18O Substitution Research Articles

Oxygen in antimony triselenide: An IR absorption study

Oxygen in single crystalline antimony triselenide (Sb2Se3) is addressed by infrared (IR) absorption spectroscopy. Measurements conducted on Sb2Se3 samples doped—during growth, post-growth annealing in the O2 ambient, or by O ion implantation—with 16O reveal an IR absorption line at 527 cm−1 (10 K). Substitution of 16O by 18O “red”-shifts the signal down to 500 cm−1 based on which the line is assigned to a local vibrational mode of an isolated oxygen defect. Annealing of O-enriched samples in hydrogen atmosphere at temperatures above 380 °C results in the suppression of the 527-cm−1 line and concurrent appearance of the signals due to hydroxyl groups, suggesting formation of oxygen-hydrogen complexes. The configuration of the 527-cm−1 oxygen center is discussed.

Kinetics of enzyme‐catalysed oxygen isotope exchange between phosphate and water revealed by Raman spectroscopy

Pyrophosphatases (EC 3.6.1.1) are ubiquitous enzymes that catalyse the hydrolysis of pyrophosphate, a byproduct of many biochemical reactions. The hydrolysis leads to an oxygen isotope exchange between the newly formed phosphate molecules and water. Here, we applied Raman spectroscopy to monitor the oxygen isotope exchange reaction in presence of pyrophosphatase from baker's yeast. For this purpose, enzymatic assays consisting of 0.8 m 18O‐enriched phosphate were prepared under pH‐buffered conditions. Upon addition of pyrophosphatase, the Raman spectrum of the solution immediately started to shift to higher wavenumbers, indicating the progressive substitution of 18O in phosphate by 16O from water. The analytical results were quantified by fitting a Voigt function to the measured Raman spectra that allowed to determine the relative contribution of each phosphate isotopologue in solution over time. Based on the relative contribution of the different phosphate species, the apparent overall oxygen exchange rate could be calculated assuming a first‐order kinetic. The progressive formation and disappearance of the different phosphate isotopologues were then modelled by applying a consecutive reaction scheme with first‐order steps. The results of our experiments show that Raman spectroscopy can be used to study the kinetics of enzyme‐catalysed oxygen isotope exchange in the phosphate–water system. Copyright © 2016 John Wiley & Sons, Ltd.

Local Structure Studies of Ti for SrTi16O3 and SrTi18O3 by Advanced X-ray Absorption Spectroscopy Data Analysis

Strontium titanate is a model quantum paraelectric in which in the region of dominating quantum statistics the ferroelectric instability is inhibited due to nearly complete compensation of the harmonic contribution into ferroelectric soft mode frequency by the zero-point motion contribution. The enhancement of atomic masses by the substitution of 18O for 16O decreases the zero-point atomic motion and low-T ferroelectricity in SrTi18O3 is realized. In this study we report on the local structure of Ti in SrTi16O3 and SrTi18O3 by Ti K-edge extended x-ray absorption fine structure measurements in temperature range 6 – 300 K.

Influence of the isotopic substitution 16O → 18O on the magnetic, electrical, and thermal properties of manganite La0.8Ag0.1MnO3

The results of the influence of the isotopic substitution of oxygen 16O → 18O on the susceptibility, electrical resistance, heat capacity, and the magnetocaloric effect in univalently doped manganite La0.8Ag0.1MnO3 are presented. It has been shown that the substitution of 16O by a heavier isotope 18O leads to a shift in T C and affects all the measured coefficients. This result indicates the necessity of accounting the lattice dynamics when explaining the mechanism of colossal magnetoresistance in manganites.

Vibrational spectrum and structural features of the high-temperature superconductor YBa2Cu3O7

The structure of YBa2Cu3O7 has been represented as consisting of molecular ions CuO22− and described in space group P4m2. This structure provides a satisfactory interpretation of IR-and Raman-active bands. The calculated isotope shifts of the bands induced by the substitution of 18O for 16O are well consistent with the measured shifts. A possible mechanism of the motion of electron pairs is suggested.

