Bulk Modulus K0 Research Articles

Phase comparison and equation of state for Ta2O5

Tantalum pentoxide (Ta2O5) is among the most technologically useful heavy transition metal oxides. Unfortunately, its crystal structure is the subject of long-standing and unresolved disagreement. Among other consequences, this uncertainty has made it impossible to formulate a robust high pressure equation of state for Ta2O5. Here, we report the results of high pressure x-ray diffraction experiments indexed against a comprehensive list of proposed Ta2O5 phases. Five of the proposed phases produce good matches to experimental observations, and the compressibility parameters for these phases are all consistent within uncertainty. This means that regardless of the particular phase chosen, the Ta2O5 equation of state can be described with bulk modulus K0=138±3.68 GPa and pressure derivative K0′=1.82±0.45 . Combining these experimental results with new density-functional theory calculations allows us to identify the λ phase as the best structural model of Ta2O5 at ambient conditions.

A comparative study of magnetic behaviours in bulk and ribbon samples of PrMn2Ge2 compound

The magnetic properties of PrMn2Ge2 samples in both the as-cast bulk and melt-spun ribbon forms have been investigated in detail by a comprehensive set of x-ray and neutron powder diffraction, magnetic and heat capacity measurements and corresponding sets of data analyses. Thermal expansion measurements indicate the presence of magnetoelastic coupling effects around all transition temperatures in the bulk sample. The bulk modulus K0 = 42.0 GPa and its first derivative K0’ = 18.7 have been derived from the pressure-volume data. The Curie temperature from the intralayer antiferromagnetism (AFl) of PrMn2Ge2 to a canted spin structure (Fmc) is TCinter = 332 K for the bulk sample, decreasing to TCinter = 320 K for the ribbon sample. The critical components γ, β and δ of this second order magnetic transition as determined from Kouvel-Fisher analyses, indicate long range magnetic interactions around TCinter. Based on these critical exponents the magnetization, field and temperature data around TCinter collapse onto two curves obeying the single scaling equation M(H,ε)=εβf±(Hεβ+γ). The Debye temperatures and the density of states at the Fermi level are θD=306K and N(EF)=3.47state/eVatom for the bulk sample and θD=320K and (EF)=2.18state/eVatom for the ribbon sample with the nuclear specific heat coefficient for bulk PrMn2Ge2 derived from the splitting of the nuclear hyperfine levels as CN = 517 mJ mol−1 K−1. With a field change of ΔB = 5 T, the maximum values of the magnetic entropy changes -ΔSmax in the region around TCinter are -ΔSmax = 3.00 J/kg K and 2.35 J/kg K for the bulk and ribbon samples respectively, while the relative cooling power (RCP) for the ribbon-spun sample, RCP = 135.9 J/kg, is significantly higher than the value of RCP = 116.6 J/kg for the bulk sample. These findings indicate that PrMn2Ge2 could be a promising candidate for magnetic refrigeration applications in the room temperature region.

Quantifying uncertainty in analysis of shockless dynamic compression experiments on platinum. I. Inverse Lagrangian analysis

Absolute measurements of solid-material compressibility by magnetically driven shockless dynamic compression experiments to multi-megabar pressures have the potential to greatly improve the accuracy and precision of pressure calibration standards for use in diamond anvil cell experiments. To this end, we apply characteristics-based inverse Lagrangian analysis (ILA) to 11 sets of ramp-compression data on pure platinum (Pt) metal and then reduce the resulting weighted-mean stress–strain curve to the principal isentrope and room-temperature isotherm using simple models for yield stress and Grüneisen parameter. We introduce several improvements to methods for ILA and quasi-isentrope reduction, the latter including calculation of corrections in wave speed instead of stress and pressure to render results largely independent of initial yield stress while enforcing thermodynamic consistency near zero pressure. More importantly, we quantify in detail the propagation of experimental uncertainty through ILA and model uncertainty through quasi-isentrope reduction, considering all potential sources of error except the electrode and window material models used in ILA. Compared to previous approaches, we find larger uncertainty in longitudinal stress. Monte Carlo analysis demonstrates that uncertainty in the yield-stress model constitutes by far the largest contribution to uncertainty in quasi-isentrope reduction corrections. We present a new room-temperature isotherm for Pt up to 444 GPa, with 1-sigma uncertainty at that pressure of just under ±1.2%; the latter is about a factor of three smaller than uncertainty previously reported for multi-megabar ramp-compression experiments on Pt. The result is well represented by a Vinet-form compression curve with (isothermal) bulk modulus K0=270.3±3.8 GPa, pressure derivative K0′=5.66±0.10, and correlation coefficient RK0,K0′=−0.843.

