Nonhydrostatic Compression Research Articles

Effects of Nonhydrostatic Stress on Structural and Optoelectronic Properties of Methylammonium Lead Bromide Perovskite.

We report synchrotron X-ray diffraction, photoconductivity, and photoluminescence investigations of methylammonium-lead-bromide (MAPbBr3) under various stress conditions, supported by density-functional-theory (DFT) calculations. The properties of MAPbBr3 show substantial dependence on the hydrostatic conditions. While nonhydrostatic compression of MAPbBr3 leads to amorphization above 2.4 GPa, under quasi-hydrostatic (Ar) and hydrostatic (He) pressure, the sample remains in crystalline phases. A sequence of phase transitions between two cubic phases and orthorhombic Pnma phase is observed when using Ar, or no pressure-transmitting-medium (PTM). In helium-PTM only transitions between the two cubic structures and a new isostructural phase transition with a large volume collapse to a third cubic-phase at 2.7 GPa was observed. The photoluminescence measurements indicate a pressure-induced band gap-narrowing in the cubic phase I, and a blue-shift in the orthorhombic structure. DFT calculations illustrate that the dynamics of the organic molecules and the inorganic lattice, coupled via the N-H···Br hydrogen-bonding interactions, affect the Pb-Br distance and the bandgap evolution under pressure.

Strength and equation of state of molybdenum triboride under 80 GPa pressure, using radial X-ray diffraction

ABSTRACTThe strength and equation of state of molybdenum triboride have been determined under nonhydrostatic compression up to 80 GPa, using an angle-dispersive radial X-ray diffraction technique in a diamond anvil cell (DAC). The RXD data yield a bulk modulus and its pressure derivative as K0 = 342(6) GPa with K0′ = 2.11(17) at ψ = 54.7°. Analysis of diffraction data using the strain theory indicates that the ratio of differential stress to shear modulus (t/G) ranges from 0.002 to 0.050 at pressures of 4–80 GPa. Together with theoretical results on the high pressure shear modulus, our results here show that molybdenum triboride sample under uniaxial compression can support a differential stress of ∼10 GPa when it started to yield with plastic deformation at ∼30 GPa. In addition, we draw a conclusion that MoB3 is not a superhard material but a hard material.

Pressure-induced permanent metallization with reversible structural transition in molybdenum disulfide

This report presents a pressure-induced permanent metallization for MoS2 under non-hydrostatic conditions. Impedance and Raman spectra were measured to study the pressure-induced structural and electronic transformations of MoS2 at up to ∼25 GPa in diamond anvil cells under both non-hydrostatic and hydrostatic conditions. The results show evidence for isostructural hexagonal distortion from 2Hc to 2Ha and metallization at ∼17 GPa and ∼20 GPa under non-hydrostatic and hydrostatic conditions, respectively. Interestingly, the metallization is irreversible only under non-hydrostatic compression. We attribute this phenomenon to the incorporation of molecules of pressure medium between layers, which mitigate compressed stress and reduce interlayer interaction.

ScVO4 under non-hydrostatic compression: a new metastable polymorph

The high-pressure (HP) behaviour of scandium vanadate (ScVO4) is investigated under non-hydrostatic compression. The compound is studied by means of synchrotron-based powder x-ray diffraction (XRD) and optical-absorption techniques. The occurrence of a non-reversible phase transition is detected. The transition is from the zircon structure to the fergusonite-type structure and takes place around 6 GPa with nearly 10% volume discontinuity. XRD measurements on the pressure cycled sample confirm for the first time that the fergusonite-type ScVO4 can be recovered as the metastable phase at ambient conditions. Raman spectroscopic measurements verify the metastable phase to be of a fergusonite-type phase. Theoretical calculations also corroborate the experimental findings. The fergusonite phase is found to be stiffer than the ambient-pressure zircon phase, as indicated by the observed experimental and theoretical bulk moduli. The optical properties and lattice-dynamics calculation of the fergusonite ScVO4 are discussed. At ambient pressure the band gap of the zircon (fergusonite)-type ScVO4 is 2.75 eV (2.3 eV). This fact suggests that the novel metastable polymorph of ScVO4 can have applications in green technologies; for instance, it can be used as photocatalytic material for hydrogen production by water splitting.

