Volume Collapse Research Articles

Pressure-induced polyamorphism by quantitative structure factor and pair distribution function analysis in two Ce-based metallic glasses

We utilized the pair distribution function method to characterize the pressure-induced polyamorphic transition in Ce60Al20Cu20 and Ce55Al45 metallic glass at room temperature. Using synchrotron high-energy x-ray diffraction we collected scattering information from a large Q-space coverage, which in turn gave a high resolution g(r) that provided accurate local structure information. We observed a sudden change in compressibility and the nearest neighbor distance at 3.50–6.32 GPa for Ce60Al20Cu20 and 2.20–6.89 GPa for Ce55Al45. The origin of the volume collapse seemed to be pressure-induced qualitative changes in bond shortening that corresponded to different coordination spheres. The polyamorphic transitions in these two systems from low-density glass (LDG) to high-density glass (HDG) are associated with local atomic rearrangements.

Polymorphism in Strontium Tungstate SrWO4 under Quasi-Hydrostatic Compression.

The structural and vibrational properties of SrWO4 have been studied experimentally up to 27 and 46 GPa, respectively, by angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy measurements as well as using ab initio calculations. The existence of four polymorphs upon quasi-hydrostatic compression is reported. The three phase transitions were found at 11.5, 19.0, and 39.5 GPa. The ambient-pressure SrWO4 tetragonal scheelite-type structure (S.G. I41/a) undergoes a transition to a monoclinic fergusonite-type structure (S.G. I2/a) at 11.5 GPa with a 1.5% volume decrease. Subsequently, at 19.0 GPa, another structural transformation takes place. Our calculations indicate two possible post-fergusonite phases, one monoclinic and the other orthorhombic. In the diffraction experiments, we observed the theoretically predicted monoclinic LaTaO4-type phase coexisting with the fergusonite-type phase up to 27 GPa. The coexistence of the two phases and the large volume collapse at the transition confirm a kinetic hindrance typical of first-order phase transitions. Significant changes in Raman spectra suggest a third pressure-induced transition at 39.5 GPa. The conclusions extracted from the experiments are complemented and supported by ab initio calculations. Our data provides insight into the structural mechanism of the first transition, with the formation of two additional W-O contacts. The fergusonite-type phase can be therefore considered as a structural bridge between the scheelite structure, composed of [WO4] tetrahedra, and the new higher pressure phases, which contain [WO6] octahedra. All the observed phases are compatible with the high-pressure structural systematics predicted for ABO4 compounds using crystal-chemistry arguments such as the diagram proposed by Bastide.

Mixing and residence time distribution in ultrasonic microreactors

Intensification of liquid mixing was investigated in domestic fabricated ultrasonic microreactors. Under the ultrasonic field, cavitation bubbles were generated, which undergo vigorous translational motion and surface oscillation with different modes (volume, shape oscillation, and transient collapse). These cavitation phenomena induce intensive convective mixing and reduce the mixing time from 24–32 s to 0.2–1.0 s. The mixing performance decreases with the channel size, due to the weaker cavitation activity in smaller channel. The energy efficiency is comparable to that of the conventional T‐type and higher than the Y‐type and Caterpillar microreactors. Residence time distribution was also measured by a stimulus‐response experiment and analyzed with axial dispersion model. Axial dispersion was significantly reduced by the ultrasound‐induced radial mixing, leading to the increasing of Bo number with ultrasound power. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1404–1418, 2017

Enhanced high-pressure superconductivity and local structure of theBa8Si46clathrate

