Anomalous Pressure Dependence Research Articles

Explaining an anomalous pressure dependence of shear modulus in germanate glasses based on Reverse Monte Carlo modelling

Unlike traditional silicate glasses, germanate glasses often feature non-monotonic variations in material properties (e.g., elastic moduli and glass transition temperature) with varying chemical composition, temperature, and pressure. However, the underlying atomic-scale structural origins remain poorly understood. This is because, in most oxide glasses, the structural changes are quantified through solid-state NMR spectroscopy, but unfortunately the only NMR active germanium isotope (73Ge) has very unfavorable NMR properties. Here, we circumvent this problem by using high-energy X-ray and neutron total scattering coupled with ab initio molecular dynamics simulations as input for Reverse Monte Carlo modeling. In detail, we study the structure and properties of two sodium germanate glasses (10Na2O-90GeO2 and 20Na2O-80GeO2) subjected to permanent densification through hot compression up to 2 GPa at the glass transition temperature. While density as well as Young's and bulk modulus increase with pressure as expected, shear modulus first increases and then decreases slightly at higher pressures. The refined atomistic structure models suggest that the glasses feature a distribution of 4, 5, and 6 coordinated Ge with a majority of 4 and 5 coordinated species. Only minor changes in the Ge–O coordination occur upon hot compression, but a notable transformation of edge- to corner-sharing Ge-polyhedra is found. This anomalous polyhedral packing causes a lower number of angular constraints upon higher pressure treatment, explaining the non-monotonic trend of shear modulus with pressure. We also find that the rings become smaller and less circular upon compression, contributing to the volumetric compaction. These findings may aid the future design of germanate glasses with tailored properties and the general understanding of structure-property relations in oxide glasses.

Correction: Anomalous pressure dependence of phonon line widths in metallic pyrochlore iridates (Eu$$_{1-x}$$Bi$$_x$$)$$_2$$Ir$$_2$$O$$_7$$: crossover from incoherent to coherent metal

Correction: Anomalous pressure dependence of phonon line widths in metallic pyrochlore iridates (Eu$$_{1-x}$$Bi$$_x$$)$$_2$$Ir$$_2$$O$$_7$$: crossover from incoherent to coherent metal

Anomalous pressure dependence of phonon line widths in metallic pyrochlore iridates (Eu$$_{1-x}$$Bi$$_x$$)$$_2$$Ir$$_2$$O$$_7$$: crossover from incoherent to coherent metal

Anomalous pressure dependence of phonon line widths in metallic pyrochlore iridates (Eu$$_{1-x}$$Bi$$_x$$)$$_2$$Ir$$_2$$O$$_7$$: crossover from incoherent to coherent metal

High Pressure-Driven Magnetic Disorder and Structural Transformation in Fe3 GeTe2 : Emergence of a Magnetic Quantum Critical Point.

Among the recently discovered 2D intrinsic van der Waals (vdW) magnets, Fe3 GeTe2 (FGT) has emerged as a strong candidate for spintronics applications, due to its high Curie temperature (130 - 220 K) and magnetic tunability in response to external stimuli (electrical field, light, strain). Theory predicts that the magnetism of FGT can be significantly modulated by an external strain. However, experimental evidence is needed to validate this prediction and understand the underlying mechanism of strain-mediated vdW magnetism in this system. Here, the effects of pressure (0 - 20GPa) are elucidated on the magnetic and structural properties of Fe3 GeTe2 by means of synchrotron Mössbauer source spectroscopy, X-ray powder diffraction and Raman spectroscopy over a wide temperature range of 10 - 290 K. A strong suppression of ferromagnetic ordering is observed with increasing pressure, and a paramagnetic ground state emerges when pressure exceeds a critical value, PPM ≈ 15GPa. The anomalous pressure dependence of structural parameters and vibrational modes is observed at PC ≈ 7GPa and attributed to an isostructural phase transformation. Density functional theory calculations complement these experimental findings. This study highlights pressure as a driving force for magnetic quantum criticality in layered vdW magnetic systems.

