Pressure-induced Decrease Research Articles

Viscosity and Stokes-Einstein relation in deeply supercooled water under pressure.

We report measurements of the shear viscosity η in water up to 150MPa and down to 229.5K. This corresponds to more than 30K supercooling below the melting line. The temperature dependence is non-Arrhenius at all pressures, but its functional form at 0.1MPa is qualitatively different from that at all pressures above 20MPa. The pressure dependence is non-monotonic, with a pressure-induced decrease of viscosity by more than 50% at low temperature. Combining our data with literature data on the self-diffusion coefficient Ds of water, we check the Stokes-Einstein relation which, based on hydrodynamics, predicts constancy of Dsη/T, where T is the temperature. The observed temperature and pressure dependence of Dsη/T is analogous to that obtained in simulations of a realistic water model. This analogy suggests that our data are compatible with the existence of a liquid-liquid critical point at positive pressure in water.

Pressure-Induced Phase Transition and Band Gap Decrease in Semiconducting β-Cu2V2O7.

The understanding of the interplay between crystal structure and electronic structure in semiconductor materials is of great importance due to their potential technological applications. Pressure is an ideal external control parameter to tune the crystal structures of semiconductor materials in order to investigate their emergent piezo-electrical and optical properties. Accordingly, we investigate here the high-pressure behavior of the semiconducting antiferromagnetic material β-Cu2V2O7, finding it undergoes a pressure-induced phase transition to γ-Cu2V2O7 below 4000 atm. The pressure-induced structural and electronic evolutions are investigated by single-crystal X-ray diffraction, absorption spectroscopy and ab initio density functional theory calculations. β-Cu2V2O7 has previously been suggested as a promising photocatalyst for water splitting. Now, these new results suggest that β-Cu2V2O7 could also be of interest with regards to barocaloric effects, due to the low phase -transition pressure, in particular because it is a multiferroic material. Moreover, the phase transition involves an electronic band gap decrease of approximately 0.2 eV (from 1.93 to 1.75 eV) and a large structural volume collapse of approximately 7%.

Effect of hydrostatic pressure on magneto-crystalline anisotropy of Heusler Ni2MnSn-based alloy

ABSTRACT Single crystal of the stoichiometric Ni2MnSn alloy (cubic L21 crystal structure) was prepared by the Czochralski method. The values of the magneto-crystalline anisotropy constant K 1 have been determined at temperature 10 K under ambient and high hydrostatic pressures, K 1 = + 0.17 × 104 and +1.96 × 104 J/m3 (0.7 GPa), respectively. The pressure-induced decrease of magnetization was confirmed and hence the significant non-trivial increase of uniaxial anisotropy with increasing pressure points to a possible distortion of the cubic structure of the single crystal under hydrostatic pressure. Simultaneously, the more pronounced and pressure almost insensitive magneto-crystalline anisotropy, K 1 = + 9.1 × 104 J/m3, has been observed in the martensite phase (orthorhombic structure) of the off-stoichiometric Ni2Mn1.43Sn0.57 alloy. The effect of a directional dependence of the Young modulus that was theoretically derived in the case of the Ni2MnSn-based alloys is discussed.

Unconventional stable stoichiometry of vanadium peroxide.

Peroxides have attracted considerable attention due to their intriguing electronic properties and diverse applications. However, only a few transition metal peroxides have been known thus far, limiting the variety of peroxide examples. Here, we demonstrate the stabilization of peroxides in the O-rich V-O system through first-principles calculations coupled with a swarm-intelligence structure search. As well as reproducing the known stoichiometries of VO, V2O3, VO2, and V2O5, two hitherto unknown V2O and VO4 stoichiometries are predicted to be thermodynamically stable at megabar pressures. VO4 has the highest oxygen content among the known peroxides to date. More interestingly, its electronic band gap increases with pressure, originating from the pressure-induced decrease of O-O bonding length in the peroxide group. V-rich V2O exhibits superconductivity, becoming the first example in the V-O system. Our work not only unravels the unusual vanadium peroxide, but also provides further insight into the diverse electronic properties of vanadium oxides under high pressure.