The high-pressure superconducting properties and the local structure of the ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$ clathrate have been studied using electrical resistance and x-ray absorption spectroscopy measurements up to more than 20 GPa. At pressures above 10--13 GPa, corresponding to a well-known volume collapse phase transformation, a sudden increase in the critical temperature leads to a maximum value of the superconducting critical temperature ${T}_{c}$ of $\ensuremath{\sim}8.5--9$ K at 20 GPa. In the low-pressure clathrate phase, the superconducting critical temperature decreases as pressure is applied with an electron-phonon coupling constant $\ensuremath{\lambda}=1.1$ derived from the temperature evolution of the electrical resistance at different pressures. A progressive disorder in the Ba-Si correlations in the ${\mathrm{Ba}@\mathrm{Si}}_{24}$ cages of the structure is observed for pressures beyond $\ensuremath{\sim}5$ GPa. These observations exclude a structural homothecy at the volume collapse transition. The high-pressure collapsed phase of ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$ is then associated with local structural changes and shows enhanced superconducting properties.

Temperature and pressure investigation of Hf x Ta1–x C and Zr x Nb1–x C carbide alloys

Structural mechanical and thermal properties of refractory carbides have been investigated using the Realistic Interaction Potential Approach (RIPA) model. The study has been extended to mixed crystals of Hf x Ta1–x C (0 ≤ x ≤ 1) and Zr x Nb1–x C (0 ≤ x ≤ 1) alloys and the effect of composition on structural and elastic properties are investigated. Phase transition pressure and associated volume collapses (ΔV(P t )/V(0)) calculated from this approach are in good agreement with available literature for the parent compounds (x = 0 and x = 1). The results for the mixed crystal counter parts are also in fair agreement with experimental data generated from the application of Vegard’s law to data for the parent compounds.

Compression and phase diagram of lithium hydrides at elevated pressures and temperatures by first-principles calculation

High pressure and high temperature properties of AB (A = 6Li, 7Li; B = H, D, T) are comprehensively investigated with first-principles methods. It is found that H−sublattice features in the low-pressure electronic structure near the Fermi level of LiH are shifted to that dominated by the Li+ sublattice under compression. The lattice dynamics are studied in quasi-harmonic approximation, from which the phonon contribution to the free energy and the isotopic effects are accurately modelled with the aid of a parameterized double-Debye model. The equation of state (EOS) obtained matches perfectly with available static experimental data. The calculated principal Hugoniot is also in accordance with that derived from shock wave experiments. Using the calculated principal Hugoniot and the previous theoretical melting curve, we predict a shock melting point at 56 GPa and 1923 K. In order to establish the phase diagram for LiH, the phase boundaries between the B1 and B2 solid phases are explored. The B1–B2-liquid triple point is determined at about 241 GPa and 2413 K. The remarkable shift in the phase boundaries with isotopic effect and temperature reveal the significant role played by lattice vibrations. Furthermore, the Hugoniot of the static-dynamic coupling compression is assessed. Our EOS suggests that a precompression of the sample to 50 GPa will allow the shock Hugoniot to pass through the triple point and enter the B2 solid phase. This transition leads to a discontinuity with 4.6% volume collapse—about four times greater than the same B1–B2 transition at zero temperature.

Giant Pressure‐Driven Lattice Collapse Coupled with Intermetallic Bonding and Spin‐State Transition in Manganese Chalcogenides

AbstractMaterials with an abrupt volume collapse of more than 20 % during a pressure‐induced phase transition are rarely reported. In such an intriguing phenomenon, the lattice may be coupled with dramatic changes of orbital and/or the spin‐state of the transition metal. A combined in situ crystallography and electron spin‐state study to probe the mechanism of the pressure‐driven lattice collapse in MnS and MnSe is presented. Both materials exhibit a rocksalt‐to‐MnP phase transition under compression with ca. 22 % unit‐cell volume changes, which was found to be coupled with the Mn2+(d5) spin‐state transition from S=5/2 to S=1/2 and the formation of Mn−Mn intermetallic bonds as supported by the metallic transport behavior of their high‐pressure phases. Our results reveal the mutual relationship between pressure‐driven lattice collapse and the orbital/spin‐state of Mn2+ in manganese chalcogenides and also provide deeper insights toward the exploration of new metastable phases with exceptional functionalities.