P-V-T equation of state of boron carbide.

We report the P-V-T equation of state measurements of B4C to 50 GPa and approximately 2500 K in laser-heated diamond anvil cells. We obtain an ambient temperature, third-order Birch-Murnaghan fit to the P-V data that yields a bulk modulus K0 of 221(2) GPa and derivative, (dK/dP)0 of 3.3(1). These were used in fits with both a Mie-Grüneisen-Debye model and a temperature-dependent, Birch-Murnaghan equation of state that includes thermal pressure estimated by thermal expansion (α) and a temperature-dependent bulk modulus (dK0/dT). The ambient pressure thermal expansion coefficient (α0 + α1T), Grüneisen γ(V) = γ0(V/V0)q and volume-dependent Debye temperature, were used as input parameters for these fits and found to be sufficient to describe the data in the whole P-T range of this study. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

Synthesis and Ultrahigh Pressure Compression of High-Entropy Boride (Hf0.2Mo0.2Nb0.2Ta0.2Zr0.2)B2 to 220 GPa.

The high-entropy boride (Hf0.2Mo0.2Nb0.2Ta0.2Zr0.2)B2 material was synthesized under high-pressures and high-temperatures in a large-volume Paris-Edinburgh (PE) press from a ball-milled powder mix of HfO2, MoO3, Nb2O5, Ta2O5, ZrO2, carbon black, and boron carbide. The transformation process was monitored in situ by energy-dispersive x-ray diffraction with conversion starting at 1100 °C and completed by 2000 °C with the formation of a single hexagonal AlB2-type phase. The synthesized sample was recovered, powdered, and mixed with platinum pressure marker and studied under high pressure by angle-dispersive x-ray diffraction in a diamond anvil cell. The hexagonal AlB2-type phase of (Hf0.2Mo0.2Nb0.2Ta0.2Zr0.2)B2 was found to be stable up to the highest pressure of 220 GPa reached in this study (volume compression V/V0 = 0.70). The third order Birch-Murnaghan equation of state fit to the high-pressure data up to 220 GPa results in an ambient pressure unit cell volume V0=28.16±0.04 Å3, bulk modulusKo = 407 ± 6 GPa, pressure derivative of bulk-modulus K0′ = 2.73 ± 0.045 GPa. Our study indicates that this high-entropy boride (Hf0.2Mo0.2Nb0.2Ta0.2Zr0.2)B2 material is stable to ultrahigh pressures and temperatures and exhibit high bulk modulus similar to other incompressible transition metal borides like ReB2 and Os2B3.

Revealing Phosphorus Nitrides up to the Megabar Regime: Synthesis of α'-P3 N5, δ-P3 N5 and PN2.

Non-metal nitrides are an exciting field of chemistry, featuring a significant number of compounds that can possess outstanding material properties. These properties mainly rely on maximizing the number of strong covalent bonds, with crosslinked XN6 octahedra frameworks being particularly attractive. In this study, the phosphorus-nitrogen system was studied up to 137 GPa in laser-heated diamond anvil cells, and three previously unobserved phases were synthesized and characterized by single-crystal X-ray diffraction, Raman spectroscopy measurements and density functional theory calculations. δ-P3 N5 and PN2 were found to form at 72 and 134 GPa, respectively, and both feature dense 3D networks of the so far elusive PN6 units. The two compounds are ultra-incompressible, having a bulk modulus of K0 =322 GPa for δ-P3 N5 and 339 GPa for PN2 . Upon decompression below 7 GPa, δ-P3 N5 undergoes a transformation into a novel α'-P3 N5 solid, stable at ambient conditions, that has a unique structure type based on PN4 tetrahedra. The formation of α'-P3 N5 underlines that a phase space otherwise inaccessible can be explored through materials formed under high pressure.