Anomalous thermal transport under high pressure in boron arsenide.

High pressure represents extreme environments and provides opportunities for materials discovery1-8. Thermal transport under high hydrostatic pressure has been investigated for more than 100 years and all measurements of crystals so far have indicated a monotonically increasing lattice thermal conductivity. Here we report in situ thermal transport measurements in the newly discovered semiconductor crystal boron arsenide, and observe an anomalous pressure dependence of the thermal conductivity. We use ultrafast optics, Raman spectroscopy and inelastic X-ray scattering measurements to examine the phonon bandstructure evolution of the optical and acoustic branches, as well as thermal conductivity under varied temperatures and pressures up to 32 gigapascals. Using atomistic theory, we attribute the anomalous high-pressure behaviour to competitive heat conduction channels from interactive high-order anharmonicity physics inherent to the unique phonon bandstructure. Our study verifies ab initio theory calculations and we show that the phonon dynamics-resulting from competing three-phonon and four-phonon scattering processes-are beyond those expected from classical models and seen in common materials. This work uses high-pressure spectroscopy combined with atomistic theory as a powerful approach to probe complex phonon physics and provide fundamental insights for understanding microscopic energy transport in materials of extreme properties.

Anomalous elasticity of talc at high pressures: Implications for subduction systems

Talc is a layered hydrous silicate mineral that plays a vital role in transporting water into Earth’s interior and is crucial for explaining geophysical observations in subduction zone settings. In this study, we explored the structure, equation of state, and elasticity of both triclinic and monoclinic talc under high pressures up to 18 GPa using first principles simulations based on density functional theory corrected for dispersive forces. Our results indicate that principal components of the full elastic constant tensor C11 and C22, shear components C66, and several off-diagonal components show anomalous pressure dependence. This non-monotonic pressure dependence of elastic constant components is likely related to the structural changes and is often manifested in a polytypic transition from a low-pressure polytype talc-I to a high-pressure polytype talc-II. The polytypic transition of talc occurs at pressures within its thermodynamic stability. However, the bulk and shear elastic moduli show no anomalous softening. Our study also shows that talc has low velocity, extremely high anisotropy, and anomalously high VP/VS ratio, thus making it a potential candidate mineral phase that could readily explain unusually high VP/VS ratio and large shear wave splitting delays as observed from seismological studies in many subduction systems.

Carbon bearing aluminosilicate melt at high pressure

It is well known that volatiles such as water has significant influence on the properties of silicate melts. Carbon dioxide (CO2) is also an abundant volatile in deep Earth, however the effect of CO2 on the properties of polymerized melts, particularly the transport properties, are poorly understood. This is crucial for better understanding of the generation and migration of carbon bearing silicate magma in deep crustal and mantle settings. In this study, we explore the structure and properties of carbon bearing aluminosilicate melt up to a pressure of ~25 GPa and temperature range of 2500–4000 K using first principles molecular dynamics (FPMD) simulation. Our results show that CO2 in the aluminosilicate melts dissolves as molecular CO2 and carbonate (CO32-) at lower pressures (~0–3 GPa). However, at higher pressures (>3 GPa) relevant to most of the upper mantle, CO32- is predominant carbon species along with carbon in 4-fold coordination (CO4). Fraction of CO32- increases with decreasing temperature and increasing pressure. We find that at the reference isotherm (2500 K), the density of the aluminosilicate melt is reduced by addition of CO2 (in wt.%) with dρdXCO2 = −0.0214. Effect of water on the density of melt is more pronounced with dρdXH2O = −0.0422. Thus, the gravity-driven buoyancy of volatile rich magma will be greater than that of the magma without volatile components. The compressibility of the aluminosilicate melt is also affected by volatiles. For instance, both bulk modulus (KT0) and its pressure derivative (KT0') for volatile bearing melts are respectively lower and higher than that of dry melt. Aluminosilicate melts are highly polymerized and show an anomalous pressure dependence of melt viscosity. Pressure dependence of viscosity at low pressure regime (P < 5 GPa) is significant with dlog(η)dP=-2.06. Melt viscosity is further reduced by the addition of volatiles. Effect of volatiles on viscosity is more pronounced at low pressures and it minimizes at pressures where the melt viscosity exhibits minima. Due to the lower viscosity and larger buoyancy for both the H2O and CO2 bearing aluminosilicate melts at crustal and upper mantle depths, magma mobility (Δρη) is greater for volatile bearing melts. Thus, the residence time of such melts are expected to be shorter than volatile free melts in those settings. Like the mobility, electrical conductivity of aluminosilicate melts also increases when volatile is present in the silicate melt. We find that that as low as <1 vol.% volatile bearing aluminosilicate melt mixed with mantle matrix could be sufficient to explain the observed electrical conductivity anomalies in the upper mantle.