Element-selective elastic properties of Fe65Ni35 Invar alloy and Fe72Pt28 alloy studied by extended X-ray absorption fine structure

ABSTRACTWe present the element-selective local structure analysis of ferromagnetic FeNi Invar alloy and FePt alloy under high pressures as a recent application of nano-polycrystalline diamond (NPD) to extended X-ray absorption fine structure (EXAFS). The Invar effect is a phenomenon where thermal expansion becomes nearly zero in a wide range of temperatures. FePt is an alloy exhibiting negative thermal expansion. Our EXAFS analysis successfully revealed significant differences in element-selective elastic properties between local structures around Fe and Ni/Pt. The local structure around Fe has a smaller bulk modulus than that of Ni. Hence, the more compressive behavior around Fe is responsible for the anomalously soft elastic property that is characteristic of Invar alloy. In the case of FePt, Fe has a smaller bulk modulus than Pt; however, the difference in bulk modulus in the ferromagnetic phase is small compared with that of Invar alloy. We also report different trends of pressure-induced decrease in Debye-Waller factors toward paramagnetic phases.

Effect of high pressure on magnetic properties of CrMnFeCoNi high entropy alloy

Abstract The temperature and magnetic field dependences of magnetization of the Cantor CrMnFeCoNi alloy were studied at ambient and high (up to 1 GPa) external pressure. The low-temperature spin-glass-like behavior and the paramagnetic behavior at temperatures above 100 K with effective paramagnetic moment of the alloy, meff = 2.71 μB/f.u., were observed. A negative value of the paramagnetic Curie temperature, θC = −210 K, was found that confirms a presence of strong antiferromagnetic interactions of moments in the alloy. A model of the magnetic clusters with the effective moment of mcl = 0.275 μB/f.u. provides a possibility to describe qualitatively the pronounced difference between the pressure-induced decrease of magnetization of the ZFC and the FC alloys, dlnM/dP(5 K) = −15.5*10−3 GPa−1 and −39.2*10−3 GPa−1, respectively. The former value is in an excellent agreement with the value of −16*10−3 GPa−1 obtained from our quantum–mechanical calculations. The negative pressure shift of the ordering temperature of the alloy, dTord/dP = −9 K/GPa, and the pressure-induced decrease of magnetization of the alloy in paramagnetic state, dlnM/dP(90 K) = −70*10−3 GPa−1, indicate a possible itinerant character of magnetism of the alloy. The results of theoretical calculations of the total moment of the CrMnFeCoNi fcc-alloy are discussed.

Cyanin Chloride Inhibits Hyperbaric Pressure-Induced Decrease of Intracellular Glutamate-Aspartate Transporter in Rat Retinal Müller Cells.

Purpose Glaucoma is the leading cause of irreversible blindness throughout the world. The pathogenesis of glaucoma is complex, and neuroprotection is a crucial aspect of therapy. High concentrations of extracellular glutamate are toxic to the optic nerve. The glutamate-aspartate transporter (GLAST) in retinal Müller cells is involved in the development of glaucoma. Anthocyanin has been reported to protect retinal neurons. We hypothesize that cyanin chloride, a type of anthocyanin, can inhibit hyperbaric pressure-induced GLAST decreases in cultured rat retinal Müller cells and may serve as a potential neuroprotective agent in glaucoma treatment. Materials and Methods Sprague Dawley rat Müller cells were cultured in a hyperbaric pressure device at 60 mmHg additional pressure and treated with cyanin chloride (10 μmol/L, 30 μmol/L, or 50 μmol/L) or vehicle for 2 hours. Cell survival rates (SRs) were evaluated by an MTT assay. GLAST mRNA and protein expression were determined by western blot and RT-PCR analyses, respectively. Results Cell SR was significantly decreased in the 60 mmHg additional hyperbaric pressure group compared to the control group (P < 0.01). Cyanin chloride treatment significantly improved SR under 60 mmHg additional pressure (P < 0.01). GLAST mRNA and protein expression levels in Müller cells were significantly reduced in the 60 mmHg hyperbaric pressure group compared to the control group (P < 0.01), but cyanin chloride significantly inhibited hyperbaric pressure-induced decreases in GLAST expression (P < 0.01). Conclusion Our results support our hypothesis and demonstrate that cyanin chloride can protect rat retinal Müller cells from hyperbaric pressure-induced decreases of GLAST.