Giant Pressure-Driven Lattice Collapse Coupled with Intermetallic Bonding and Spin-State Transition in Manganese Chalcogenides.

Materials with an abrupt volume collapse of more than 20 % during a pressure-induced phase transition are rarely reported. In such an intriguing phenomenon, the lattice may be coupled with dramatic changes of orbital and/or the spin-state of the transition metal. A combined in situ crystallography and electron spin-state study to probe the mechanism of the pressure-driven lattice collapse in MnS and MnSe is presented. Both materials exhibit a rocksalt-to-MnP phase transition under compression with ca. 22 % unit-cell volume changes, which was found to be coupled with the Mn(2+) (d(5) ) spin-state transition from S=5/2 to S=1/2 and the formation of Mn-Mn intermetallic bonds as supported by the metallic transport behavior of their high-pressure phases. Our results reveal the mutual relationship between pressure-driven lattice collapse and the orbital/spin-state of Mn(2+) in manganese chalcogenides and also provide deeper insights toward the exploration of new metastable phases with exceptional functionalities.

Superconductivity in a heavy fermion system: CeCu2Si2

Electron elementary excitations in certain inter-metallic rare-earth compounds are heavy electrons with physical characteristics completely different from that of metallic electrons. Using this knowledge, superconductivity has been studied exploiting heavy electron-phonon interaction brought about by Kondo volume collapse. Superconducting transition temperature, Tc, has been computed, in particular, for CeCu2Si2 under different values of observed Kondo temperature and Gruneisen parameter. It is found that one can explain the experimental Tc=0.6K, which is consistent with the description of heavy electron elementary excitations.

Magnetic correlations in a periodic Anderson model with nonuniform conduction electron coordination

The Periodic Anderson Model (PAM) is widely studied to understand strong correlation physics and especially the competition of antiferromagnetism and singlet formation. Quantum Monte Carlo (QMC) studies have focused both on issues such as the nature of screening and locating the quantum critical point (QCP) at zero temperature and also on possible experimental connections to phenomena ranging from the Cerium volume collapse to the relation of the magnetic susceptibility and Knight shift in heavy fermions. In this paper we extend QMC work to lattices in which the conduction electron sites can have variable coordination. This situation is relevant both to recently discovered magnetic quasicrystals and also to magnetism in doped heavy fermion systems.

Geomorphology and volcanology of Maat Mons, Venus

Full-resolution (FMIDR) Magellan radar backscatter images have been used to characterize the geology and volcanology of the volcano Maat Mons on Venus. This volcano has often been identified by remote sensing techniques as one of the volcanoes on the planet that could have been recently active, and is the highest volcano on Venus with a relief of ∼9km. The summit of Maat Mons is characterized by a caldera complex ∼26×30km in diameter with at least six remnant pit craters ∼10km in diameter preserved in the walls of the caldera, suggesting that multiple small volume (<16km3) collapse events formed the caldera. Four lava flow types, described as “digitate flows”, “sheet flows”, “fan flows” and “filamentary flows”, can be identified on the flanks. Three rift zones can be identified from the distribution of 217 pit craters >1km in diameter on the flanks. These pits appear to have formed by collapse with no effusive activity associated with their formation. No evidence for explosive volcanism can be identified, despite the (relatively) low atmospheric pressure (∼55bar) near the summit. There is also a lack of evidence for lava channels, deformation features within the caldera, and thrust faults on the flanks, indicating that the physical volcanology of Maat Mons is simpler than that of typical martian and terrestrial shield volcanoes. Preservation of fine-scale (3–4 pixels) structures within the pit craters and summit pits is consistent with geologically very recent activity, but no evidence for current activity can be identified.