Cyclic Phase Transition from Hexagonal to Orthorhombic Then Back to Hexagonal of EuF3 While Loading Uniaxial Pressure and under High Temperature

The structure and photoluminescence properties are investigated under high pressure and high temperature for pure orthorhombic and hexagonal EuF3 nanocrystals. Under hydrostatic compression, the hexagonal EuF3 remains stable at pressures up to 26 GPa. Under nonhydrostatic compression, a cyclic phase transition from hexagonal to orthorhombic and then back to hexagonal is observed for the first time. When loading uniaxial compression, the pure hexagonal EuF3 partly transforms to orthorhombic at 70 MPa, then the orthorhombic EuF3 transforms to hexagonal at about 3 GPa, and the transition is completed at about 10 GPa. The cyclic phase transition is also observed during the heating process; the hexagonal transforms to orthorhombic at 550 °C and then to hexagonal at 855 °C. The content phase diagrams are obtained under high pressure and at high temperature.

The α → ω phase transformation in zirconium followed with ms-scale time-resolved X-ray absorption spectroscopy

ABSTRACTThe conditions of the pressure-induced phase transformation in zirconium have been reported to be influenced by the sample purity, the pressurizing conditions, and the deformation rate. Here, we study this transformation using a dynamic diamond-anvil cell compression setup and ms-scale time-resolved X-ray absorption spectroscopy. The sample pressure is also monitored at the same timescale using the ruby luminescence method. For the samples used in this study (polycrystal of 99.9+ purity, hydrostatically compressed in neon pressure transmitting medium), the phase transformation pressure is very close in static (11 GPa) and in dynamic (12±1 GPa) compression and the transformation is achieved in less than a few ms at 12 GPa. Comparison with literature studies suggest that the kinetics and the mechanism of this martensitic phase transformation are different under hydrostatic and non-hydrostatic compression.

Metastable structural transformations and pressure-induced amorphization in natural (Mg,Fe)2SiO4olivine under static compression: A Raman spectroscopic study

Raman spectroscopic data were obtained for (Mg,Fe)2SiO4 samples during compression to 57 GPa. Single crystals of San Carlos olivine compressed hydrostatically above 41 GPa showed appearance of a new “defect” peak in the 820–840 cm−1 region associated with SiOSi linkages appearing between adjacent Embedded Image tetrahedra to result in five- or sixfold-coordinated silicate species. Appearance of this feature is accompanied by a broad amorphous background. The changes occur at lower pressure than metastable crystalline transitions of end-member Mg2SiO4 forsterite (Fo-I) into Fo-II and Fo-III phases described recently. We complemented our experimental study using density functional theory (DFT) calculations and anisotropic ion molecular dynamics (AIMD) simulations to study the Raman spectra and vibrational density of states (VDOS) of metastably compressed Mg2SiO4 olivine, Fo-II and Fo-III, and quenched melts at high and low pressures. By 54 GPa all sharp crystalline peaks disappeared from observed Raman spectra indicating complete pressure-induced amorphization (PIA). The amorphous (Mg,Fe)2SiO4 spectrum contains Si-O stretching bands at lower wavenumber than expected for Embedded Image indicating high coordination of the silicate units. The amorphous spectrum persisted on decompression to ambient conditions but with evidence for reappearance of tetrahedrally coordinated units. Non-hydrostatic compression of polycrystalline olivine samples showed similar appearance of the defect feature and broad amorphous features between 43–44 GPa. Both increased in intensity as the sample was left at pressure overnight but they disappeared during decompression below 17 GPa with recovery of the starting olivine Raman signature. A hydrated San Carlos olivine sample containing 75–150 ppm OH was also studied. Significant broadening of the Embedded Image stretching peaks was observed above 43 GPa but without immediate appearance of the defect or broad amorphous features. However, both of these characteristics emerged after leaving the sample at 47 GPa overnight followed by complete amorphization that occurred upon subsequent pressurization to 54 GPa. During decompression the high-density amorphous spectrum was retained to 3 GPa but on final pressure release a spectrum similar to thermally quenched low-pressure olivine glass containing isolated Embedded Image groups was obtained. Leaving this sample overnight resulted in recrystallization of olivine. Our experimental data provide new insights into the metastable structural transformations and relaxation behavior of olivine samples including material recovered from meteorites and laboratory shock experiments.