The hydrostatic pressure dependence of the phase transitions and dielectric properties for a potassium niobate crystal

The phase transitions, spontaneous polarizations, and dielectric properties of a potassium niobate (KNbO3) ferroelectric crystal were investigated under hydrostatic pressure from 0 to 21.3 GPa using the Landau–Ginsburg–Devonshire phenomenological theory. It was shown that the phase transition temperatures of cubic to tetragonal, tetragonal to orthorhombic, and orthorhombic to rhombohedric decreased as the hydrostatic pressure was increased and the derivative of the phase transition temperatures with respect to hydrostatic pressure could be estimated to be −33.1 °C/GPa. The spontaneous polarizations decreased nonlinearly with increasing hydrostatic pressure which could be attributed to the hydrostatic pressure-induced decrease of the interionic distance. The dielectric susceptibility ε33 increased by about 85% and the dielectric susceptibility ε11 decreased by about 50% when hydrostatic pressure increased from 0 to 4 GPa at 260 °C. It provides a new effective way to improve the pressure sensitivity in acoustic transducer and the energy storage performance of capacitor. The changes of dielectric susceptibilities induced by hydrostatic pressure had been explained by the flattening or steepness of Gibbs free energy profile. As the properties of the ceramics are related to those of the single crystal, results in this paper can be used to explore the general rule of performance change induced by pressure for lead free KNbO3-based ceramics.

Pressure-Induced Decrease in the Curie Temperature of Gd2Fe17: LSDA+U Calculations

To explain the magnetic properties of advanced ferromagnetic intermetallic compounds of the R2Fe17 (R is a rare-earth element) class, experimentalists often use the hypothesis of competition between ferromagnetic exchange and antiferromagnetic exchange between four types of the nearest iron atoms in nonequivalent lattice sites. For the rhombohedral Gd2Fe17 ferromagnet, we calculate the magnetic moments of iron and gadolinium ions, the parameters of exchange between Fe atoms, and Curie temperature TC at a zero pressure and during hydrostatic lattice compression. The magnetic moment of the unit cell of Gd2Fe17 is shown to decrease under pressure, and this decrease is almost completely associated with a decrease in the magnetic moments of Fe rather than Gd ions, the pressure dependence of the magnetic moments of which is weaker by an order of magnitude. In contrast to the hypothesis regarding the competition of exchange interactions between different kinds of Fe atoms, the parameters of exchange between the nearest iron atoms in different crystallographic sites are found to be positive ferromagnetic (at a zero pressure and during compression), and a ferromagnetic character of interaction is shown to remain unchanged under pressure even for Fe atoms in the so-called dumbbell sites with the nearest interatomic distances. The Curie temperature TC of Gd2Fe17 is shown to decrease with increasing pressure. The changes in the exchange parameters and the magnetic moments of Gd2Fe17 during compression are found to be mainly related to a change in the position of energy spectrum branches with respect to each other and the Fermi level ϵF rather than to a change in the overlapping of wavefunctions, which play a minor role.

High-pressure effects on isotropic superconductivity in the iron-free layered pnictide superconductor BaPd2As2

While the layered 122 iron arsenide superconductors are highly anisotropic, unconventional, and exhibit several forms of electronic orders that coexist or compete with superconductivity in different regions of their phase diagrams, we find in the absence of iron in the structure that the superconducting characteristics of the end member BaPd2As2 are surprisingly conventional. Here we report on complementary measurements of specific heat, magnetic susceptibility, resistivity measurements, Andreev spectroscopy, and synchrotron high pressure x-ray diffraction measurements supplemented with theoretical calculations for BaPd2As2. Its superconducting properties are completely isotropic as demonstrated by the critical fields, which do not depend on the direction of the applied field. Under the application of high pressure, Tc is linearly suppressed, which is the typical behavior of classical phonon-mediated superconductors with some additional effect of a pressure-induced decrease in the electronic density of states and the electron-phonon coupling parameters. Structural changes in the layered BaPd2As2 have been studied by means of angle-dispersive diffraction in a diamond-anvil cell. At 12 GPa and 24.2 GPa we observed pressure induced lattice distortions manifesting as the discontinuity and, hence discontinuity in the Birch-Murnaghan equation of state. The bulk modulus is B0=40(6) GPa below 12 GPa and B0=142(3) GPa below 27.2 GPa.