In situ IR and pulse reaction studies on the active oxygen species over SrF 2/Nd 2O 3 catalyst for oxidative coupling of methane

Pulse reaction method and in situ IR spectroscopy were used to characterize the active oxygen species for oxidative coupling of methane (OCM) over SrF 2/Nd 2O 3 catalyst. It was found that OCM activity of the catalyst was very low in the absence of gas phase oxygen, which indicated that lattice oxygen species contributed little to the yield of C 2 hydrocarbons. IR band of superoxide species (O 2 −) was detected on the O 2-preadsorbed SrF 2/Nd 2O 3. The substitution of 18O 2 isotope for 16O 2 caused the IR band of O 2 − at 1128 cm −1 to shift to lower wavenumbers (1094 and 1062 cm −1), consistent with the assignment of the spectra to the O 2 − species. A good correlation between the rate of disappearance of surface O 2 − and the rate of formation of gas phase C 2H 4 was observed upon interaction of CH 4 with O 2-preadsorbed catalyst at 700 °C. The O 2 − species was also observed on the catalyst under working condition. These results suggest that O 2 − species is the active oxygen species for OCM reaction on SrF 2/Nd 2O 3 catalyst.

Structural aspects of the giant oxygen isotope effect in perovskite manganese oxides

The structure of manganites R 1−x A x MnO3 (R = La, Sm, or a mixture of rare earth cations; A = Ca, Sr; x = 0.3, 0.5) has been studied by neutron diffraction to find out the reasons for the giant oxygen isotope effect—the transition from a low-temperature metallic state to an insulating state as a result of replacement of 16O with 18O. It is shown that this effect is observed in compositions in which the two phases P1 and P2 coexist at low temperatures. They have the same crystal symmetry (sp. gr. Pnma) but different types of Jahn-Teller distortions of oxygen octahedra and different magnetic structure. Phase P1 has a conductivity of the metallic type, weakly distorted MnO6 octahedra, and a ferromagnetic structure. Phase P2 is insulating, with MnO6 octahedra extended or compressed in the apical direction and the moments of Mn ions forming an antiferromagnetic structure. The relative volume of phases P1 and P2 in samples depends on the average radius of the A cation and changes occurring upon replacement of 16O with 18O. The percolation transition from the metallic to insulating state upon substitution of 16O with 18O is caused by the sharp decrease in the volume of the ferromagnetic metallic phase in favor of the insulating antiferromagnetic phase. An effect of the sample microstructure on the formation of the two-phase state is found.

Interplay of electron–lattice interactions and superconductivity in Bi2Sr2CaCu2O8+δ

Formation of electron pairs is essential to superconductivity. For conventional superconductors, tunnelling spectroscopy has established that pairing is mediated by bosonic modes (phonons); a peak in the second derivative of tunnel current d2I/dV2 corresponds to each phonon mode. For high-transition-temperature (high-T(c)) superconductivity, however, no boson mediating electron pairing has been identified. One explanation could be that electron pair formation and related electron-boson interactions are heterogeneous at the atomic scale and therefore challenging to characterize. However, with the latest advances in d2I/dV2 spectroscopy using scanning tunnelling microscopy, it has become possible to study bosonic modes directly at the atomic scale. Here we report d2I/dV2 imaging studies of the high-T(c) superconductor Bi2Sr2CaCu2O8+delta. We find intense disorder of electron-boson interaction energies at the nanometre scale, along with the expected modulations in d2I/dV2 (refs 9, 10). Changing the density of holes has minimal effects on both the average mode energies and the modulations, indicating that the bosonic modes are unrelated to electronic or magnetic structure. Instead, the modes appear to be local lattice vibrations, as substitution of 18O for 16O throughout the material reduces the average mode energy by approximately 6 per cent--the expected effect of this isotope substitution on lattice vibration frequencies. Significantly, the mode energies are always spatially anticorrelated with the superconducting pairing-gap energies, suggesting an interplay between these lattice vibration modes and the superconductivity.

When Is an Isotope Effect Non-Mass Dependent?

In the formation of ozone by an electric discharge in molecular oxygen, substitution of 18O and 17O for 16O produces equal kinetic isotope effects. This may be taken as a paradigm of non-mass dependent isotope effects. However, a number of other reactions have been thus labeled, even when deviations from the expected ratio of ~0.5 for (17O/16O)/(18O/16O) are a few parts per thousand. Several equilibrium constants are calculated for isotopic exchange reactions involving oxygen and sulfur isotope effects, and it is shown that exact calculations can differ from this approximate ratio by a few percent. The conclusion is that the label “non-mass dependent” should be applied with care.

Magnetostructural phase separation and giant isotope effect in R0.5Sr0.5MnO3

Results of neutron diffraction studies of R0.5Sr0.5MnO3 manganites (R = Sm, Nd0.772Tb0.228, and Nd0.544Tb0.456) performed to reveal the microscopic origins of the giant oxygen isotope effect recently discovered in Sm0.5Sr0.5MnO3 are presented. It is shown that two crystalline phases differing in the type of Jahn-Teller distortions of oxygen octahedra and in the type of magnetic ordering coexist at low temperatures in all the studied compositions. A scenario for the observed phase transitions is suggested based on the diffraction data. It is found that the percolation transition from the metallic to insulating state in compositions with Sm upon substitution of 18O for 16O is associated with a sharp (from 65 to 13%) decrease in the volume of the ferromagnetic metallic phase.

