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Cerium oxide (ceria, CeO2) is frequently used as a standard in applications such as synchrotron and x-ray free electron lasers for calibrating x-ray wavelengths and offers the potential for understanding the high pressure properties and deformation mechanisms in a wide range of similar face centered cubic (fcc) materials. In this study, the pressure dependence of the strength of ceria was investigated up to 38 GPa using angle dispersive x-ray diffraction in a radial geometry in a diamond anvil cell. In this experiment, the difference in the stress along the axis of compression and perpendicular to the direction of compression can be determined, giving a quantity known as the differential stress. It was found that the differential stress (t), a measure of the lower bound for yield strength, initially increases rapidly from 0.35 ± 0.06 GPa to 2.2 ± 0.4 GPa at pressures of 1.8 and 3.8 GPa, respectively. Above 4 GPa, t increases more slowly to 13.8 ± 2.6 GPa at a pressure of 38 GPa. The changes in the preferred orientation (texture) of CeO2 with pressure were also measured, allowing for the determination of active deformation mechanisms using an elasto-viscoplastic self-consistent model (EVPSC). It was found that as pressure increased, the [001] direction had a slight preferred orientation along the axis of compression. Our EVPSC model of experimental fiber (cylindrically symmetric) textures and lattice strains were most consistent with dominant slip activity along { 111 } ⟨ 1 1 ¯ 0 ⟩. [ABSTRACT FROM AUTHOR]

Tantalum (Ta) is a metal that has useful properties that make it useful in extreme environments. It is, therefore, important to understand how Ta performs in such extreme conditions by accurately measuring its properties. In this work, the yield strength of tantalum has been measured at pressures up to 276 GPa using axial and radial x-ray diffraction (XRD) methods in diamond anvil cells (DACs). We measured strength using XRD in a radial DAC to 50 GPa, in an axial DAC to 60 GPa using diamonds with standard flat culets, and in a final experiment to 276 GPa using toroidal diamond anvils. The radial XRD data were refined using the Material Analysis Using Diffraction Rietveld software package to extract lattice strain and the yield strength. The axial data were refined using the General Structure Analysis System II and a linewidth method was used to calculate the yield strength. The yield strength measured near ambient pressure was found to be 0.5 GPa and increased with a pressure of up to 50 GPa, where the yield strength plateaued at a value of 2.4 GPa. At pressures above 60 GPa, the strength increased again to a maximum value of 9 GPa at the highest pressure of 276 GPa. The data from the three experiments show good agreement between the methods and previously reported experimental data. This agreement illustrates the value of axial diffraction data for material strength determination and allows for measurements at multi-hundreds of GPa using toroidal DACs. [ABSTRACT FROM AUTHOR]

We utilize synchrotron X-ray diffraction measurements, conducted inside a diamond anvil cell, to analyze the high-pressure stability of Mg/Nb multilayered nanocomposites of equal (1:1) and unequal (1:10) thickness ratios. At larger layer thicknesses, Mg in these nanocomposites exists in its traditional hexagonal close packed (hcp) structure, while below a critical layer thickness of 7–8 nm, the Mg structure is found to transform into an interface strain-induced metastable pseudomorphic body center cubic (bcc) crystal structure. The hcp Mg present in the larger layer thicknesses exhibits an hcp-to-bcc phase transformation at pressures greater than 44 GPa, and this pressure value is found to vary between the equal and unequal Mg/Nb nanocomposite thickness ratios. On the other hand, the pseudomorphic bcc Mg structure is stable up to pressures of 60 GPa. Additionally, the compressibility of the pseudomorphic bcc Mg structure under pressure is shown to be fundamentally different from the bulk (nonlaminated) bcc Mg structure formed under high pressures. These results indicate that interface strain engineering, and an appropriate choice of the adjacent layer material, might be a viable pathway for tuning the structure and properties of the pseudomorphic bcc Mg structure. [ABSTRACT FROM AUTHOR]