Photoelastic response of permanently densified oxide glasses

The stress-induced birefringence (photoelastic response) in oxide glasses has important consequences for several applications, including glass for flat panel displays, chemically strengthened cover glass, and advanced optical glasses. While the effect of composition on the photoelastic response is relatively well documented, the effect of pressure has not been systematically studied. In this work, we evaluate the effect of hot isostatic compression on the photoelastic response of ten oxide glasses within two commonly used industrial glass families: aluminosilicates and boroaluminosilicates. Hot isostatic compression generally results in decreasing modifier-oxygen bond lengths and increasing network-former coordination numbers. These structural changes should lead to an increase in the stress optic coefficient (C) according to the model of Zwanziger et al., which can successfully predict the composition and structure dependence of C. However, in compressed glasses, we observe the opposite trend, viz., a decrease in the stress optic coefficient as a result of pressurization. We discuss this result based on measured changes in refractive index and elastic moduli within the context of atomic and lattice effects, building on the pioneering work of Mueller. We propose that the pressure-induced decrease in C is a result of changes in the shear modulus due to underlying topological changes in the glass network.

TRPV4 participates in pressure-induced inhibition of renin secretion by juxtaglomerular cells.

Increase in blood pressure in the renal afferent arteriole is known to induce an increase in cytosolic calcium concentration ([Ca2+ ]i ) of juxtaglomerular (JG) cells and to result in a decreased secretion of renin. Mechanical stimulation of As4.1 JG cells induces an increase in [Ca2+ ]i that is inhibited by HC067047 and RN1734, two inhibitors of TRPV4, or by siRNA-mediated repression of TRPV4. Inhibition of TRPV4 impairs pressure-induced decrease in renin secretion. Compared to wild-type mice, Trpv4-/- mice present increased resting plasma levels of renin and aldosterone and present a significantly altered pressure-renin relationship. We suggest that TRPV4 channel participates in mechanosensation at the juxtaglomerular apparatus. The renin-angiotensin system is a crucial blood pressure regulation system. It consists of a hormonal cascade where the rate-limiting enzyme is renin, which is secreted into the blood flow by renal juxtaglomerular (JG) cells in response to low pressure in the renal afferent arteriole. In contrast, an increase in blood pressure results in a decreased renin secretion. This is accompanied by a transitory increase in [Ca2+ ]i of JG cells. The inverse relationship between [Ca2+ ]i and renin secretion has been called the 'calcium paradox' of renin release. How increased pressure induces a [Ca2+ ]i transient in JG cells, is however, unknown. We observed that [Ca2+ ]i transients induced by mechanical stimuli in JG As4.1 cells were completely abolished by HC067047 and RN1734, two inhibitors of TRPV4. They were also reduced by half by siRNA-mediated repression of TRPV4 but not after repression or inhibition of TRPV2 or Piezo1 ion channels. Interestingly, the stimulation of renin secretion by the adenylate cyclase activator forskolin was totally inhibited by cyclic stretching of the cells. This effect was mimicked by stimulation with GSK1016790A and 4αPDD, two activators of TRPV4 and inhibited in the presence of HC067047. Moreover, in isolated perfused kidneys from Trpv4-/- mice, the pressure-renin relationship was significantly altered. In vivo, Trpv4-/- mice presented increased plasma levels of renin and aldosterone compared to wild-type mice. Altogether, our results suggest that TRPV4 is involved in the pressure-induced entry of Ca2+ in JG cells, which inhibits renin release and allows the negative feedback regulation on blood pressure.

Effect of High Pressure on the Transformations of Ferrocene-Filled, Single-Wall, Carbon Nanotubes: Density Functional Theory and Raman Spectroscopy Studies

Metallocene-filled single-walled carbon nanotubes (SWCNTs) have tunable electronic properties and a large potential in creating new structures due to their tunable electron doping and unique chemical reaction in a nanoscale confinement environment. Here we study the effect of high pressure on the transformations of ferrocene (FeCp2)-filled SWCNTs (FC@SWCNTs) by theoretical simulation and Raman spectroscopy. It is found that the filling of FeCp2 into carbon nanotube leads to higher transition pressures for the nanotube cross section changes and the intertubular bonding compared with the unfilled nanotubes. These results can be explained by the different charge distribution on the nanotubes due to charge transfer and the effect from host–guest interactions. Band structure analysis shows a pressure-induced decrease of band gap below 11 GPa, a transformation into a semimetal structure at 11 GPa, and subsequently, an abrupt increase of band gap due to the formation of intertube bonding. The increased host–guest interaction also leads to a decomposition of FeCp2 and the formation of a new 3D zeolite-like structure, which is quenchable to ambient conditions. Our preliminary experimental results confirm that the encapsulation of FeCp2 affects the transformations of nanotubes, which can be explained by the host–guest interactions, as demonstrated by our simulations.