Thermoelasticity of Fe3+‐ and Al‐bearing bridgmanite: Effects of iron spin crossover

AbstractWe report ab initio (LDA + Usc) calculations of thermoelastic properties of ferric iron (Fe3+)‐ and aluminum (Al)‐bearing bridgmanite (MgSiO3 perovskite), the main Earth forming phase, at relevant pressure and temperature conditions and compositions. Three coupled substitutions, namely, [Al]Mg‐[Al]Si, [Fe3+]Mg‐[Fe3+]Si, and [Fe3+]Mg‐[Al]Si have been investigated. Aggregate elastic moduli and sound velocities are successfully compared with limited experimental data available. In the case of [Fe3+]Mg‐[Fe3+]Si substitution, the high‐spin (S = 5/2) to low‐spin (S = 1/2) crossover in [Fe3+]Si induces a volume collapse and elastic anomalies across the transition region. However, the associated anomalies should disappear in the presence of aluminum in the most favorable substitution, i.e., [Fe3+]Mg‐[Al]Si. Calculated elastic properties along a lower mantle model geotherm suggest that the elastic behavior of bridgmanite with simultaneous substitution of Fe2O3 and Al2O3 in equal proportions or with Al2O3 in excess should be similar to that of (Mg,Fe2+)SiO3 bridgmanite. However, excess of Fe2O3 should produce elastic anomalies in the crossover pressure region.

Pressure dependence of the structure and electronic properties ofSr3Ir2O7

We study the structural evolution of Sr$_3$Ir$_2$O$_7$ as a function of pressure using x-ray diffraction. At a pressure of 54 GPa at room temperature, we observe a first-order structural phase transition, associated with a change from tetragonal to monoclinic symmetry, and accompanied by a 4% volume collapse. Rietveld refinement of the high-pressure phase reveals a novel modification of the Ruddlesden-Popper structure, which adopts an altered stacking sequence of the perovskite bilayers. As the positions of the oxygen atoms could not be reliably refined from the data, we use density functional theory (local-density approximation+$U$+spin orbit) to optimize the crystal structure, and to elucidate the electronic and magnetic properties of Sr$_3$Ir$_2$O$_7$ at high pressure. In the low-pressure tetragonal phase, we find that the in-plane rotation of the IrO$_6$ octahedra increases with pressure. The calculations further indicate that a bandwidth-driven insulator-metal transition occurs at $\sim$20 GPa, along with a quenching of the magnetic moment. In the high-pressure monoclinic phase, structural optimization resulted in complex tilting and rotation of the oxygen octahedra, and strongly overlapping $t_{2g}$ and $e_g$ bands. The $t_{2g}$ bandwidth renders both the spin-orbit coupling and electronic correlations ineffectual in opening an electronic gap, resulting in a robust metallic state for the high-pressure phase of Sr$_3$Ir$_2$O$_7$.

Ice Ih anomalies: Thermal contraction, anomalous volume isotope effect, and pressure-induced amorphization.

Ice Ih displays several anomalous thermodynamic properties such as thermal contraction at low temperatures, an anomalous volume isotope effect (VIE) rendering the volume of D2O ice greater than that of H2O ice, and a pressure-induced transition to the high-density amorphous (HDA) phase. Furthermore, the anomalous VIE increases with temperature, despite its quantum-mechanical origin. Here, embedded-fragment ab initio second-order many-body perturbation (MP2) theory in the quasiharmonic approximation (QHA) is applied to the Gibbs energy of an infinite, proton-disordered crystal of ice Ih at wide ranges of temperatures and pressures. The quantum effect of nuclei moving in anharmonic potentials is taken into account from first principles without any empirical or nonsystematic approximation to either the electronic or vibrational Hamiltonian. MP2 predicts quantitatively correctly the thermal contraction at low temperatures, which is confirmed to originate from the volume-contracting hydrogen-bond bending modes (acoustic phonons). It qualitatively reproduces (but underestimates) the thermal expansion at higher temperatures, caused by the volume-expanding hydrogen-bond stretching (and to a lesser extent librational) modes. The anomalous VIE is found to be the result of subtle cancellations among closely competing isotope effects on volume from all modes. Consequently, even ab initio MP2 with the aug-cc-pVDZ and aug-cc-pVTZ basis sets has difficulty reproducing this anomaly, yielding qualitatively varied predictions of the sign of the VIE depending on such computational details as the choice of the embedding field. However, the temperature growth of the anomalous VIE is reproduced robustly and is ascribed to the librational modes. These solid-state MP2 calculations, as well as MP2 Born-Oppenheimer molecular dynamics, find a volume collapse and a loss of symmetry and long-range order in ice Ih upon pressure loading of 2.35 GPa or higher. Concomitantly, rapid softening of acoustic phonons is observed starting around 2 GPa. They constitute a computational detection of a mechanical instability in ice Ih and the resulting pressure-induced amorphization to HDA.