The nanomechanical properties of non-crosslinked calcium aluminosilicate hydrate: The influences of tetrahedral Al and curing age

Calcium aluminosilicate hydrate (C-A-S-H) is the binding phase of both blended cement-based and alkali-activated materials. The intrinsic mechanical properties of non-cross-linked C-A-S-H are important while experimentally unvalidated. Here, the properties are for the first time measured using high-pressure X-ray diffraction. The incompressibility and bulk modulus K0 of C-A-S-Hs are correlated to their nanostructure and stability using nuclear magnetic resonance and X-ray absorption spectroscopies. Al coordination in stable C-A-S-H (Al/Si = 0.1) cured for 546 days is purely tetrahedral (AlIV), while in metastable C-A-S-H (Al/Si = 0.05) cured for only 182 days is both AlIV and pentahedral (AlV). The stable C-A-S-H is stiffer along the a,b,c-axis with higher K0 relative to C-S-H. Short-curing-induced metastable C-A-S-H (Al/Si = 0.05) shows expanded interlayer and softer c-axis, thus lower K0 than C-S-H and the stable C-A-S-H. Our results highlight the stiffening effect of AlIV incorporation and the negative influences of insufficient curing on the nanomechanical properties of non-cross-linked C-A-S-H at Ca/Si = 1.

Ab initio simulations of α- and β-ammonium carbamate (NH4·NH2CO2), and the thermal expansivity of deuterated α-ammonium carbamate from 4.2 to 180 K by neutron powder diffraction.

Experimental and computational studies of ammonium carbamate have been carried out, with the objective of studying the elastic anisotropy of the framework manifested in (i) the thermal expansion and (ii) the compressibility; furthermore, the relative thermodynamic stability of the two known polymorphs has been evaluated computationally. Using high-resolution neutron powder diffraction data, the crystal structure of α-ammonium carbamate (ND4·ND2CO2) has been refined [space group Pbca, Z = 8, with a = 17.05189 (15), b = 6.43531 (7), c = 6.68093 (7) Å and V = 733.126 (9) Å3 at 4.2 K] and the thermal expansivity of α-ammonium carbamate has been measured over the temperature range 4.2-180 K. The expansivity shows a high degree of anisotropy, with the b axis most expandable. The ab initio computational studies were carried out on the α- and β-polymorphs of ammonium carbamate using density functional theory. Fitting equations of state to the P(V) points of the simulations (run athermally) gave the following values: V0 = 744 (2) Å3 and bulk modulus K0 = 16.5 (4) GPa for the α-polymorph, and V0 = 713.6 (5) Å3 and K0 = 24.4 (4) GPa for the β-polymorph. The simulations show good agreement with the thermoelastic behaviour of α-ammonium carbamate. Both phases show a high-degree of anisotropy; in particular, α-ammonium carbamate shows unusual compressive behaviour, being determined to have negative linear compressibility (NLC) along its a axis above 5 GPa. The thermodynamically stable phase at ambient pressure is the α-polymorph, with a calculated enthalpy difference with respect to the β-polymorph of 0.399 kJ mol-1; a transition to the β-polymorph could occur at ∼0.4 GPa.

Equations of state of iron and nickel to the pressure at the center of the Earth

Synchrotron radiation x-ray diffraction investigations of iron (Fe) and nickel (Ni) are conducted at pressures up to 354 and 368 GPa, respectively, and the equations of state (EOSs) at 298 K for the two elements are obtained for data extending to pressures as high as those at the center of the Earth, using the latest Pt-EOS pressure scale. From a least-squares fit to the Vinet equation using the observed pressure–volume data, the isothermal bulk modulus K0 and its pressure derivative K0′ are estimated to be 159.27(99) GPa and 5.86(4) for hcp-Fe, and 173.5(1.4) GPa and 5.55(5) for Ni. By comparing the present EOSs and extrapolated EOSs reported in the literature for Fe and Ni, the volumes of Fe and Ni at 365 GPa are found to be 2.3% and 1.5% larger than those estimated from extrapolated EOSs in previous studies, respectively. It is concluded that these discrepancies are due to the pressure scale. The present results suggest that the densities of Fe and Ni at a pressure of 365 GPa corresponding to the center of the Earth are 2.3% and 1.5%, respectively, lower than previously thought.