Theoretical study on structural and electronic properties of solid anthracene under high pressure by density functional theory

ABSTRACTBased on the density functional theory, a theoretical determination method is applied to investigate structural and electronic properties of anthracene up to 27 GPa. The lattice parameters a, b, c, decrease by 1.53 (−18.2%), 0.76 (−12.8%), and 0.93 (−8.4%), respectively, while the monoclinic angle β increased by 4.65° in this pressure region. At the highest pressure of 27 GPa the unit cell volume is decreased by 58.9%. These findings are shown to be in agreement with experimental results and hint towards the evolution of intermolecular interaction with pressure. The calculated electronic band splitting and the band gap reduce smoothly to some extent with the pressure increasing. Moreover, a pressure-induced decrease of the band gap is observed.

Direct measurement of magnetization isotherms of Fe64Ni36 Invar alloy in a diamond anvil cell

Magnetization isotherms of the Fe64Ni36 Invar alloy have been measured under pressure up to 5.3 GPa in magnetic field up to 5 T using a diamond anvil cell and SQUID magnetometer. The unambiguous change of the pressure parameter dlnMS/dP (from −9 to −13×10−2 GPa−1) has been observed in a narrow pressure interval from 2.5 to 3.5 GPa at all temperatures in the range from 5 to 300 K. The pressure interval, where the sharp decrease in magnetization was observed, coincides with the critical pressures of the pressure-induced decrease in Fe-moment that were determined by the X-ray Magnetic Circular Dichroism and the X-ray Emission Spectra studies, recently. The pronounced decrease in the Curie temperature of the Fe64Ni36 alloy under pressure, dTC/dP = −44 ±2 K/GPa, has been confirmed.

Optic Nerve Head Blood Flow Response to Reduced Ocular Perfusion Pressure by Alteration of Either the Blood Pressure or Intraocular Pressure

Purpose: To test the hypothesis that blood flow autoregulation in the optic nerve head has less reserve to maintain normal blood flow in the face of blood pressure-induced ocular perfusion pressure decrease than a similar magnitude intraocular pressure-induced ocular perfusion pressure decrease.Materials and methods: Twelve normal non-human primates were anesthetized by continuous intravenous infusion of pentobarbital. Optic nerve blood flow was monitored by laser speckle flowgraphy. In the first group of animals (n = 6), the experimental eye intraocular pressure was maintained at 10 mmHg using a saline reservoir connected to the anterior chamber. The blood pressure was gradually reduced by a slow injection of pentobarbital. In the second group (n = 6), the intraocular pressure was slowly increased from 10 mmHg to 50 mmHg by raising the reservoir. In both experimental groups, optic nerve head blood flow was measured continuously. The blood pressure and intraocular pressure were simultaneously recorded in all experiments.Results: The optic nerve head blood flow showed significant difference between the two groups (p = 0.021, repeat measures analysis of variance). It declined significantly more in the blood pressure group compared to the intraocular pressure group when the ocular perfusion pressure was reduced to 35 mmHg (p < 0.045) and below. There was also a significant interaction between blood flow changes and the ocular perfusion pressure treatment (p = 0.004, adjusted Greenhouse & Geisser univariate test), indicating the gradually enlarged blood flow difference between the two groups was due to the ocular perfusion pressure decrease.Conclusions: The results show that optic nerve head blood flow is more susceptible to an ocular perfusion pressure decrease induced by lowering the blood pressure compared with that induced by increasing the intraocular pressure. This blood flow autoregulation capacity vulnerability to low blood pressure may provide experimental evidence related to the hemodynamic pathophysiology in glaucoma.

