Nonhydrostatic Compression Research Articles

Negative differential stresses in niobium: Analysis of x-ray measured pressure–volume data

The macro-stresses that develop in a sample under nonhydrostatic compression in a diamond anvil cell (DAC) are axially symmetric about the load axis, the axial stress component being larger than the radial stress component. The difference between the two, termed differential stress t, is a positive quantity. The lattice strain theory (LST) suggests that the measured unit cell volume using conventional x-ray diffraction geometry of the DAC is overestimated in the presence of a positive t. In an earlier study on niobium, one set of x-ray measured compression data was found to yield negative t. It is suggested that the inward collapse of the gasket leads to negative t. It is shown that the volume-effect produced by negative t is also in accordance with the existing LST.

Magnetic Network on Demand: Pressure Tunes Square Lattice Coordination Polymers Based on {[Cu(pyrazine)2]2+}n.

We report the pressure-induced structural and magnetic changes in [CuCl(pyz)2](BF4) (pyz = pyrazine) and [CuBr(pyz)2](BF4), two members of a family of three-dimensional coordination polymers based on square mesh {[Cu(pyz)2]2+}n layers. High-pressure X-ray diffraction and density functional theory calculations have been used to investigate the structure–magnetic property relationship. Although structurally robust and almost undeformed within a large pressure range, the {[Cu(pyz)2]2+}n network can be electronically modified by adjusting the interaction of the apical linkers interconnecting the layers, which has strong implications for the magnetic properties. It is then demonstrated that the degree of covalent character of the apical interaction explains the difference in magnetic exchange between the two species. We have also investigated the mechanical deformation of the network induced by nonhydrostatic compression that affects the structure depending on the crystal orientation. The obtained results suggest the existence of “Jahn–Teller frustration” triggered at the highest hydrostatic pressure limit.

Electronic structure and anisotropic compression of Os2B3 to 358 GPa

High pressure study on ultra-hard transition-metal boride Os2B3 was carried out in a diamond anvil cell under isothermal and non-hydrostatic compression with platinum as an x-ray pressure standard. The ambient-pressure hexagonal phase of Os2B3 is found to be stable with a volume compression V/V0 = 0.670 ± 0.009 at the maximum pressure of 358 ± 7 GPa. Anisotropic compression behavior is observed in Os2B3 to the highest pressure, with the c-axis being the least compressible. The measured equation of state using the 3rd-order Birch–Murnaghan fit reveals a bulk modulus K0 = 397 GPa and its first pressure derivative = 4.0. The experimental lattice parameters and bulk modulus at ambient conditions also agree well with our density-functional-theory (DFT) calculations within an error margin of ∼1%. DFT results indicate that Os2B3 becomes more ductile under compression, with a strong anisotropy in the axial bulk modulus persisting to the highest pressure. DFT further enables the studies of charge distribution and electronic structure at high pressure. The pressure-enhanced electron density and repulsion along the Os and B bonds result in a high incompressibility along the crystal c-axis. Our work helps to elucidate the fundamental properties of Os2B3 under ultrahigh pressure for potential applications in extreme environments.

Stress‐Driven Phase Transitions of SrI2: A First‐Principles Investigation

The structural and electronic properties of strontium iodide (SrI2) under hydrostatic and nonhydrostatic compressions up to 20 GPa are investigated using first‐principles calculations. Upon hydrostatic compression, SrI2 undergoes a phase transition from the Pbca structure to a quasilayered Pbcm structure around 13.2 GPa, and the bandgap narrows with increasing pressure. Upon uniaxial compression, the system exhibits strong anisotropy. Along [010], a Pbca → Cmcm phase transition is observed at about 16.8 GPa via an intermediate metastable Pbcm structure, whereas along [100] and [001], the Pbca structure remains with cell shape changed. These phase transitions are systematically analyzed with shear moduli, band structures, and densities of states. The suitable bandgaps manifested during its uniaxial compression are exploited for optical and semiconductor applications.

Experimental and Computational Studies on Superhard Material Rhenium Diboride under Ultrahigh Pressures.