Anomalous pressure-dependence in surface-modified silicon-derived nanoparticles

Surfaces can significantly alter the optical properties of nanomaterials, but they are difficult to control and their roles are hard to understand in highly reactive materials such as silicon nanomaterials. In this work, we investigate the role of the surface in controlling the optical transitions in highly luminescent silicon-derived nanoparticles. By combining high-pressure and low-temperature experiments, we experimentally correlate the anomalously intense and narrow transitions in the UV range with the surface oxides, while the visible transition and the photoluminescence (PL) are verified to originate from the Si-ligand charge transfer band. We find that the high-pressure absorption and PL depends on the rigidity of the surface ligand. This indicates that the surface plays a dominant role on the optical properties of these silicon-derived nanoparticles, and is different than other semiconductor nanomaterials, in which pressure-dependent optical transitions depend on lattice strain or phase transformations. This work presents a comprehensive understanding of the optical transitions and the effect of surface ligands and surface oxidation in these highly luminescent Si-derived nanoparticles. The new insight into the oxidation-activated and ligand-mediated transitions, and the pressure-dependent PL may help with engineering the band structure of other highly-reactive optical nanomaterials.

A combined synchrotron diffraction and first-principles investigation on structural properties of Co(OH)2under pressure up to 7 GPa

An H–H repulsion induced structural anomaly in Co(OH)2 at pressure around 2.5 GPa is reported using systematic synchrotron X-ray powder diffraction and density functional theory (DFT) based first-principles calculations. The diffraction pattern evidences anomalous pressure dependence of Bragg peaks (003) and (013) compared to others indicating a much stronger change in the hydrogen sublattice with pressure. Analysis of the lattice parameters reveals a subtle but clear change in the trend of the out-of-plane to in-plane lattice constants' ratio above 2.5 GPa. Two independent pressure runs using two different pressure transmitting media yielded similar results, thus proving the observations to be independent of the minor variations in the quasi-hydrostatic conditions during the pressure run. DFT calculations were performed on an extended unit cell of Co(OH)2 which enables the description of hydrogen in either ordered or disordered state. Calculations showed that only the hydrogen disordered structure is energetically favourable at high pressures. We attribute the anomalous behaviour of the (003) and (013) Bragg peaks above 2.5 GPa to the charge delocalization along hydrogen sublattice coming due to the hydrogen ordered-disordered transition, in agreement with DFT results showing the H disordered state to be energetically more favourable at higher pressure. A second-order Brich-Murnaghan equation of state (EoS) can well describe the low- and high-pressure regime separately with a 20% higher bulk modulus of the latter, further supporting the subtle transition around 2.5 GPa. Present results also complement the previous Raman spectroscopic study on Co(OH)2 which reported anomalous behaviour in the (2) mode around 2.5 GPa.