Pressure-induced structural transformation of CaC2

The high pressure structural changes of calcium carbide CaC2 have been investigated with Raman spectroscopy and synchrotron X-ray diffraction (XRD) techniques in a diamond anvil cell at room temperature. At ambient conditions, two forms of CaC2 co-exist. Above 4.9 GPa, monoclinic CaC2-ii diminished indicating the structural phase transition from CaC2-ii to CaC2-i. At about 7.0 GPa, both XRD patterns and Raman spectra confirmed that CaC2-i transforms into a metallic Cmcm structure which contains polymeric carbon chains. Along with the phase transition, the isolated C2 dumbbells are polymerized into zigzag chains resulting in a large volume collapse with 22.4%. Above 30.0 GPa, the XRD patterns of CaC2 become featureless and remain featureless upon decompression, suggesting an irreversible amorphization of CaC2.

High-pressure and temperature-induced structural, elastic, and thermodynamical properties of strontium chalcogenides

Pressure- and temperature-dependent mechanical, elastic, and thermodynamical properties of rock salt to CsCl structures in semiconducting SrX (X = O, S, Se, and Te) chalcogenides are presented based on model interatomic interaction potential with emphasis on charge transfer interactions, covalency effect, and zero point energy effects apart from long-range Coulomb, short-range overlap repulsion extended and van der Waals interactions. The developed potential with non-central forces validates the Cauchy discrepancy among elastic constants. The volume collapse (V P/V 0) in terms of compressions in SrX at higher pressure indicates the mechanical stiffening of lattice. The expansion of SrX lattice is inferred from steep increase in V T/V 0 and is attributed to thermal softening of SrX lattice. We also present the results for the temperature-dependent behaviors of hardness, heat capacity, and thermal expansion coefficient. From the Pugh’s ratio (ϕ = B T /G H), the Poisson’s ratio (ν) and the Cauchy’s pressure (C 12–C 44), we classify SrO as ductile but SrS, SrSe, and SrTe are brittle material. To our knowledge these are the first quantitative theoretical prediction of the pressure and temperature dependence of mechanical stiffening, thermally softening, and brittle nature of SrX (X = O, S, Se, and Te) and still await experimental confirmations.

A detailed first principle study on the structural, elastic, and electronic properties of indium arsenide (InAs) under induced pressure

An ab initio calculation of the structural, elastic, and electronic properties of indium arsenide (InAs) under induced pressure is investigated using density functional theory with modified Becke–Johnson potential within the generalised gradient approximation of the Perdew–Burke–Ernzerhof scheme. The lattice parameters are found to be in good agreement with experimental and other theoretical data. The pressure-induced structural phase transition of InAs zinc blende to rock salt structure is found to occur at 4.7 GPa pressure with a 17.2% of volume collapse. The elastic properties of both the zinc blende and rock salt structures at different pressures are studied. The electronic band structures at different pressures for both the structures are investigated using the total and partial density of states. The energy band gap of the InAs zinc blende phase is increased with increasing pressure while in rock salt the phase the conduction band crosses towards the valence band and thus shows metallic behaviour.