Experimental and Computational Studies of Compression and Deformation Behavior of Hafnium Diboride to 208 GPa.

The compression behavior of the hexagonal AlB2 phase of Hafnium Diboride (HfB2) was studied in a diamond anvil cell to a pressure of 208 GPa by axial X-ray diffraction employing platinum as an internal pressure standard. The deformation behavior of HfB2 was studied by radial X-ray diffraction technique to 50 GPa, which allows for measurement of maximum differential stress or compressive yield strength at high pressures. The hydrostatic compression curve deduced from radial X-ray diffraction measurements yielded an ambient-pressure volume V0 = 29.73 Å3/atom and a bulk modulus K0 = 282 GPa. Density functional theory calculations showed ambient-pressure volume V0 = 29.84 Å3/atom and bulk modulus K0 = 262 GPa, which are in good agreement with the hydrostatic experimental values. The measured compressive yield strength approaches 3% of the shear modulus at a pressure of 50 GPa. The theoretical strain-stress calculation shows a maximum shear stress τmax~39 GPa along the (1−10) [110] direction of the hexagonal lattice of HfB2, which thereby can be an incompressible high strength material for extreme-environment applications.

Natural cubic perovskite, Ca(Ti,Si,Cr)O3–δ, a versatile potential host for rock-forming and less-common elements up to Earth’s mantle pressure

Abstract Perovskite, CaTiO3, originally described as a cubic mineral, is known to have a distorted (orthorhombic) crystal structure. We herein report on the discovery of natural cubic perovskite. This was identified in gehlenite-bearing rocks occurring in a pyrometamorphic complex of the Hatrurim Formation (the Mottled Zone), in the vicinity of the Dead Sea, Negev Desert, Israel. The mineral is associated with native α-(Fe,Ni) metal, schreibersite (Fe3P), and Si-rich fluorapatite. The crystals of this perovskite reach 50 μm in size and contain many micrometer-sized inclusions of melilitic glass. The mineral contains significant amounts of Si substituting for Ti (up to 9.6 wt% SiO2), corresponding to 21 mol% of the davemaoite component (cubic perovskite-type CaSiO3), in addition to up to 6.6 wt% Cr2O3. Incorporation of trivalent elements results in the occurrence of oxygen vacancies in the crystal structure; this is the first example of natural oxygen-vacant ABO3 perovskite with the chemical formula Ca(Ti,Si,Cr)O3–δ (δ ~0.1). Stabilization of cubic symmetry (space group Pm3m) is achieved via the mechanism not reported so far for CaTiO3, namely displacement of an O atom from its ideal structural position (site splitting). The mineral is stable at atmospheric pressure to 1250 ± 50 °C; above this temperature, its crystals fuse with the embedded melilitic glass, yielding a mixture of titanite and anorthite upon melt solidification. The mineral is stable upon compression to at least 50 GPa. The a lattice parameter exhibits continuous contraction from 3.808(1) Å at atmospheric pressure to 3.551(6) Å at 50 GPa. The second-order truncation of the Birch-Murnaghan equation of state gives the initial volume V0 equal to 55.5(2) Å3 and room temperature isothermal bulk modulus K0 of 153(11) GPa. The discovery of oxygen-deficient single perovskite suggests previously unaccounted ways for incorporation of almost any element into the perovskite framework up to pressures corresponding to those of the Earth’s mantle.

Structural transition and ductility enhancement of a tungsten heavy alloy under high pressure