An emerging class of superhard materials for extreme environment applications are compounds formed by heavy transition metals with light elements. In this work, ultrahigh pressure experiments on transition metal rhenium diboride (ReB2) were carried out in a diamond anvil cell under isothermal and non-hydrostatic compression. Two independent high-pressure experiments were carried out on ReB2 for the first time up to a pressure of 241 GPa (volume compression V/V0 = 0.731 ± 0.004), with platinum as an internal pressure standard in X-ray diffraction studies. The hexagonal phase of ReB2 was stable under highest pressure, and the anisotropy between the a-axis and c-axis compression increases with pressure to 241 GPa. The measured equation of state (EOS) above the yield stress of ReB2 is well represented by the bulk modulus K0 = 364 GPa and its first pressure derivative K0´ = 3.53. Corresponding density-functional-theory (DFT) simulations of the EOS and elastic constants agreed well with the experimental data. DFT results indicated that ReB2 becomes more ductile with enhanced tendency towards metallic bonding under compression. The DFT results also showed strong crystal anisotropy up to the maximum pressure under study. The pressure-enhanced electron density distribution along the Re and B bond direction renders the material highly incompressible along the c-axis. Our study helps to establish the fundamental basis for anisotropic compression of ReB2 under ultrahigh pressures.

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 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 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.

Phase-pure bulk BC8 silicon prepared through secondary phase transition method under high pressure

Phase-pure bulk samples of silicon in the body-centered cubic structure (BC8-Si) were prepared via secondary phase transition method under high pressure and room temperature. The bulk samples of BC-Si, with diameter of 3 mm and thickness of 0.5 mm, were synthesized via high-pressure phase transitions of single crystal silicon using the large volume cubic press. The formation mechanism of phase-pure BC8-Si via phase transitions of normal single crystal silicon was investigated using in situ synchrotron X-ray diffraction. The phase-pure BC8-Si was observed through primary phase transition under hydrostatic high-pressure condition. However, the secondary phase transformation was needed to obtain the phase-pure BC8-Si sample at non-hydrostatic compression using the large volume cubic press. The structure and purity of bulk BC8-Si were verified using powder X-ray diffraction, Raman spectroscopy, and energy dispersive spectra. Our experimental results show that the secondary phase transformation method under high pressure is an effective way for phase-pure bulk sample preparation of BC8-Si, which provides an opportunity for further studies of its intrinsic physical properties.

Experimental evidence of crystal symmetry protection for the topological nodal line semimetal state in ZrSiS

Tunable symmetry breaking plays a crucial role for the manipulation of topological phases of quantum matter. Here, through combined high-pressure magneto-transport measurements, Raman spectroscopy, and X-ray diffraction, we demonstrate a pressure-induced topological phase transition in nodal-line semimetal ZrSiS. Symmetry analysis and first-principles calculations suggest that this pressure-induced topological phase transition may be attributed to weak lattice distortions by non-hydrostatic compression, which breaks some crystal symmetries, such as the mirror and inversion symmetries. This finding provides some experimental evidence for crystal symmetry protection for the topological semimetal state, which is at the heart of topological relativistic fermion physics.

Synthesis and High-Pressure Mechanical Properties of Superhard Rhenium/Tungsten Diboride Nanocrystals.

Rhenium diboride is an established superhard compound that can scratch diamond and can be readily synthesized under ambient pressure. Here, we demonstrate two synergistic ways to further enhance the already high yield strength of ReB2. The first approach builds on previous reports where tungsten is doped into ReB2 at concentrations up to 48 at. %, forming a rhenium/tungsten diboride solid solution (Re0.52W0.48B2). In the second approach, the composition of both materials is maintained, but the particle size is reduced to the nanoscale (40-150 nm). Bulk samples were synthesized by arc melting above 2500 °C, and salt flux growth at ∼850 °C was used to create nanoscale materials. In situ radial X-ray diffraction was then performed under high pressures up to ∼60 GPa in a diamond anvil cell to study mechanical properties including bulk modulus, lattice strain, and strength anisotropy. The differential stress for both Re0.52W0.48B2 and nano ReB2 (n-ReB2) was increased compared to bulk ReB2. In addition, the lattice-preferred orientation of n-ReB2 was experimentally measured. Under non-hydrostatic compression, n-ReB2 exhibits texture characterized by a maximum along the [001] direction, confirming that plastic deformation is primarily controlled by the basal slip system. At higher pressures, a range of other slip systems become active. Finally, both size and solid-solution effects were combined in nanoscale Re0.52W0.48B2. This material showed the highest differential stress and bulk modulus, combined with suppression of the new slip planes that opened at high pressure in n-ReB2.

