due to nonhydrostatic compression. The mode Gruneisen parameters of the four Raman-active modes are determined and shown to be in reasonable agreement with those obtained by density functional perturbation theory (DFPT) calculations. The temperature behavior of the Raman-active modes was studied, allowing us to determine the implicit volume and explicit anharmonic contributions. Above 400 K, the implicit volume contribution dominates the temperature-induced variation of the four Raman-active modes, whereas, below this temperature, the explicit anharmonic contribution is the dominant contributor to the highest energy mode. Our results underline the importance of anharmonicity in vibration-related properties of Sr2TiO4.

Strength enhancement of nanocrystalline tungsten under high pressure

Three tungsten powder samples—one coarse grained (c-W; grain size: 1 µm–3 µm) and two nanocrystalline (n-W; average grain sizes: 10 nm and 50 nm)—are investigated under nonhydrostatic compression in a diamond anvil cell in separate experiments, and their in situ X-ray diffraction patterns are recorded. The maximum microscopic deviatoric stress in each tungsten sample, a measure of the yield strength, is determined by analyzing the diffraction line width. Over the entire pressure range, the strength of tungsten increases noticeably as the grain size is decreased from 1 µm–3 µm to 10 nm. The results show that the yield strength of tungsten with an average crystal size of 10 nm is around 3.5 times that of the sample with a grain size of 1 µm–3 µm.

Strong variant selection observed in the α−ε martensitic transition of iron under quasihydrostatic compression along [111]α

The $\ensuremath{\alpha}$ (bcc) phase of single-crystalline iron was compressed along the ${[111]}_{\ensuremath{\alpha}}$ direction under quasihydrostatic and nonhydrostatic conditions. The emergence of the $\ensuremath{\epsilon}$ (hcp) phase variants via the pressure-induced martensitic transformation was investigated using x-ray diffraction and optical observations of the sample surface. In quasihydrostatic compression, the $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\epsilon}$ transition occurred at approximately 14 GPa accompanied by the sudden appearance of texture structures on the mirror-polished sample surface. The texture structure divided the sample surface into two characteristic regions: distorted regions with undulations and flat regions without undulations. Although martensitic transformation based on the Burgers model allows for the emergence of 12 $\ensuremath{\epsilon}$ variants from one single-crystalline phase, the transition proceeded according to the Burgers model, but selected variants preferably emerged depending on the regions in the texture structure. The flat region consisted of only three variants whose $c$ axes were parallel to the sample surface. The region near the undulation exhibited a large number of variants whose $c$ axes aligned parallel and nonparallel to the sample surface. The $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\epsilon}$ transition under nonhydrostatic compression deviated from the Burgers model, and the variant selection was not observed. This study demonstrates that the quasihydrostatic compression along ${[111]}_{\ensuremath{\alpha}}$ reduces the number of variants via strong variant selection. The strong preferred orientation is useful for investigating the anisotropy of physical properties in the $\ensuremath{\epsilon}$ phase.