EPR study of the spin dynamics of the (La1−y Pry)0.7Ca0.3MnO3 system

The EPR of Mn ions in the (La1−y Pry)0.7Ca0.3MnO3 system has been studied within a broad range of temperatures (4<T<600 K) and Pr concentrations (0≤y≤1), as well as under isotope substitution of 18O for 16O. All compositions were shown to undergo transitions to a magnetically ordered state with decreasing temperature. Magnetic phase diagrams were constructed for systems with different oxygen isotopes. The diagrams include paramagnetic, ferromagnetic, and antiferromagnetic regions. In the paramagnetic region, at temperatures not too close to the phase transition points, the Mn ion linewidth ΔH pp (T) is related to the magnetic susceptibility χ(T) through the relation ΔH pp (T) = [χ0/χ(T)]ΔH pp (∞) + ΔH 0, where ΔH pp (∞) is the width of the exchange-narrowed line in the high-temperature approximation, χ0 ∝ 1/T is the susceptibility of noninteracting ions, and ΔH 0 is the residual width originating from the sample porosity and resonance-field scatter in unoriented grains of a powder sample. An analysis of the data on ΔH pp (∞), ΔH 0, and χ(T) made it possible to estimate the symmetric and antisymmetric exchange interaction of Mn ions and of the noncubic crystal-field component of the oxygen ions. These parameters were found to be independent of the oxygen isotope species to within experimental error.

Large tunability driven by low electric field in oxygen-isotope-exchanged strontium titanate at cryogenic temperatures

The substitution of heavy isotope 18O for 16O in SrTiO3 simultaneously results in a significant enhancement of tunability and a large reduction of driving electric fields at cryogenic temperatures. For a 36% oxygen isotope exchanged SrTiO3 at 2 K, the driving electric field for the tunability of 0.82 is about half an order lower than that for an unexchanged one. The tunability and the figure of merit could be tailored by the oxygen isotope exchange rate. It is considered that the phenomena are closely related to the appearance of ferroelectric microregions and the “domain state” in this system [R. Wang and M. Itoh, Phys. Rev. B 62, R731 (2000)].

Crossover between Ferroelectric and Quantum Paraelectric in SrTiO3 both by Isotopic Substitution and Hydrostatic Pressure

AbstractQuantum paraelectric SrTiO3 undergoes a transition to ferroelectric by the substitution of 18O for 16O. The Tc vs. x in SrTi(16O1-x18Ox)3 follows Tc = 30.4(x - 0.33)1/2. Application of the hydrostatic pressure on the SrTiO3 and SrTi(16O0.0718O0.93)3 have the effects of decreasing ε and depressing Tc, respectively. Above the critical pressure pc, ferroelectricity of SrTi(16O0.0718O0.93)3 disappears. Fitting the data to the Barrett's formula, ε , elucidated that T0 and T1 for both compounds changes linearly with pressure.

Thermal conductivity of (La0.25Pr0.75)0.7Ca0.3MnO3 under giant isotope effect conditions

The thermal conductivity of (La0.25Pr0.75)0.7Ca0.3MnO3 manganite has been studied. The isotope substitution of 18O for 16O in this compound leads to a ferromagnetic-antiferromagnetic phase transition at low temperatures. It has been found that the thermal conductivity in the ferromagnetic state is approximately two times higher than in the antiferromagnetic state. It has been shown that the small value of thermal conductivity and its temperature dependence can be due to strong phonon scattering from crystal lattice defects, which are thought of as Jahn-Teller distortions. The parameters of this scattering can be determined within the Debye model of thermal conductivity from a comparison of samples differing in their isotope composition.

Evolution of the magnetic phase diagram of CMR manganites after oxygen isotope substitution

Oxygen isotope effect in magnetic properties of CMR manganites of the (La 1− y Pr y ) 0.7Ca 0.3MnO 3 system was systematically studied. Substitution of 16O by 18O was found to stabilize insulating state and suppress ferromagnetic (FM) component. The AC susceptibility data revealed a pronounced temperature hysteresis. The hysterisis became more pronounced with increase of y up to 0.75. The width of the hysteretic loops was strongly affected by the 16O→ 18O substitution. The T– y phase diagram and its evolution due to oxygen isotope substitution are analyzed.

