Oxygen Isotope Effect Research Articles

Anomalous oxygen-isotope effect on the in-plane far-infrared conductivity of detwinnedYBa2Cu316,18O6.9

We observe an anomalous oxygen isotope effect on the a-axis component of the far-infrared electronic response of detwinned YBa2Cu3$^{16,18}$O$_{6.9}$. For $^{18}$O a pronounced low-energy electronic mode (LEM) appears around 240 cm$^{-1}$. This a-axis LEM exhibits a clear aging effect, after one year it is shifted to 190 cm$^{-1}$. For $^{16}$O we cannot resolve a corresponding a-axis LEM above 120 cm$^{-1}$. We interpret the LEM in terms of a collective electronic mode that is pinned by `isotopic defects', i.e. by the residual $^{16}$O in the matrix of $^{18}$O.

Site-selective oxygen isotope effect on the magnetic-field penetration depth in underdopedY0.6Pr0.4Ba2Cu3O7−δ

We report site-selective oxygen isotope ${(}^{16}\mathrm{O}{/}^{18}\mathrm{O})$ effect (OIE) measurements on the in-plane penetration depth ${\ensuremath{\lambda}}_{\mathrm{ab}}$ in underdoped ${\mathrm{Y}}_{0.6}{\mathrm{Pr}}_{0.4}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}},$ using the muon-spin rotation technique. A pronounced OIE on the transition temperature ${T}_{c}$ as well as on ${\ensuremath{\lambda}}_{\mathrm{ab}}^{\ensuremath{-}2}(0)$ was observed, which mainly arises from the oxygen sites within the superconducting ${\mathrm{CuO}}_{2}$ planes (100% within error bar). The values of the corresponding relative isotope shifts were found to be $\ensuremath{\Delta}{T}_{c}{/T}_{c}=\ensuremath{-}3.7(4)%$ and $\ensuremath{\Delta}{\ensuremath{\lambda}}_{\mathrm{ab}}^{\ensuremath{-}2}(0)/{\ensuremath{\lambda}}_{\mathrm{ab}}^{\ensuremath{-}2}(0)=\ensuremath{-}6.2(1.0)%.$ Our results imply that in this compound the phonon modes involving the movement of planar oxygen are dominantly coupled to the electrons.

Modelling nitrogen and oxygen isotope fractionation during denitrification in a lacustrine redox-transition zone

The stable isotope composition (δ 15N and δ 18O) of nitrate was measured during Summer 1999 in the anaerobic hypolimnion of eutrophic Lake Lugano (Switzerland). Denitrification was demonstrated by a progressive nitrate depletion coupled to increasing δ 15N and δ 18O values for residual nitrate. Maximum δ 15N and δ 18O values amounted to 27.2 and 15.7‰, respectively. 15N and 18O enrichment factors for denitrification (ε) were estimated using a closed-system model and a dynamic diffusion-reaction model. Using the Rayleigh equation (closed-system approach), we obtained ε values of −11.2 and −6.6‰ for nitrogen and oxygen, respectively. The average ε values derived using the diffusion-reaction model were determined to be −20.7 ± 3.8 for nitrogen and −11.0 ± 1.7 for oxygen. Both N and O isotope fractionation appeared to be lower when denitrification rates where high, possibly in association with high organic carbon availability. In addition, variations in the isotope effects may be attributed to the variable importance of sedimentary denitrification having only a small isotope effect on the water column. The combined measurement of N and O isotope ratios in nitrate revealed that coupled nitrification-denitrification in the open-water was of minor importance. This is the first study of nitrogen and oxygen isotope effects associated with microbial denitrification in a natural lake. Moreover, this study confirms the high potential of δ 18O of nitrate as a valuable biogeochemical tracer in aquatic systems, complementing nitrate δ 15N.

The Online 18O/ 16O Analysis: Development and application

Oxygen is a globally important element, in inorganic compounds it is involved in key structural compounds of the earth, and it is providing functional groups for most organic compounds. Oxygen isotope ratio analyses on these compounds provide information on the oxygen sources and (formation) reactions in which the compound was involved. Therefore generally applicable analytical methods for the δ 18O determination are highly desirable. After a description of online high temperature "carbon reduction" methods for the conversion of oxygen in the compounds mentioned into CO and discussion of problems involved, especially in analysis of inorganic substances like carbonates and silicates, selected examples for the application of these carbon reduction methods will be presented. The well-known correlation of δ 18O values of cellulose (in tree rings) to that of (leaf) water is used since the 1970's as climate indicator. The observed enrichment in 18O in cellulose by about 27 in comparison to leaf water is attributed to an oxygen equilibrium isotope effect between carbonyl groups and water. But not all carbohydrates show this "general" 18O enrichment relative to water. By means of a recently published model a non-statistical oxygen isotope distribution in carbohydrates is postulated. Oxygen isotope discrimination has been recognized long ago as an important principle of authenticity evaluation in food and beverages. Though many basic mechanisms of isotope discrimination in nature are known, the corresponding application of data is often based only on empirical observations in this field. The fact that oxygen in natural organic plant material is derived from three sources (CO2, H2O, O2) with distinct differences in δ 18O and that their incorporation by different reactions implies oxygen isotope effects of different sizes, is therefore of special importance, and it is attempted by means of individual isotopic increments of functional groups to predict global δ 18O values of natural and nature-identical compounds. Vice versa δ 18O values can also give hints on the elucidation of biosynthetic pathways.