Features of the Electrical-Conductivity Mechanism in γ-Irradiated TlInSe2 Single Crystals under Hydrostatic Pressure

The effect of hydrostatic pressure to 10 kbar on the conductivity of γ-irradiated chain TlInSe2 single crystals with a resistivity of ~108 Ω cm is studied. It is found that the observed conductivity anomalies of TlInSe2 samples irradiated at doses of D 100 krad are associated with the ordered transformation of defects including interstitial cations and anions. The nature of the anomalous change and pressure dependences of the resistivity σ(P) in the irradiated samples suggest that pressure causes local energy levels due to macroscopic clusters of radiation-induced defects, with the result that the Fermi-level energy and carrier concentration change.

Understanding how water models affect the anomalous pressure dependence of their diffusion coefficients

The self-diffusion coefficient of water shows an anomalous increase with increasing hydrostatic pressure up to a broad maximum (PmD) near 1 kbar at 298 K, which has been attributed to pressure effects on the tetrahedral hydrogen bond network of water. Moreover, the ability of a water model to reproduce anomalous properties of water is a signature that it is reproducing the network. Here, water was simulated between 1 bar and 5 kbar using three water models, two four-site (with all charges in the molecular plane) and one single-site multipole (which accounts for out-of-molecular plane charge), that have reasonable pressure-temperature properties. For these three models, the diffusion coefficients display a maximum in the pressure dependence and the radial distribution functions show good agreement with the limited experimental structural data at high pressure that are available. In addition, a variety of properties associated with the network are examined, including hydrogen bond lifetimes and occupancies, three-body angle distributions, and tetrahedral order parameters. Results suggest that the initial increasing diffusion with pressure is because hydrogen bonds are distorted and thus weakened by pressure, but above PmD, the hydrogen bonds are weakened to the point it behaves more like a normal liquid. In other words, the PmD may be a measure of the angular strength of hydrogen bonds. In addition, since the four-site models over-predict the values of PmD while the multipole model under-predicts it, out-of-plane charge may improve four-site models.

Anomalous pressure dependence of the electronic transport and anisotropy in SrIrO3 films

Iridate oxides display exotic physical properties that arise from the interplay between a large spin–orbit coupling and electron correlations. Here, we present a comprehensive study of the effects of hydrostatic pressure on the electronic transport properties of SrIrO3 (SIO), a system that has recently attracted a lot of attention as potential correlated Dirac semimetal. Our investigations on untwinned thin films of SIO reveal that the electrical resistivity of this material is intrinsically anisotropic and controlled by the orthorhombic distortion of the perovskite unit cell. These effects provide another evidence for the strong coupling between the electronic and lattice degrees of freedom in this class of compounds. Upon increasing pressure, a systematic increase of the transport anisotropies is observed. The anomalous pressure-induced changes of the resistivity cannot be accounted for by the pressure dependence of the density of the electron charge carriers, as inferred from Hall effect measurements. Moreover, pressure-induced rotations of the IrO6 octahedra likely occur within the distorted perovskite unit cell and affect electron mobility of this system.

Anomalous pressure dependence of the electronic properties of molecular crystals explained by changes in intermolecular electronic coupling

Optimization of the electronic properties of crystalline organic semiconductors is important for the evolution of organic optoelectronic devices. One method of tuning the electronic properties is by applying external pressure to semiconducting molecular crystals. A recent dispersion-inclusive density functional theory (DFT) investigation regarding the effect of pressure up to 20 GPa on herringbone polycyclic aromatic hydrocarbons (PAHs) has revealed anomalous pressure dependence of the electronic properties of six systems [J. Phys. Chem. C 122, 23828 (2018)]. Here, we use intermolecular electronic coupling values (Hab), calculated by fragment orbital DFT (FO-DFT) to elucidate the pressure dependence of the electronic properties. We show that discontinuities in the pressure dependence of the band structures are correlated with pressure-dependent discontinuities in the electronic couplings of certain molecular dimers. Hab is shown to be a useful descriptor for gaining insight into the pressure dependence of the electronic properties of crystalline organic semiconductors.