Ultrahigh pressure equation of state of tantalum to 310 GPa

ABSTRACTThe isothermal compression of transition metal tantalum (Ta) was studied in a diamond anvil cell by X-ray diffraction utilizing rhenium (Re) and gold (Au) as internal X-ray pressure standards. The Re pressure marker was employed during non-hydrostatic compression to pressures up to 310 GPa while the Au pressure marker was used during quasi-hydrostatic compression in a neon pressure-transmitting medium to 80 GPa. Two ultra-high pressure experiments were conducted on Ta and Re mixtures utilizing focused-ion beam machined toroidal diamond anvils with central flats varying from 8 microns to 16 microns in diameter. The Ta metal was observed to be stable in the body-centered-cubic phase to a volume compression V/V0 = 0.581. The measured equations of state (EOS) of Ta using two different calibrations of the Re pressure marker are compared with the ambient temperature isotherm derived from shock compression data. We provide a detailed analysis of EOS fit parameters for Ta under quasi-hydrostatic and non-hydrostatic conditions.

Radial X‐Ray Diffraction Study of Superhard Early Transition Metal Dodecaborides under High Pressure

AbstractThe deformation behavior of the three metal dodecaborides (YB12, ZrB12, and Zr0.5Y0.5B12) is investigated using radial X‐ray diffraction under nonhydrostatic compression up to ≈60 GPa with a goal of understanding how bonding and metal composition control hardness. Zr0.5Y0.5B12, which has the highest Vickers hardness (Hv = 45.8 ± 1.3 GPa at 0.49 N load), also shows the highest bulk modulus (K0 = 320 ± 5 GPa). The 0.49 N hardness for ZrB12 and YB12 are both lower and very similar, and both show lower bulk moduli (K0 = 276 ± 7 GPa, and K0 = 238 ± 6 GPa, respectively). Differential stress is then measured to study the strength and strength anisotropy. Zr0.5Y0.5B12 supports the highest differential stress, in agreement with its high hardness, a fact that likely arises from atomic size mismatch between Zr and Y combined with the rigid network of boron cages. The (200) plane for all samples supports the largest differential strain, while the (111) plane supports the smallest, consistent with the theoretically predicted slip system of {111} [ ]. Strain softening is also observed for ZrB12. Finally, the full elastic stiffness tensors for ZrB12 and YB12 are solved. ZrB12 is the most isotropic, but the extent of elastic anisotropy for all dodecaborides studied is relatively low due to the highly symmetric boron cage network.

Fluorite Phase Transition in SnO2 under Uniaxial Compression and at 500 K

The fluorite phase of SnO2 is discovered to demonstrate superior electrical conductivity compared to its rutile phase. Achieving SnO2’s fluorite phase at reduced pressure and intercepting it to ambient condition is of major significance. In this paper, we performed systematic investigations on SnO2’s phase transformations and physical properties under heating and nonhydrostatic compression using in situ X-ray diffraction (XRD) and electrical measurements. Compared to previous XRD experiments (21 GPa, room temperature), it is discovered that cubic fluorite SnO2 can be achieved and retained to ambient condition at reduced pressure, 12.0 GPa, after heating to 500 K. The electrical measurements show that the fluorite phase of SnO2 has about 10 times higher conductivity than its rutile phase and can be quenched to ambient condition after the sample was heated to 500 K at 12.0 GPa.

Fragmentation and stress diversification in diamond powder under high pressure

Studying the fragmentation and refinement of diamond powder as well as the diversification in the intergranular stress is crucial to produce a high-quality polycrystalline diamond. In this paper, using different micron-size diamond powders as the initial materials, the samples were compressed under different pressures at ambient temperature. The fragmentation behavior of the diamond powder was investigated by scanning electron microscopy and with a laser particle size analyzer. The results show that the fragmentation of diamond comprises three stages with increasing pressure: (i) fracturing of edges and corners, (ii) cracking of the crystal plane, and (iii) refinement of particle disorder; the particle deformation tends to become relatively stable after a certain pressure. In situ high-pressure synchrotron X-ray diffraction was used to study the intergranular stress distribution under non-hydrostatic compression to 35.1 GPa. A heterogeneous stress distribution was found in compressed diamond bulk, in which under the highest load, the maximum stress reached 69.5 GPa, whereas the minimum stress was only 18.8 GPa.