We present a portable CO2 laser heating system for in situ x-ray absorption spectroscopy (XAS) studies at 16-BM-D (High Pressure Collaborative Access Team, Advanced Photon Source, Argonne National Laboratory). Back scattering optical measurements are made possible by the implementation of a Ge beamsplitter. Optical pyrometry is conducted in the near-infrared, and our temperature measurements are free of chromatic aberration due to the implementation of the peak-scaling method [A. Kavner and W. R. Panero, Phys. Earth Planet. Inter. 143–144, 527–539 (2004) and A. Kavner and C. Nugent, Rev. Sci. Instrum. 79, 024902 (2008)] and mode scrambling of the input signal. Laser power stabilization is established using electronic feedback, providing a steady power over second timescales [Childs et al., Rev. Sci. Instrum. 91, 103003 (2020)]—crucial for longer XAS collections. Examples of in situ high pressure–temperature extended x-ray absorption fine structure measurements of ZrO2 are presented to demonstrate this new capability. [ABSTRACT FROM AUTHOR]

The high-entropy transition metal borides containing a random distribution of five or more constituent metallic elements offer novel opportunities in designing materials that show crystalline phase stability, high strength, and thermal oxidation resistance under extreme conditions. We present a comprehensive theoretical and experimental investigation of prototypical high-entropy boride (HEB) materials such as (Hf, Mo, Nb, Ta, Ti)B2 and (Hf, Mo, Nb, Ta, Zr)B2 under extreme environments of pressures and temperatures. The theoretical tools include modeling elastic properties by special quasi-random structures that predict a bulk modulus of 288 GPa and a shear modulus of 215 GPa at ambient conditions. HEB samples were synthesized under high pressures and high temperatures and studied to 9.5 GPa and 2273 K in a large-volume pressure cell. The thermal equation of state measurement yielded a bulk modulus of 276 GPa, in excellent agreement with theory. The measured compressive yield strength by radial X-ray diffraction technique in a diamond anvil cell was 28 GPa at a pressure of 65 GPa, which is a significant fraction of the shear modulus at high pressures. The high compressive strength and phase stability of this material under high pressures and high temperatures make it an ideal candidate for application as a structural material in nuclear and aerospace fields. [ABSTRACT FROM AUTHOR]

A state of the art, custom-built direct-metal deposition (DMD)-based additive manufacturing (AM) system at the University of Michigan was used to manufacture 50Cu–50Fe alloy with tailored properties for use in high strain/deformation environments. Subsequently, we performed preliminary high-pressure compression experiments to investigate the structural stability and deformation of this material. Our work shows that the alpha (BCC) phase of Fe is stable up to ~16 GPa before reversibly transforming to HCP, which is at least a few GPa higher than pure bulk Fe material. Furthermore, we observed evidence of a transition of Cu nano-precipitates in Fe from the well-known FCC structure to a metastable BCC phase, which has only been predicted via density functional calculations. Finally, the metastable FCC Fe nano-precipitates within the Cu grains show a modulated nano-twinned structure induced by high-pressure deformation. The results from this work demonstrate the opportunity in AM application for tailored functional materials and extreme stress/deformation applications. [ABSTRACT FROM AUTHOR]

A high-entropy transition metal boride (Hf0.2 Ti0.2 Zr0.2 Ta0.2 Mo0.2)B2 sample was synthesized under high-pressure and high-temperature starting from ball-milled oxide precursors (HfO2, TiO2, ZrO2, Ta2O5, and MoO3) mixed with graphite and boron-carbide. Experiments were conducted in a large-volume Paris–Edinburgh press combined with in situ energy dispersive x-ray diffraction. The hexagonal AlB2 phase with an ambient pressure volume V0 = 27.93 ± 0.03 Å3 was synthesized at a pressure of 0.9 GPa and temperatures above 1373 K. High-pressure high-temperature studies on the synthesized high-entropy transition metal boride sample were performed up to 7.6 GPa and 1873 K. The thermal equation of state fitted to the experimental data resulted in an ambient pressure bulk-modulus K0 = 344 ± 39 GPa, dK/dT = −0.108 ± 0.027 GPa/K, and a temperature dependent volumetric thermal expansion coefficient α = α0 + α1T + α2T−2. The thermal stability combined with a high bulk-modulus establishes this high-entropy transition metal boride as an ultrahard high-temperature ceramic material. [ABSTRACT FROM AUTHOR]