High pressure structural transformation of 95 W-3.5Ni-1.5Fe alloy was investigated by in-situ high pressure synchrotron radiate-on X-ray diffraction technique. Starting sample was compressed to 52.1 GPa in a diamond anvil cell under hydrostatic condition at ambient temperature. Experimental results demonstrated that the extreme compressive stress led to a set of reversible structural evolution. The grain size, atomic positions, bond distance and unit cell volume exhibited remarkable dependence on high pressure. The equation of state (EOS) together with bulk modulus K0 = 409(11) GPa and K0’ = 6.4(0.8) GPa of tungsten phase and K0 = 221(9) GPa and K0′=7.2(0.8) GPa of matrix phase were obtained based on the unit cell volume data varies with pressure for the first time. In addition, the structure responses of tungsten heavy alloy under high pressure and high temperature were revealed by large volume cubic press. Results indicated that the thermo-mechanical coupling effect facilitated the dynamic recrystallization process. Three-dimensional discrete dislocation plasticity simulations were proposed to illustrate the formation and evolution of screw dislocation in body-center cubic under various hydrostatic pressures. Results demonstrated that the extreme compression conditions fueled the nucleation and propagation of screw dislocations, which can be looked upon as the significant contributors to dislocation density increasing and ductility strengthening.

New Cage-Like Cerium Trihydride Stabilized at Ambient Conditions

New Cage-Like Cerium Trihydride Stabilized at Ambient Conditions

High-pressure high-temperature synthesis and thermal equation of state of high-entropy transition metal boride

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 + α2 T−2. The thermal stability combined with a high bulk-modulus establishes this high-entropy transition metal boride as an ultrahard high-temperature ceramic material.

Investigation of the itinerant metamagnetic system Hf0.75Ta0.25Fe2 under extreme conditions of pressure or magnetic field

The magnetic and structural properties of Hf0.75Ta0.25Fe2 have been investigated under the duplicate conditions, i.e., high magnetic fields and high pressures, by combining magnetization measurements up to 26 T, neutron powder diffraction experiments up to 14.5 T, and angle dispersive synchrotron x-ray diffraction up to 10 GPa. The intermetallic compound Hf0.75Ta0.25Fe2 exhibits an antiferromagnetic (AFM) ground state below the Néel temperature TN = 342 K. In the AFM structure, the Fe-6h magnetic moments (1.02 μΒ at 2 K) align along the c axis while Fe atoms at 2a sites are not magnetically ordered. We further show that the AFM ground state gets transformed into a ferromagnetic (FM) state via a field-induced metamagnetic phase transition. Magnetic-field dependent neutron diffractograms demonstrate that FM phase sets in with a change of the easy magnetization direction from axial to basal-plane and an extremely anisotropic lattice expansion. The unit cell expands drastically in the basal plane upon increasing the applied magnetic field, while its dimension along c-axis shrinks slightly; resulting in a huge positive volume magnetostriction ΔV/V = 0.78%. The application of hydrostatic pressure leads to the continuous decrease of the cell parameters. The shrinkage of the hexagonal cell is anisotropic with a larger compression along the six-fold symmetry axis c. A bulk modulus of K0 = 247 GPa has been deduced from the pressure-volume relationship.

Topaz, a Potential Volatile-Carrier in Cold Subduction Zone: Constraint from Synchrotron X-ray Diffraction and Raman Spectroscopy at High Temperature and High Pressure

The equation of state and stability of topaz at high-pressure/high-temperature conditions have been investigated by in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy in this study. No phase transition occurs on topaz over the experimental pressure–temperature (P-T) range. The pressure–volume data were fitted by the third-order Birch–Murnaghan equation of state (EoS) with the zero-pressure unit–cell volume V0 = 343.86 (9) Å3, the zero-pressure bulk modulus K0 = 172 (3) GPa, and its pressure derivative K’0 = 1.3 (4), while the obtained K0 = 155 (2) GPa when fixed K’0 = 4. In the pressure range of 0–24.4 GPa, the vibration modes of in-plane bending OH-groups for topaz show non-linear changes with the increase in pressure, while the other vibration modes show linear changes. Moreover, the temperature–volume data were fitted by Fei’s thermal equation with the thermal expansion coefficient α300 = 1.9 (1) × 10−5 K−1 at 300 K. Finally, the P-T stability of topaz was studied by a synchrotron-based single-crystal XRD at simultaneously high P-T conditions up to ~10.9 GPa and 700 K, which shows that topaz may maintain a metastable state at depths above 370 km in the upper mantle along the coldest subducting slab geotherm. Thus, topaz may be a potential volatile-carrier in the cold subduction zone. It can carry hydrogen and fluorine elements into the deep upper mantle and further affect the geochemical behavior of the upper mantle.