Electronic and structural properties of rare earth pnictides

Abstract In the present paper structural and electronic properties of rare earth pnictides have been presented. The present calculation has been performed using self-consistent tight binding linear muffin tin orbital (TB-LMTO) method within the local density approximation (LDA). The studied compounds undergo a structural phase transition from NaCl-type structure to CsCl-type structure. The electronic band structure and density of states of the pnictides have been reported. The equilibrium lattice parameter a (Å), bulk modulus B (GPa), number of f-states at the Fermi level Nf (states/Ry cell) and volume collapse of AmBi and CmBi have also been reported. The calculated equilibrium structural parameters are in good agreement with the available experimental results.

High pressure structural, electronic, and optical properties of polymorphic InVO4 phases

In the present work, we report a detailed density functional theory calculation on polymorphic InVO4 phases by means of projector augmented wave method. The computed first-order structural phase transformation from orthorhombic (Cmcm) to monoclinic (P2/c) structure is found to occur around 5.6 GPa along with a large volume collapse of 16.6%, which is consistent with previously reported experimental data. This transformation also leads to an increase in the coordination number of vanadium atom from 4 to 6. The computed equilibrium and high pressure structural properties of both InVO4 phases, including unit cell parameters, equation of state, and bulk moduli, are in good agreement with the available experimental data. In addition, compressibility is found to be highly anisotropic and the b-axis being more compressible than the other for both the structures. Electronic band structures for both the phases were calculated, and the band gaps for orthorhombic and monoclinic InVO4 are found to be 4.02 and 1.67 eV, respectively, within the Tran-Blaha Modified Becke-Johnson potential as implemented in linearized augmented planewave method. We further examined the optical properties such as dielectric function, refractive index, and absorption spectra for both the structures. From the implications of these results, it can be proposed that the high pressure InVO4 phase can be more useful than orthorhombic phase for photo catalytic applications.

Comparison of the high-pressure behavior of the cerium oxidesCe2O3andCeO2

The high-pressure behavior of $\mathrm{C}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$ was studied using angle-dispersive x-ray diffraction to 70 GPa and compared with that of $\mathrm{Ce}{\mathrm{O}}_{2}$. Up to the highest pressure $\mathrm{C}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$ remains in the hexagonal phase (space group 164, $P\overline{3}2/m1)$ typical for the lanthanide sesquioxides. A theoretically predicted phase instability for 30 GPa is not observed. The isothermal bulk modulus and its pressure derivative for the quasihydrostatic case are ${B}_{0}=111\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.28em}{0ex}}\mathrm{GPa},{B}_{0}^{\ensuremath{'}}=4.7\ifmmode\pm\else\textpm\fi{}0.3,$ and for the case without pressure-transmitting medium ${B}_{0}=104\ifmmode\pm\else\textpm\fi{}4\phantom{\rule{0.28em}{0ex}}\mathrm{GPa},{B}_{0}^{\ensuremath{'}}=6.5\ifmmode\pm\else\textpm\fi{}0.4$. Starting from ambient-pressure magnetic susceptibility measurements for both oxides in highly purified form, we find that the Ce atom in $\mathrm{C}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$ behaves like a trivalent $\mathrm{C}{\mathrm{e}}^{3+}$ ion ($2.57{\ensuremath{\mu}}_{\mathrm{B}}$ per Ce atom) in contrast to previously published data. Since x-ray emission spectroscopy of the $L\ensuremath{\gamma}$ $(4{d}_{3/2}\ensuremath{\rightarrow}2{p}_{1/2})$ transition is sensitive to the $4f$ -electron occupancy, we also followed the high-pressure dependence of this line for both oxides up to 50 GPa. No change of the respective line shape was observed, indicating that the $4f$-electron configuration is stable for both materials. We posit from this data that the $4f$ electrons do not drive the volume collapse of $\mathrm{Ce}{\mathrm{O}}_{2}$ from the high-symmetry, low-pressure fluorite structure to the lower-symmetry orthorhombic phase.