High-pressure strength and compressibility of titanium diboride (TiB2) studied under non-hydrostatic compression

The high-pressure strength and plastic properties of titanium diboride (TiB2) were investigated using synchrotron angle-dispersive x-ray diffraction (AXRD) under non-hydrostatic compression up to 42 GPa in a diamond-anvil cell (DAC). The AXRD data yielded a bulk modulus K0 = 308 ± 10 GPa with a pressure derivative K′0 = 3.4 ± 1. The experimental data are discussed and compared to the results of first-principles calculations. The compressibility of TiB2 demonstrates a strongly anisotropic property with increasing pressure. In addition, the microscopic deviatoric stress and grain size (crystallite size) were determined as a function of pressure from the line-width analysis. We can seen that the strength increases while the crystalline size decreases steeply as the pressure is raised from ambient to about 22 GPa. In other words, TiB2 starts to yield a plastic deformation at around 22 GPa, and the yield strength of TiB2 increases with pressure, reaching a value of ∼27 GPa at the highest pressure in our experiment.

Lattice Finite Strain Theory for Non-hydrostatically Compressed Materials

The purpose of this paper is to describe the nonlinear behaviour of geomaterials within the principles of thermodynamics. The main components of this contribution are (1) a new method to estimate the properties of minerals subjected to the non-hydrostatic compression in diamond anvil cell using the finite strain theory is introduced and (2) a proper measure of deformation that applies to a wide range of minerals is identified. This research work shows that the logarithmic (Hencky) strain produces a good agreement with experiments for a wide range of materials.

Understanding How Bonding Controls Strength Anisotropy in Hard Materials by Comparing the High-Pressure Behavior of Orthorhombic and Tetragonal Tungsten Monoboride

In this work, we investigate the high-pressure behavior of the stabilized high-temperature (HT) orthorhombic phase of WB using radial X-ray diffraction in a diamond-anvil cell at room temperature. The experiments were performed under nonhydrostatic compression up to 52 GPa. For comparison, the low-temperature (LT) tetragonal phase of WB was also compressed nonhydrostatically to 36 GPa to explore structurally induced changes to its mechanical properties. Although our microindentation hardness tests indicate that the HT WB possesses slightly higher hardness, synchrotron-based high-pressure compression data yield significant distinct incompressibilities. The ambient pressure bulk modulus of the HT phase of WB is 341 ± 5 GPa obtained by using the second-order Birch–Murnaghan equation of state, while for the LT phase of WB the incompressibility increased to 381 ± 3 GPa. The elastically supported differential stress was measured in a lattice-specific manner and analyzed by using lattice strain theory. Greater s...

Structural phase transition, strength, and texture in vanadium at high pressure under nonhydrostatic compression**Project supported by the National Natural Science Foundation of China (Grant Nos. 10875142 and 11079040) the Project of Sichuan University of Arts and Science, China (Grant No. 2017KZ001Z), and the Program of Education Department of Sichuan Province, China (Grant No. 18ZB0506). This work was performed at 4W2

The structural phase transition, strength, and texture of vanadium have been studied under nonhydrostatic compression up to 70 GPa using an angle-dispersive radial x-ray diffraction technique in a 2-fold paranomic diamond anvil cell and up to 38 GPa using an angle-dispersive x-ray diffraction technique in a modified Mao–Bell diamond anvil cell at room temperature. We have confirmed a phase transition from body-centered cubic structure to rhombohedral structure at 27–32 GPa under nonhydrostatic compression. The radial x-ray diffraction data yields a bulk modulus GPa and its pressure derivative for the bcc phase and with for the rhombohedral phase at ψ = 54.7°. The nonhydrostatic x-ray diffraction data of both bcc and rhombohedral phases yields a bulk modulus with . Combined with the independent constraints on the high-pressure shear modulus, it is found that the vanadium sample can support a differential stress of ∼1.6 GPa when it starts to yield with plastic deformation at ∼36 GPa. A maximum differential stress as high as ∼1.7 GPa can be supported by vanadium at the pressure of ∼47 GPa. In addition, we have investigated the texture up to 70 GPa using the software package MAUD. It is convinced that the body-centered cubic to rhombohedral phase transition and plastic deformation due to stress under high pressures are responsible for the development of texture.