Interplay between the Coulomb Interaction and Hybridization in Ca and Anomalous Pressure Dependence of the Resistivity

Increasing external pressure gives rise to s–d electron transfer in calcium that results in the localization of the charge density in the interstices of the crystal structure, i.e., the formation of an electride. The corresponding electronic states are partially filled and localized and, hence, electronic correlations could arise. We have carried out theoretical calculations for the high-pressure phases of Ca taking into account the Coulomb interactions between the electronic states centered on the interstitial site. The results of our calculations and proposed microscopic model showed that the structural phase transition under high pressure is due to an interplay of hybridization and correlation effects. Furthermore, it was found that the Coulomb repulsion can explain the experimentally observed anomalous increase in resistivity of the simple cubic phase of calcium under pressure.

Anisotropic lattice compression of α- and β-CePdZn

The lattice constants of ZrNiAl type α-CePdZn and TiNiSi type β-CePdZn were measured at high pressure up to 14GPa at room temperature using X-ray diffraction (XRD) and a diamond anvil cell. The pressure dependence of lattice constants and volume of α-CePdZn were found to be smooth without any discontinuity, and having a bulk modulus, B0, and its pressure derivative, B0′, of 67GPa and 5.1, respectively. On the other hand, the a and b axes as well as volume of β-CePdZn were found to show anomalous pressure dependence at around 8GPa. B0 and B0′ of β-CePdZn were 90GPa and 2.1, respectively. These results suggest that a crossover in the electronic states is induced by applying pressure to β-CePdZn. The origins of these anomalous behaviors are discussed in connection with crossover and change in the topology of Fermi surface.

Molecular dynamics simulations of propane in slit shaped silica nano-pores: direct comparison with quasielastic neutron scattering experiments.

Molecular motion under confinement has important implications for a variety of applications including gas recovery and catalysis. Propane confined in mesoporous silica aerogel as studied using quasielastic neutron scattering (QENS) showed anomalous pressure dependence in its diffusion coefficient (J. Phys. Chem. C, 2015, 119, 18188). Molecular dynamics (MD) simulations are often employed to complement the information obtained from QENS experiments. Here, we report an MD simulation study to probe the anomalous pressure dependence of propane diffusion in silica aerogel. Comparison is attempted based on the self-diffusion coefficients and on the time scales of the decay of the simulated intermediate scattering functions. While the self-diffusion coefficients obtained from the simulated mean squared displacement profiles do not exhibit the anomalous pressure dependence observed in the experiments, the time scales of the decay of the intermediate scattering functions calculated from the simulation data match the corresponding quantities obtained in the QENS experiment and thus confirm the anomalous pressure dependence of the diffusion coefficient. The origin of the anomaly in pressure dependence lies in the presence of an adsorbed layer of propane molecules that seems to dominate the confined propane dynamics at low pressure, thereby lowering the diffusion coefficient. Further, time scales for rotational motion obtained from the simulations explain the absence of rotational contribution to the QENS spectra in the experiments. In particular, the rotational motion of the simulated propane molecules is found to exhibit large angular jumps at lower pressure. The present MD simulation work thus reveals important new insights into the origin of anomalous pressure dependence of propane diffusivity in silica mesopores and supplements the information obtained experimentally by QENS data.

Pressure induced structural, electronic topological, and semiconductor to metal transition in AgBiSe2