Equation of state of LiNi0·5Mn1·5O4 at high pressure

Investigations of equation of state (EOS) of LiNi0·5Mn1·5O4 under high pressure have been performed using synchrotron radiation X-ray diffraction (XRD) in a diamond anvil cell (DAC) at ambient temperature, density functional theory (DFT) calculations and Raman spectroscopy. It is found that the cubic structure maintains to the maximum pressure of 27.3 GPa by XRD and 21.0 GPa by Raman scattering. The XRD data yields a bulk modulus K0 = 146.2(7.5) GPa with K0' = 14.6(1.7). In addition, the high-pressure compression behavior of LiNi0·5Mn1·5O4 has been studied by first principles calculations. And the derived bulk modulus of LiNi0·5Mn1·5O4 is 131(1) GPa.

Thermoelasticity and stability of natural epidote at high pressure and high temperature: Implications for water transport during cold slab subduction

Epidote is a typical hydrous mineral in subduction zones. Here, we report a synchrotron-based single-crystal X-ray diffraction (XRD) study of natural epidote [Ca1.97Al2.15Fe0.84(SiO4)(Si2O7)O(OH)] under simultaneously high pressure-temperature (high P-T) conditions to ~17.7 ​GPa and 700 ​K. No phase transition occurs over this P-T range. Using the third-order Birch-Murnaghan equation of state (EoS), we fitted the pressure-volume-temperature (P-V-T) data and obtained the zero-pressure bulk modulus K0 ​= ​138(2) GPa, its pressure derivative K0' ​= ​3.0(3), the temperature derivative of the bulk modulus ((∂K/∂T)P ​= ​−0.004(1) GPa/K), and the thermal expansion coefficient at 300 ​K (α0 ​= ​3.8(5) ​× ​10−5 ​K−1), as the zero-pressure unit-cell volume V0 was fixed at 465.2(2) Å3 (obtained by a single-crystal XRD experiment at ambient conditions). This study reveals that the bulk moduli of epidote show nonlinear compositional dependence. By discussing the stabilization of epidote and comparing its density with those of other hydrous minerals, we find that epidote, as a significant water transporter in subduction zones, may maintain a metastable state to ~14 ​GPa along the coldest subducting slab geotherm and promote slab subduction into the upper mantle while favoring slab stagnation above the 410 ​km discontinuity. Furthermore, the water released from epidote near 410 ​km may potentially affect the properties of the 410 ​km seismic discontinuity.

Giant negative thermal expansion across the first-order magnetoelastic transition in Hf0.86Ta0.14Fe2

With the use of neutron powder diffraction, we have discovered and characterized an extremely anisotropic thermal expansion, including negative thermal expansion (NTE), in an itinerant-electron system Hf0.86Ta0.14Fe2. It is revealed that the intermetallic compound Hf0.86Ta0.14Fe2 exhibits temperature-induced magnetic phase transitions from ferromagnetic (FM) to antiferromagnetic (AFM) order and then to the paramagnetic (PM) state upon heating. The FM-AFM transformation proceeds in a stepwise fashion, as a first-order phase transition, and is accompanied by an isomorphic (without change of symmetry) lattice collapse. The unit cell shrinks abruptly in the basal plane only, while it’s dimension along the six-fold symmetry axis c changes continuously. The thermal evolution of the a lattice constant is found to drive the change from FM to AFM magnetic order. Hf0.86Ta0.14Fe2 shows a 0.41% spontaneous volume reduction across the FM-AFM first-order magnetic transition, where a giant NTE, with a crystallographic volume thermal expansion coefficient −164 × 10−6 K−1, is observed. We further show that the AFM state can be transformed into a FM state by few-tesla magnetic fields, which results in a large positive magnetostriction. A remarkably large adiabatic temperature change of ΔTad = 2.2 K is obtained for a magnetic field change of 3 T around the FM-AFM transition temperature. Using angle dispersive synchrotron x-ray diffraction, the evolution of the lattice was investigated at room temperature under high pressures up to 25 GPa. The application of external pressure leads to the monotonous decrease of the unit-cell parameters. The contraction of the hexagonal lattice is anisotropic with a larger compression along the high-symmetry direction c. A bulk modulus of K0 = 223 GPa has been determined from the pressure-volume relationship.

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.