A broadband wavelet approach to ultrasonic pulse-echo time-of-flight measurements is described. The broadband approach significantly reduces the time required for frequency-dependent pulse-echo measurements, enabling studies of dynamic systems ranging from biological systems to solid-state phase transitions. The described broadband approach is demonstrated in parallel with the more traditional frequency stepping approach to perform ultrasound time-of-flight measurements inside a large volume Paris–Edinburgh press in situ at a synchrotron source. The broadband wavelet data acquisition process was found to be 1–2 orders of magnitude faster than the stepped-frequency approach, with no compromise on data quality or determined results. [ABSTRACT FROM AUTHOR]

The pressure-temperature (P-T) phase diagram of ammonium nitrate (AN) [NH4NO3] has been determined using synchrotron x-ray diffraction (XRD) and Raman spectroscopy measurements. Phase boundaries were established by characterizing phase transitions to the high temperature polymorphs during multiple P-T measurements using both XRD and Raman spectroscopy measurements. At room temperature, the ambient pressure orthorhombic (Pmmn) AN-IV phase was stable up to 45 GPa and no phase transitions were observed. AN-IV phase was also observed to be stable in a large P-T phase space. The phase boundaries are steep with a small phase stability regime for high temperature phases. A P-V-T equation of state based on a high temperature Birch-Murnaghan formalism was obtained by simultaneously fitting the P-V isotherms at 298, 325, 446, and 467 K, thermal expansion data at 1 bar, and volumes from P-T ramping experiments. Anomalous thermal expansion behavior of AN was observed at high pressure with a modest negative thermal expansion in the 3-11 GPa range for temperatures up to 467 K. The role of vibrational anharmonicity in this anomalous thermal expansion behavior has been established using high P-T Raman spectroscopy. [ABSTRACT FROM AUTHOR]

An isotopically enriched 13C homoepitaxial diamond layer of 6±1 μm thickness was grown on top of a brilliant cut diamond anvil by a microwave plasma chemical vapor deposition process for application as a pressure sensor. This isotopically enriched diamond tip was then used in conjunction with a natural isotopic abundance diamond anvil to generate high pressure on the sample. We provide a calibration for the 13C Raman mode of this extremely thin epitaxial layer to 156 GPa using ruby fluorescence and the equation of state of copper as secondary pressure standards. The nonlinear calibration of the 13C Raman mode pressure sensor is compared with similar calibrations of 12C Raman edge and a good agreement is obtained. The Raman signal from the 13C epitaxial layer remained a distinct singlet to 156 GPa, and pressure calibration is independent of sample mechanical strength or the diamond anvil geometry. The use of even thinner layer would allow calibration further into ultrahigh pressure regime where the use of other optical sensors has proven to be difficult. [ABSTRACT FROM AUTHOR]

The 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. [ABSTRACT FROM AUTHOR]

Cerium-12at%Praseodymium(Ce0.88Pr0.12) and Ce-50at%Praseodymium(Ce0.50Pr0.50) alloy samples that contain a random solid-solution of Ce (4f1 (J = 5/2)) and Pr (4f2 (J = 4)) localized f-states have been studied by angle dispersive x-ray diffraction in a diamond anvil cell to a pressure of 65 GPa and 150 GPa respectively using a synchrotron source. Ce0.88Pr0.12 alloy crystallizes in a face-centered cubic (γ-phase) structure at ambient conditions, while Ce0.50Pr0.50 alloy crystallizes in the double hexagonal close packed (dhcp) structure at ambient conditions. Two distinct volume collapse transitions are observed in Ce0.88Pr0.12 alloy at 1.5 GPa and 18 GPa with volume change of 8.5% and 3% respectively. In contrast, Ce0.50Pr0.50 alloy shows only a single volume collapse of 5.6% at 20 GPa on phase transformation to α-Uranium structure under high pressure. Electrical transport measurements under high pressure show anomalies in electrical resistance at phase transitions for both compositions of this alloy. [ABSTRACT FROM AUTHOR]