We report the effect of strong spin orbit coupling inducing electronic topological and semiconductor to metal transitions on the thermoelectric material AgBiSe2 at high pressures. The synchrotron X-ray diffraction and the Raman scattering measurement provide evidence for a pressure induced structural transition from hexagonal (α-AgBiSe2) to rhombohedral (β-AgBiSe2) at a relatively very low pressure of around 0.7 GPa. The sudden drop in the electrical resistivity and clear anomalous changes in the Raman line width of the A1g and Eg(1) modes around 2.8 GPa was observed suggesting a pressure induced electronic topological transition. On further increasing the pressure, anomalous pressure dependence of phonon (A1g and Eg(1)) frequencies and line widths along with the observed temperature dependent electrical resistivity show a pressure induced semiconductor to metal transition above 7.0 GPa in β-AgBiSe2. First principles theoretical calculations reveal that the metallic character of β-AgBiSe2 is induced mainly due to redistributions of the density of states (p orbitals of Bi and Se) near to the Fermi level. Based on its pressure induced multiple electronic transitions, we propose that AgBiSe2 is a potential candidate for the good thermoelectric performance and pressure switches at high pressure.

Efficient Computation of Difference Vibrational Spectra in Isothermal-Isobaric Ensemble.

Difference spectroscopy between two close systems is widely used to augment its selectivity to the different parts of the observed system, though the molecular dynamics calculation of tiny difference spectra would be computationally extraordinary demanding by subtraction of two spectra. Therefore, we have proposed an efficient computational algorithm of difference spectra without resorting to the subtraction. The present paper reports our extension of the theoretical method in the isothermal-isobaric (NPT) ensemble. The present theory expands our applications of analysis including pressure dependence of the spectra. We verified that the present theory yields accurate difference spectra in the NPT condition as well, with remarkable computational efficiency over the straightforward subtraction by several orders of magnitude. This method is further applied to vibrational spectra of liquid water with varying pressure and succeeded in reproducing tiny difference spectra by pressure change. The anomalous pressure dependence is elucidated in relation to other properties of liquid water.

Experimental Observation of Phase Transition in Sb2O3 under High Pressure**Supported by the National Natural Science Foundation of China under Grant No 11304114.

The in situ high-pressure behavior of the semiconductor antimony trioxide (Sb2O3) is investigated by the Raman spectroscopy techniques and angle-dispersive synchrotron x-ray powder diffraction in a diamond anvil cell up to 31.5 and 30.7 GPa, respectively. New peaks observed in the external lattice mode range in the Raman spectra at 13.5 GPa suggest that the structural phase transition occurs. The group mode (140 cm−1) in Sb2O3 exhibits anomalous pressure dependence; that is, the frequency decreases gradually with the increasing pressure. High pressure synchrotron x-ray diffraction measurements at room temperature reveal that the transition from the orthorhombic structure to high-pressure new phase occurs at about 14.2 GPa, corresponding to the softening of the group optic mode (140 cm−1).

Dynamics anomaly in high-density amorphous ice between 0.7 and 1.1 GPa

We studied high-density amorphous ices between 0.004 and 1.6 GPa by isobaric in situ volumetry and by subsequent ex situ x-ray diffraction and differential scanning calorimetry at 1 bar. Our observations indicate two processes, namely, relaxation in the amorphous matrix and crystallization, taking place at well-separated time scales. For this reason, we are able to report rate constants of crystallization ${k}_{X}$ and glass-transition temperatures ${T}_{g}$ in an unprecedented pressure range. ${T}_{g}$'s agree within \ifmmode\pm\else\textpm\fi{}3 K with earlier work in the small pressure range where there is overlap. Both ${T}_{g}$ and ${k}_{X}$ show a pressure anomaly between 0.7 and 1.1 GPa, namely, a ${k}_{X}$ minimum and a ${T}_{g}$ maximum. This anomalous pressure dependence suggests a continuous phase transition from high- (HDA) to very-high-density amorphous ice (VHDA) and faster hydrogen bond dynamics in VHDA. We speculate this phenomenology can be rationalized by invoking the crossing of a Widom line between 0.7 and 1.1 GPa emanating from a low-lying HDA-VHDA critical point. Furthermore, we interpret the volumetric relaxation of the amorphous matrix to be accompanied by viscosity change to explain the findings such that the liquid state can be accessed prior to the crystallization temperature ${T}_{X}$ at 0.4 GPa and &gt;0.8 GPa.