Shear-Induced Isostructural Phase Transition and Metallization of Layered Tungsten Disulfide under Nonhydrostatic Compression

Pressure-induced structural and electronic transformations of tungsten disulfide (WS2) have been studied to 60 GPa, in both hydrostatic and nonhydrostatic conditions, using four-probe electrical resistance measurements, micro-Raman spectroscopy, and synchrotron X-ray diffraction. The results show the evidence for an isostructural phase transition from hexagonal 2Hc phase to hexagonal 2Ha phase, which accompanies the metallization at ∼37 GPa. This isostructural transition occurs displacively over a large pressure range between 15 and 45 GPa and is driven by the presence of strong shear stress developed in the layer structure of WS2 under nonhydrostatic compression. Interestingly, this transition is absent in hydrostatic conditions using He pressure medium, underscoring its strong dependence on the state of stress. We attribute the absence to the incorporation of He atoms between the layers, mitigating the development of shear stress. We also conjecture a possibility of magnetic ordering in WS2 that may occ...

In situx-ray diffraction of fast compressed iron: Analysis of strains and stress under non-hydrostatic pressure

Series of high-pressure x-ray diffraction patterns of iron and its high-pressure polymorphs were collected with 0.1--0.2-s exposure time utilizing a membrane diamond anvil cell (DAC) for compression at various loading and unloading rates to a maximum pressure of 70 GPa. Strain rates of ${10}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at a maximum pressurization rate of 4.1 GPa/s were achieved in non-hydrostatic compression of hcp Fe. Linewidth analysis was used to retrieve strain and uniaxial stress of Fe as a function of pressure upon both compression and decompression. Analysis of the lattice parameters ratio $c/a$ of hcp Fe indicates the presence of complex non-hydrostatic stress states, which developed as a function of strain rate, relaxation time, and various levels of hydrostaticity. Our results emphasize the importance of a controlled pressurization in DACs because the experimental loading rate strongly influences the stress state of the sample, particularly on decompression. Our time-resolved x-ray diffraction of the phase transition from bcc Fe to hcp Fe reveals residual grains of bcc Fe capable of surviving to very high pressures ($>35$ GPa) for a few minutes after the transition.

New automated shear cell with diamond anvils for in situ studies of materials using X-ray diffraction

A new design of an automated shear cell with diamond anvils for X-ray in situ studies has been developed and manufactured. As compared with a previous design, the dimensions and weight of the new cell have been reduced almost by one half on retention of the pressure range up to 100 GPa. To demonstrate the efficiency of the apparatus, the investigations of the possibility to use polycrystalline NaCl as a sensor of pressure at diffraction studies of samples in nonhydrostatic compression conditions have been carried out.

Strength and structural phase transitions of gadolinium at high pressure from radial X-ray diffraction

Lattice strength and structural phase transitions of gadolinium (Gd) were determined under nonhydrostatic compression up to 55 GPa using an angle-dispersive radial x-ray diffraction technique in a diamond-anvil cell at room temperature. Three new phases of fcc structure, dfcc structure, and new monoclinic structure were observed at 25 GPa, 34 GPa, and 53 GPa, respectively. The radial x-ray diffraction data yield a bulk modulus K0 = 36(1) GPa with its pressure derivate K0′ = 3.8(1) at the azimuthal angle between the diamond cell loading axis and the diffraction plane normal and diffraction plane ψ = 54.7°. With K0′ fixed at 4, the derived K0 is 34(1) GPa. In addition, analysis of diffraction data with lattice strain theory indicates that the ratio of differential stress to shear modulus (t/G) ranges from 0.011 to 0.014 at pressures of 12–55 GPa. Together with estimated high-pressure shear moduli, our results show that Gd can support a maximum differential stress of 0.41 GPa, while it starts to yield to plastic deformation at 16 GPa under uniaxial compression. The yield strength of Gd remains approximately a constant with increasing pressure, and reaches 0.46 GPa at 55 GPa.