Adsorption and Reaction of 2-Chloroethylethyl Sulfide with Al2O3 Surfaces

The reaction of 2-chloroethylethyl sulfide (CEES) with a high-area Al2O3 surface was investigated. Two different reactive sites were created by thermally pretreating the Al2O3 powder: isolated hydroxyl sites and Lewis acid−base pair sites. The reaction of CEES with the isolated hydroxyl groups at 303 and 473 K produced a surface-bound species, which is characterized by a carbon−oxygen stretching mode near 1100 cm-1. This assignment was confirmed by isotopic substitution of 18O into the isolated hydroxyl groups prior to reaction with CEES. The other reaction product detected at 473 K was HCl. For a more highly dehydroxylated Al2O3 surface, the reaction of CEES with the Lewis acid−base pairs produced a surface−bound species also exhibiting a carbon−oxygen stretching mode near 1100 cm-1. This species is postulated to form at O2- Lewis base sites as Al3+−Cl- bonds form on adjacent Lewis acid sites. The adsorption of pyridine onto the Lewis acid sites prior to CEES exposure effectively closes this reaction pa...

Theory of ferromagnetism and colossal magnetoresistance in doped manganites

An exchange interaction of polaronic carriers with localized spins leads to a new ferromagnetic transition in doped charge-transfer insulators with strong electron–phonon coupling. The relative strength of the exchange and electron–phonon interactions determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of bipolaron formation in the paramagnetic phase, is extremely sensitive to an external magnetic field. We show that the carrier density collapse describes the colossal magnetoresistance, anomalous specific heat, and tunneling gap of doped manganites close to the transition. The dependence of the binding energy on ion mass explains the giant isotope effect in magnetization and resistivity upon substitution of 16O by 18O.

Effects induced by O-16-O-18 oxygen isotope exchange in manganite ceramics and films

Unique sensitivity of electrical resistivity to the 16O→18O isotope exchange was observed for (La1−yPry)0.7Ca0.3MnO3 ceramics and epitaxial thin films. At y=0.7, substitution of 16O by 18O in ceramic samples results in the reversible transition from a metal-like to insulating state. In thin films, this metal–insulator transition occurs at y=0.5 due to the strains induced by substrate. The applied magnetic field (H≥2 T) transforms the sample with 18O back to the metallic state. In such magnetic fields the temperature dependence of resistivity exhibits a peak corresponding to the onset of metallic state. The shift of the peak toward low temperatures after the 16O→18O exchange exceeds 50 K at 3 T for both ceramic and thin film samples. The results are interpreted in terms of the isotope dependence of the effective electron bandwidth which shifts the balance between the charge ordered insulating and the ferromagnetic metal-like states.

Changes in the magnetic structure of (La0.25Pr0.75)0.7Ca0.3MnO3 upon the isotopic substitution of 18O for 16O

A giant isotope effect, wherein the low-temperature metallic state is replaced by an insulator state, was recently observed when 18O was substituted for 16O in (La0.25Pr0.75)0.7Ca0.3MnO3 [N. A. Babushkina, L. M. Belova, O. Yu. Gorbenko et al., Nature (London) 391, 159 (1998)]. In the present work, the temperature evolution of the magnetic structure of two samples of this compound is studied by neutron diffraction. One sample contained a natural mixture of oxygen isotopes, 99.7% 16O, while the other was enriched to 75% with the isotope 18O. It is established that the samples are crystallographically identical at room temperature. As the temperature decreases, the sample with 16O undergoes successive antiferromagnetic (TAFM=5150 K) and ferromagnetic (TFM=110 K) transitions, which lead to the establishment of a noncollinear ferromagnetic structure, while a purely antiferromagnetic (TAFM=150 K) order arises in the sample with 18O. The temperature dependences of the intensities of the diffraction peaks associated with the charge ordering differ substantially in samples with 16O and 18O, and they correlate with the behavior of their electric resistance and magnetic structure.

Metal–insulator transition induced by O16−O18 oxygen isotope exchange in colossal negative magnetoresistance manganites

The effect of O16→O18 isotope exchange on the electric resistivity was studied for (La1−yPry)0.7Ca0.3MnO3 ceramic samples. It was found that at y=0.75, the substitution of O16 by O18 results in the reversible transition from a ferromagnetic metal (FM) to charge ordered (CO) insulator at zero magnetic field. The applied magnetic field (H⩾2 T) transformed the sample with O18 again to the metallic state and caused the increase in the FM transition temperature TC of the O16 sample. As a result, the isotope shift of TC at H=2 T was as high as 63 K. Such unique sensitivity of the system to oxygen isotope exchange, giving rise even to the metal–insulator transition, is discussed in terms of the isotope dependence of the effective electron bandwidth which shifts the balance between the CO and FM phases.