A new sample cell assembly design for the Paris-Edinburgh type large-volume press for simultaneous measurements of X-ray diffraction, electrical resistance, Seebeck coefficient and relative changes in the thermal conductance at high pressures has been developed. The feasibility of performing in situ measurements of the Seebeck coefficient and thermal measurements is demonstrated by observing well known solid-solid phase transitions of bismuth (Bi) up to 3 GPa and 450 K. A reversible polarity flip has been observed in the Seebeck coefficient across the Bi-I to Bi-II phase boundary. Also, successful Seebeck coefficient measurements have been performed for the classical high-temperature thermoelectric material PbTe under high pressure and temperature conditions. In addition, the relative change in the thermal conductivity was measured and a relative change in ZT, the dimensionless figure of merit, is described. This new capability enables pressure-induced structural changes to be directly correlated to electrical and thermal properties. [ABSTRACT FROM AUTHOR]

By combining mask-less lithography and chemical vapor deposition (CVD) techniques, a novel two-stage diamond anvil has been fabricated. A nanocrystalline diamond (NCD) micro-anvil 30 m in diameter was grown at the center of a [100]- oriented, diamond anvil by utilizing microwave plasma CVD method. The NCD micro-anvil has a diamond grain size of 115 nm and micro-focused Raman and X-ray Photoelectron spectroscopy analysis indicate sp³-bonded diamond content of 72%. These CVD grown NCD micro-anvils were tested in an opposed anvil configuration and the transition metals osmium and tungsten were compressed to high pressures of 264 GPa in a diamond anvil cell. [ABSTRACT FROM AUTHOR]

Ultra-high static pressures have been achieved in the laboratory using a two-stage micro-ball nanodiamond anvils as well as a two-stage micro-paired diamond anvils machined using a focused ion-beam system. The two-stage diamond anvils’ designs implemented thus far suffer from a limitation of one diamond anvil sliding past another anvil at extreme conditions. We describe a new method of fabricating two-stage diamond micro-anvils using a tungsten mask on a standard diamond anvil followed by microwave plasma chemical vapor deposition (CVD) homoepitaxial diamond growth. A prototype two-stage diamond anvil with 300 µm culet and with a CVD diamond second stage of 50 µm in diameter was fabricated. We have carried out preliminary high pressure X-ray diffraction studies on a sample of rare-earth metal lutetium sample with a copper pressure standard to 86 GPa. The micro-anvil grown by CVD remained intact during indentation of gasket as well as on decompression from the highest pressure of 86 GPa. [ABSTRACT FROM AUTHOR]

Using designer diamond anvils and angle dispersive X-ray diffraction technique at a synchrotron source, we have performed simultaneous electrical and structural studies on neodymium metal to 152 GPa in a diamond anvil cell. Four-probe electrical resistance measurement shows a 38% decrease in the electrical resistivity, associated with the delocalization of the 4f-shell electrons, starting at 100 GPa up to a final pressure of 152 GPa. The continuous decrease in electrical resistivity is consistent with the observation of a gradual phase transition to a-U structure in this pressure range. The (1 1 1) diffraction peak of a-U structure first appears at 100 GPa and increases in intensity with increasing pressure to 152 GPa. This increase in intensity is attributed to an increasing volume fraction of a-U phase and an increase in structural y -parameter from 0.07 at 118 GPa to 0.095 at 152 GPa. In contrast to the abrupt pressure-induced f-electron transition seen in cerium and praseodymium, the continuous evolution of a-U structure and electrical resistivity in neodymium confirms the gradual nature of 4f delocalization process in this element. [ABSTRACT FROM AUTHOR]

It is generally observed that the rare earth metals adapt an orthorhombic alpha-uranium (α-U) structure at high pressures following the delocalization of 4f shell under compression. We examine the stability of the α-U structure in praseodymium metal at ultrahigh pressures of 313 GPa (volume compression V / V 0 = 0.343) in a diamond anvil cell at room temperature. X-ray diffraction data show a transformation from the α-U structure to a primitive orthorhombic P 2 1 2 1 2 1 phase at 147±5 GPa, which is characterized by the anisotropic compressibility of various crystallographic axes. This anisotropic compressibility leads to an interesting situation when the b -axis and the c -axis of the orthorhombic phase become nearly equal above 260 GPa and the structure can be regarded as a pseudo-tetragonal phase. Our present study shows that the 4f band metal Pr does not adapt a body centred tetragonal phase as predicted by theory, but instead novel crystallographic phases are observed at extreme compressions. The present results have a broader impact on the stability of the α-U phase in a variety of f-band systems at high pressures. [ABSTRACT FROM AUTHOR]