Radial X-ray diffraction study of the static strength and equation of state of MoB2 to 85 GPa

Investigations of strength and equation of state of α-MoB2 have been performed under nonhydrostatic compression up to 85GPa using an angle-dispersive radial X-ray diffraction (RXD) techniques together with the lattice strain theory in a 2-fold panoramic diamond anvil cell at ambient temperature. The RXD data yields a bulk modulus and its pressure derivative as K0=323(6)GPa with K0′=4.59(27). The ratio of t/G is found to remain constant above ∼44GPa, indicating that the α-MoB2 started to experience yield with plastic deformation at this pressure. Together with theoretical results on high-pressure shear modulus, our results here show that molybdenum diborides sample could support a differential stress of ∼18GPa when it started to yield with plastic deformation at ∼44GPa under uniaxial compression. A maximum differential stress, as high as ∼25GPa can be supported by molybdenum diborides at the high pressure of ∼85GPa.

Strength of tungsten triboride under pressure up to 86 GPa from radial X-ray diffraction

The strength of tungsten triboride (WB3) was determined under nonhydrostatic compression up to 86GPa using an angle-dispersive radial X-ray diffraction technique in a diamond-anvil cell (DAC). Analyze of diffraction data using lattice strain theory indicate that the ratio of differential stress to shear modulus (t/G) changes from 0.004 at ambient conditions to 0.078 at 86GPa. Together with theoretical results on the high-pressure shear modulus, our results here show that WB3 under uniaxial compression can support a differential stress of 26GPa when it starts to yield to the plastic deformation at 40GPa. The yield strength of WB3 increases with increasing pressure, reaching a maximum value of 30GPa at 77GPa. By comparison, we find that the high-pressure strength of WB3 is comparable to those of c-BC2N, B6O, and γ-Si3N4.

Structural stability and compressive behavior of ZrH2under hydrostatic pressure and nonhydrostatic pressure

The important transition metal dihydride ZrH2 has been characterized using in situ synchrotron X-ray diffraction combined with diamond anvil cell techniques and ab initio calculations. The effect of a pressure-transmitting medium on the structural stability and compressive behavior was investigated under both hydrostatic pressure and nonhydrostatic pressure conditions. The ambient I4/mmm structure is stable under both nonhydrostatic and hydrostatic compressions. The supplementary theoretical calculations have proposed that the I4/mmm structure transformed into the P4/nmm structure above 100 GPa confirming the stability of the I4/mmm structure during the experimental runs. The difference in the volume reduction between the two compressions becomes larger with increasing pressure. Up to about 50 GPa, the volume collapse of nonhydrostatic compression is 6% relative to the hydrostatic compression. The present study offers a new approach with nonhydrostatic compression or shear stress for finding higher volumetric density hydrogen structures in metal hydrides.

Lattice stress states of superhard tungsten tetraboride from radial x-ray diffraction under nonhydrostatic compression

In this work, we examine the lattice behavior of the economically interesting superhard material, tungsten tetraboride (${\mathrm{WB}}_{4}$), in a diamond anvil cell under nonhydrostatic compression up to 48.5 GPa. From the measurements of lattice-supported differential stress, significant strength anisotropy is observed in ${\mathrm{WB}}_{4}$. The (002) planes are found to support the highest differential stress of 19.7 GPa within the applied pressure range. This result is in contrast to ${\mathrm{ReB}}_{2}$, one of the hardest transition metal borides known to date, where the same planes support the least differential stress. A discontinuous change in the slope of c/a ratio is seen at 15 GPa, suggesting a structural phase transition that has also been observed under hydrostatic compression. Speculations on the possible relationship between the observed structural changes, the strength anisotropy, and the orientation of boron-boron bonds along the $c$ direction within the ${\mathrm{WB}}_{4}$ structure are included.