Photoinduced perturbations of the magnetic superexchange in core@shell Prussian blue analogues

Cubic heterostructured core@shell particles of Prussian blue analogues, composed of a shell (∼80nm thick) of ferromagnetic K0.3Ni[Cr(CN)6]0.8·1.3H2O (A), Tc∼70K, surrounding a bulk (∼350 nm) core of photoactive ferrimagnetic Rb0.4Co[Fe(CN)6]0.8·1.2H2O (B), Tc∼20K, have been studied. Below nominally 70K, these CoFe@NiCr samples exhibit a persistent photoinduced decrease in low-field magnetization, and these results resemble data from other BA core@shell particles and analogous ABA heterostructured films. This net decrease suggests that the photoinduced lattice expansion in the B layer generates a strain-induced decrease in the magnetization of the NiCr layer, similar to a pressure-induced decrease observed by others in a similar, pure NiCr material and by us in our CoFe@NiCr cubes. Upon further examination, the data also reveal a significant portion of the NiCr shell whose magnetic superexchange, J, is perturbed by the photoinduced strain from the CoFe constituent.

Pressure effects on strained FeSe0.5Te0.5 thin films

The pressure effect on the resistivity and superconducting Tc of prestrained thin films of the iron chalcogenide superconductor FeSe0.5Te0.5 is studied. Films with different anion heights above the Fe layer showing different values of ambient pressure Tc’s are compressed up to a pressure of 1.7 GPa. All films exhibit a significant increase of Tc with pressure. The results cannot solely be explained by a pressure-induced decrease of the anion height but other parameters have to be considered to explain the data for all films.

Pressure-induced luminescence of cerium-doped gadolinium gallium garnet crystal

Studies of the spectroscopic properties of Ce${}^{3+}$ dopant in bulk Gd${}_{3}$Ga${}_{5}$O${}_{12}$:Ce crystal under pressure are presented. In spite of strong intershell $4f\ensuremath{\rightarrow}5d$ absorption bands at ambient pressure, the cerium luminescence in Gd${}_{3}$Ga${}_{5}$O${}_{12}$ is entirely quenched even at low temperature. It has been shown that applying pressure allows for recovery of the $5d\ensuremath{\rightarrow}4f$ radiative transitions. Further increase of pressure improves the emission efficiency. This effect is analyzed in terms of two possible phenomena: (i) by pressure-induced electronic crossover of the excited 5$d$ energy level of the Ce${}^{3+}$ with the conduction band bottom of the host crystal, and (ii) by decrease of electron-lattice coupling with increasing pressure, resulting in reduction of the Stokes shift and nonradiative transitions between the low vibrational levels of the 5$d$ state and high vibrational levels of the ground 4$f$ state. The results of high-pressure absorption and luminescence measurements point out that the ambient-pressure luminescence quenching is caused by the donor-like charge transfer processes due to the resonant location of the Ce${}^{3+}$ 5$d$ electronic levels with respect to the host conduction band. In such a situation, the ionization of Ce${}^{3+}$ to Ce${}^{4+}$ occurs, accompanied by large lattice relaxation, which enables the nonradiative recombination to the Ce 4$f$ state. The pressure-induced approach of the conduction-band bottom of the host crystal by the excited 5$d$ energy level of the Ce${}^{3+}$ results in mixing between conduction band states and the 5$d$ state, and the broadband luminescence from this mixed state is observed at pressures from 30 up to about 60 kbar. Then, for pressures exceeding 60 kbar, this luminescence is replaced by the classic ${\mathrm{Ce}}^{3+}\phantom{\rule{0.16em}{0ex}}5d\ensuremath{\rightarrow}4f$ transitions. Using a simple anticrossing model and configuration coordinate model in the appropriate pressure range, quantitative description of the system has been carried out, and the pressure-induced decrease of electron-lattice coupling has been shown.

Band energy structure of K2ZnCl4 crystals under uniaxial pressures

We have studied the energy band structure of uniaxially compressed single crystals K2ZnCl4. The smallest direct energy-gap for the stress-free crystal is 4.55 eV and corresponds to the Γ-point of the Brillouin zone. The pressure coefficients for the energy positions of the valence and conduction band states and the band gap are determined. The pressure-induced decrease in the band gap found by us correlates well with the increase of the refractive indices known from the earlier experiments.