Pressure-induced superconductivity in semimetallic 1T−TiTe2 and its persistence upon decompression

Pristine $1\mathit{T}\ensuremath{-}{\mathrm{TiTe}}_{2}$ single crystal has been studied for resistance and magnetoresistance behavior under quasihydrostatic and nonhydrostatic compressions. While the semimetallic state is retained in nearly hydrostatic pressures, small nonhydrostatic compression leads to an abrupt change in low-temperature resistance, a signature of possible charge density wave (CDW) ordering, that eventually collapses above 6.2 GPa. Superconductivity emerges at $\ensuremath{\sim}5$ GPa, rapidly increasing to a critical temperature (${T}_{c}$) of 5.3 K at 12 GPa, irrespective of pressure condition. Pressure studies thus evidence that $1\mathit{T}\ensuremath{-}{\mathrm{TiTe}}_{2}$ exhibits superconductivity irrespective of the formation of the CDW-like state, implying the existence of phase-separated domains. Most surprisingly, the superconducting state persists upon decompression, establishing a novel phase diagram with suppressed $P$ scale. The pressure quenchable superconductivity, of multiband nature and relatively high upper critical field, makes $1\mathit{T}\ensuremath{-}{\mathrm{TiTe}}_{2}$ unique among other layered dichalcogenides.

Radial X-ray diffraction study of the static strength and texture of tungsten to 96 GPa

Investigations of equation of state, strength and texture of tungsten (W) have been performed under nonhydrostatic compression up to 96 GPa using an angle-dispersive radial X-ray diffraction (RXRD) techniques together with the lattice strain theory in a 2-fold paranomic diamond anvil cell (DAC) at ambient temperature. The ratio of t/G is found to remain constant above ∼35 GPa, indicating that the W started to experience macro yield with plastic deformation at this pressure. Combined with independent constraints on the high-pressure shear modulus, we found that W sample could support a differential stress of ∼4.57 GPa when it started to yield with plastic deformation at ∼35 GPa under uniaxial compression. The differential stress in W ranges from 0.01 GPa to 1.46 GPa with pressure increasing from 6 GPa to 35 GPa and can be described as t = −0.904(136) + 0.154(6)P, where P is the pressure in GPa. A maximum differential stress, as high as ∼6.46 GPa can be supported by W at the highest pressure of ∼96 GPa. In addition, we have investigated the texture of W under nonhydrostatic compression up to the highest pressure of 96 GPa using the software package MAUD (Material Analysis Using Diffraction). It is convinced that the plastic deformation due to stress under high pressures is responsible for the development of texture.

Radial X-Ray Diffraction Study of Static Strength of Tantalum to 80 GPa**Supported by the National Natural Science Foundation of China under Grant Nos 10875142 and 11079040, and the Chinese Academy of Sciences under Grant Nos KJCX2-SW-N03 and KJCX2-SW-N20.

We study the strength and texture of tantalum (Ta) under uniaxial compression up to 80 GPa using an angle-dispersive radial x-ray diffraction technique together with the lattice strain theory in a diamond anvil cell at ambient temperature. The ratio of differential stress to shear modulus (t/G) is found to remain constant above ∼60 GPa, indicating that the Ta starts to experience macro yield with plastic deformation at this pressure. Combined with independent constraints on the high-pressure shear modulus, we find that the Ta sample could support a differential stress of ∼4.67 GPa when it starts to yield with plastic deformation at ∼60 GPa under uniaxial compression. The differential stress in Ta ranges from 0.216 GPa to 4.67 GPa with pressure increasing from 1 GPa to 60 GPa and can be expressed as t = 0.199(33) + 0.075(1)P, where P is the pressure in GPa. A maximum differential stress as high as ∼5.37 GPa can be supported by Ta at the high pressure of ∼80 GPa. In addition, we investigate the texture of Ta under nonhydrostatic compression to 80 GPa using the software package material analysis using diffraction. It is proven that the plastic deformation due to stress under high pressures is responsible for the development of texture.