Polycrystalline solids under nonhydrostatic compression: Determination of strength from x-ray diffraction data

A polycrystalline sample compressed in a diamond anvil cell (DAC) without any pressure transmitting medium develops a stress state at the centre of the sample that is axially symmetric about the load axis. The axial stress component is larger than the radial component and the difference t between the two is taken as a measure of the compressive strength of the sample material at a confining pressure that equals the mean normal stress. A proper analysis of the diffraction data yields t. The data taken with the radial diffraction geometry, wherein the incident x-ray beam is perpendicular to the load axis of the DAC, give reliable estimates of strength. The diffraction data obtained with the conventional geometry, wherein the incident x-ray beam passes parallel to the DAC axis, provide reasonable estimates of strength. However, even in this case, reliable strength data can be obtained by combining the measured pressure-volume data under nonhydrostatic compression and the hydrostat derived from an independent source. The determination of strength from high pressure diffraction data is discussed.

The strength of ruby from X-ray diffraction under non-hydrostatic compression to 68 GPa

Polycrystalline ruby (α-Al2O3:Cr3+), a widely used pressure calibrant in high-pressure experiments, was compressed to 68.1 GPa at room temperature under non-hydrostatic conditions in a diamond anvil cell. Angle-dispersive X-ray diffraction experiments in a radial geometry were conducted at beamline X17C of the National Synchrotron Light Source. The stress state of ruby at high pressure and room temperature was analyzed based on the measured lattice strain. The differential stress of ruby increases with pressure from ~3.4 % of the shear modulus at 18.5 GPa to ~6.5 % at 68.1 GPa. The polycrystalline ruby sample can support a maximum differential stress of ~16 GPa at 68.1 GPa under non-hydrostatic compression. The results of this study provide a better understanding of the mechanical properties of this important material for high-pressure science. From a synthesis of existing data for strong ceramic materials, we find that the high-pressure yield strength correlates well with the ambient pressure Vickers hardness.

Pressure transmitting medium-dependent structure stability of nanoanatase TiO2 under high pressure

The pressure transmitting medium (PTM)-dependent structure stability of ∼12 nm nanoanatase TiO2 under high pressure has been investigated using in situ synchrotron X-ray powder diffraction techniques at room temperature. We found that the starting anatase phase (I41/amd) transforms into the monoclinic baddeleyite phase (P21/c) regardless of embedded in PTM or not. However, the beginning and ending pressure for the transformation under quasihydrostatic compression is found to be ∼7 GPa higher than that of under nonhydrostatic compression, and amorphization of sample occurs more easily during the former condition. This result indicates that the interface energy between the PTM and nanoparticles undoubtedly modifies the phase stability during high pressure experiments.

Strength and equation of state of NaCl from radial x-ray diffraction

The strength and equation of state of NaCl were determined under nonhydrostatic compression up to 27 GPa using an energy-dispersive radial x-ray diffraction technique in a diamond-anvil cell using the lattice strain theory. Together with estimation of the high-pressure shear modulus, it is suggested that NaCl could support a maximum differential stress of 0.980 GPa at 22.6 GPa under uniaxial compression. The differential stress rapidly drops at 27.2 GPa due to the phase transition from B1 phase to B2 phase for NaCl. The hydrostatic compression data of B1 phase yield a bulk modulus K0 = 25.6(8) GPa and its pressure derivative K0′ = 5.16(20) using Pt pressure scale. In addition, a comparative study of the observed pressures from Pt scale and ruby-fluorescence scale shows that the ruby-fluorescence pressures may reflect the lower stress state under nonhydrostatic compression compared with hydrostatic compression.

High-pressure strength of nanocrystalline tantalum carbide (TaC) studied at a non-hydrostatic compression

High-pressure strength and plastic properties of nanocrystalline tantalum carbide (nano-TaC) have been investigated by using synchrotron radiation up to 65.5GPa. Nano-TaC shows a bulk modulus of K0=433±7GPa with K′0 fixed at 4.0. It is close to that of diamond and is higher than the results from micron-TaC. In its plastic deformation behavior, nano-TaC starts to yield a plastic deformation at around 20GPa, and the yield strength of nano-TaC increases with pressures and reaches a value of ~22GPa at the highest pressure in our test. The high-pressure strength of nano-TaC is comparable to that of other superhard metals such as B6O, c-BC2N and γ-Si3N4. The first-principle DFT calculations confirm a lower bulk modulus for micron-TaC.