We conducted in-situ high-pressure synchrotron x-ray experiments on beryllium metal at pressures up to 7.9 GPa and temperatures up to 1373 K. A complete pressure (P)-volume (V)-temperature (T) equation of state (EOS) is determined based on the experiment, which includes temperature derivatives of elastic bulk modulus (at both constant pressure and constant volume) and pressure dependence of thermal expansivity. From this EOS, we calculate thermal pressure, heat capacity at constant volume, and thermodynamic Grüneisen parameter as a function of temperature. Above ∼600 K, our results show notable deviation from theoretical predictions using the quasiharmonic and local-density approximations, indicating that the free energy calculations need to be further improved within the current scheme of approximations. [ABSTRACT FROM AUTHOR]

The phase stability and isothermal compression behavior of 2,4,6-trinitrotoluene (TNT) have been established to 26.5 GPa using angle-dispersive x-ray diffraction. P-V isotherms derived from the high-pressure x-ray spectra displayed a slight density hysteresis around 4.0 GPa and a sharp discontinuity at ∼20.0 GPa. The latter transition is ascribed to a monoclinic-to-orthorhombic first-order phase transition in TNT. The conversion of the isothermal P-V data to the shock velocity-particle velocity plane revealed a deviation from linearity at low up, a cusp associated with the phase transition at high up, and general agreement with the wealth of unreacted Hugoniot data on TNT. [ABSTRACT FROM AUTHOR]

Structural stability of Ti alloy, stabilized as a mixture of bcc (β) and hcp (α) phases, was investigated up to 71 GPa. Over this pressure, β phase remains stable, while for α phase, a decrease in c/a ratio from 1.601 at ambient pressure to 1.568 at 36 GPa is observed, followed by an increase to 1.613 at 44 GPa. A steady value is then observed up to 67 GPa. Above 58 GPa, the authors also observe that α phase peak intensity is decreasing. At 67 GPa, the authors only observe β phase peaks, which indicates that α→β structural phase transition is complete. [ABSTRACT FROM AUTHOR]

Isotopically pure 13C homoepitaxial diamond layer of 20±5 microns thickness was grown on top of a brilliant cut diamond anvil by a microwave plasma chemical vapor deposition process for application as a stress sensor. This isotopically pure diamond tip was then used in conjunction with a natural abundance 12C diamond anvil to generate high pressure on a soft sample containing carbon nanotubes to 100 GPa. The 13C diamond Raman signal remained a distinct single peak showing that the mean normal stress experienced by this thin layer is uniform to the highest pressure. The stress-induced shift of the 13C layer is related to the quasihydrostatic pressure in the sample chamber using a ruby pressure sensor and shows a remarkably linear behavior to extreme pressures. [ABSTRACT FROM AUTHOR]

The electrical and magnetic properties of light rare-earth metals and trans-plutonium actinide metals are of interest to study the f-shell delocalization phenomenon under high compressions. Using designer diamond anvil technology, sensitive electrical four-probe measurements were performed on light rare-earth metal praseodymium to pressures of 179 GPa at room temperature. We document an average drop in resistivity of 53% at a pressure of 20 GPa in a series of high-pressure experiments. This large drop in resistivity provides the strongest experimental evidence yet for the view that the 20 GPa phase transition is indeed associated with f-electron delocalization. Our results show that the precise electrical measurements are ideally suited for f-delocalization studies, especially where structural data do not provide clear evidence of this transition. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structural phase stability of 1,1-diamino-2,2-dinitroethylene (FOX-7) has been studied up to 10 GPa through isothermal compression at 100 °C and 200 °C using synchrotron mid- and far-infrared spectroscopy. During isothermal compression at 100 °C changes are observed in vibrational spectra with increase in pressure that are indicative of significant distortion to monoclinic α phase or a possible structural transformation to a high pressure α′ phase at 2.2 GPa and α″ phase at 6.1 GPa. At 200 °C, for the far- and mid-IR regimes, the similar changes were observed at 2.1 (2.0) GPa and 5.3 (5.5) GPa, respectively. The observed change is nearly isobaric, consistent with previously reported high pressure and room temperature values, up to the highest temperature of 200 °C reached in our experiments. Over the total P-T range investigated, up to ∼10 GPa and 200 °C, we observed no evidence of sample decomposition. The observed changes are partially reversible with only slight evidence of the high pressure distortion remaining upon complete decompression. Additional isobaric heating at 1.07 GPa was performed in the mid-IR regime, which clearly revealed an onset of decomposition at 360 °C. Further x-ray or neutron diffraction, which are needed to fully resolve the cause of observed changes above 2 and 5 GPa, are ongoing. [ABSTRACT FROM AUTHOR]

High pressure infrared (IR) measurements of the insensitive explosive, 1,1-diamino-2,2-dinitroethylene (DADNE or FOX-7), have been performed in the far- and mid-IR spectral regions up to ∼28 GPa using a synchrotron source. Petroleum jelly and KBr were employed as the pressurizing media for the far- and mid-IR experiments, respectively. In both experiments, IR spectra were collected at various pressures both in compression and decompression to determine reversibility under pressure cycling. There is evidence for at least two and possibly three phase transitions in the samples at pressures near 2, 5, and10 GPa, respectively. In the high frequency region (∼3000-3300 cm-1), the NH2 symmetric and antisymmetric vibrational modes soften with increasing pressure, suggesting strengthening intermolecular hydrogen bonding, and then stiffen with further pressure increase. At higher pressures (above 5 GPa), we suspect progressive flattening of the zig zag structure characteristic of the α phase occurs, which was observed in the low temperature, quenched γ phase. FOX-7 was also observed to survive pressure cycling up to 28 GPa contrary to earlier reports of pressure-induced molecular decomposition. [ABSTRACT FROM AUTHOR]

A bioceramic hydroxyapatite, Ca[SUB10](PO[SUB4])6(OH)[SUB2] polycrystalline sample was studied under high pressures in a diamond anvil cell to investigate its structural, electrical, and mechanical properties under compression. Anisotropic compression effects were observed in x-ray diffraction studies below 8 GPa. Nanoindentation hardness measurements on pressure-treated hydroxyapatite samples show hardness value of 4.0±0.5 GPa which is comparable to the plasma-sprayed samples. Electrical studies show that the bioceramic sample retained its insulating properties to 65 GPa. The present studies demonstrate that a fully dense and translucent hydroxyapatite sample is attained above 10 GPa at 300 K. [ABSTRACT FROM AUTHOR]

Synchrotron X-ray radiation Laue diffraction is a widely used diagnostic technique for characterizing the microstructure of materials. An exciting feature of this technique is that comparable numbers of reflections can be measured several orders of magnitude faster than using monochromatic methods. This makes polychromatic beam diffraction a powerful tool for time-resolved microstructural studies, critical for understanding pressure-induced phase transition mechanisms, by in situ and in operando measurements. The current status of this technique, including experimental routines and data analysis, is presented along with some case studies. The new experimental setup at the High-Pressure Collaborative Access Team (HPCAT) facility at the Advanced Photon Source, specifically dedicated for in situ and in operando microstructural studies by Laue diffraction under high pressure, is presented. [ABSTRACT FROM AUTHOR]

The most incompressible transition metal osmium (Os) has been studied under high pressure. There is significant interest in Os because of the structural anomalies attributed to topological transitions in the Fermi surface for valence electrons in the hexagonal close-packed phase. We report on measurements of structural parameters and equation of state on Os metal to a pressure of 207 GPa at ambient temperature using platinum as a pressure standard. We obtained angle-dispersive X-ray diffraction data at a synchrotron source with closely spaced pressure intervals to observe any discontinuities or anomalies in the axialc/aratio at high pressures. Rietveld refinements of X-ray diffraction data show a slowly varying axial ratio (c/a) with a broad minimum at 75 GPa. Our data do not provide any evidence of anomalous behavior in thec/aratio in Os at 25 or 150 GPa as have been reported in previous studies. Our experimental results are in agreement with theoretical calculations that do not predict any anomalous behavior inc/aratio in Os under extreme conditions. We present an equation of state for Os to 207 GPa (V/V0 = 0.761) at ambient temperature and compare our results with the previously published data. [ABSTRACT FROM PUBLISHER]