Oxygen‐isotope effect on the magnetic properties of Pr0.7Ca0.3MnO3 under strong pulsed magnetic fields

AbstractMagnetization and susceptibility measurements have been performed on oxygen‐isotope substituted (16O and 18O) samples of Pr0.7Ca0.3MnO3 under pulsed magnetic fields up to 35 T. Experiments were carried out within the temperature range 78 to 300 K. Magnetic‐field‐induced phase transition into ferromagnetic state has been obtained in both samples at temperatures up to 200 K. Isotope substitution has been proved to have a significant impact on the temperature and field of this transition. Phase diagrams of the samples obtained from magnetization measurements are presented.

Oxygen and hydrogen isotope effects in an active site tyrosine to phenylalanine mutant of peptidylglycine alpha-hydroxylating monooxygenase: mechanistic implications.

Peptidylglycine alpha-hydroxylating monooxygenase (PHM) and dopamine beta-monooxygenase (DbetaM) are homologous copper-containing enzymes that catalyze an oxygen-dependent hydroxylation of peptide-extended glycine residues and phenethylamines, respectively. The mechanism whereby these enzymes activate molecular oxygen and the C-H bond of substrate has been the subject of numerous studies, and various mechanisms have been put forth. From the magnitude of (18)O isotope effects as a function of substrate structure in DbetaM, an active site tyrosine had been proposed to function in the reductive activation of Cu(II)-OOH to generate a reactive copper-oxo species [Tian et al. (1994) Biochemistry 33, 226]. The presence of a tyrosine residue, Y318, in the active site of PHM was subsequently confirmed from crystallographic studies [Prigge et al. (1997) Science 278, 1300]. We now report extensive kinetic and isotope effect studies on the Y318F mutant form of PHM, analyzing the role of this tyrosine in the catalytic mechanism. It is found that the Y318F mutant has intrinsic hydrogen and (18)O isotope effects that are within experimental error of the wild-type enzyme and that the mutation causes only a slight reduction in the rate constant for C-H bond cleavage. These findings, together with the recent demonstration that C-H activation in PHM is dominated by quantum mechanical tunneling [Francisco et al. (2002) J. Am. Chem. Soc. 124, 8194], necessitate a reexamination of plausible mechanisms for this unique class of copper enzymes.

Possible electron-phonon interaction-driven microscopic origin of the phase transitions in β-Na 0.33 V 2 O 5 and analogues

The microscopic origin of the various phase transitions which have been observed in g-Na 0.33 V 2 O 5 and its analogues can be explored experimentally by investigating the oxygen isotope effect on the phase transition temperatures. An absent oxygen isotope effect on the sodium-ordering transition would point to an antiferroelectrically driven instability, while a large isotope effect on the charge ordering and spin ordering transition temperatures would suggest the formation of a charge-density wave instability and phonon mediated long-range magnetic ordering. All instabilities can be attributed to strong electron-phonon interaction effects, which dominate the ground-state properties.

Unconventional isotope effects in cuprate high-temperature superconductors

A brief review on unconventional oxygen-isotope ( 16 O/ 18 O ) effects (OIE) in cuprate high-temperature superconductors (HTSC) is presented. First the doping dependence of the OIE on the superconducting transition T c in various HTSC and investigations of the site-selective OIE in Y 1− x Pr x Ba 2Cu 3O 7− δ are discussed. These experiments clearly demonstrate that the planar oxygen ions mainly (⩾80%) contribute to the total OIE on T c at all doping levels. Magnetization and magnetic torque OIE studies of the in-plane penetration depth λ ab (0) in La 2− x Sr x CuO 4 indicate a substantial oxygen-mass dependence of λ ab −2(0) which increases with reduced doping. These results are in agreement with recent μSR OIE studies of λ ab −2(0) in polycrystalline samples of Y 1− x Pr x Ba 2Cu 3O 7− δ and La 2− x Sr x CuO 4. The OIE exponents of T c and λ ab −2(0) exhibit a relation that appears to be generic for various families of HTSC. The unusual isotope effects observed in HTSC strongly suggest that lattice effects play an essential role.

Oxygen stoichiometry and isotope effect inLa1−xCaxMnO3+δ

Oxygen stoichiometry in the colossal magnetoresistive La 1 - x Ca x MnO 3 + Φ has been measured using hydrogen reduction on thermobalance. With a thoughtful measurement procedure, the excess oxygen content can be determinedwith an uncertainty of Δδ= ′ 0.003. Applying this method to the oxygen-isotope exchanged samples, we find that the oxygen stoichiometries of the 1 6 O- and 1 8 O-substituted samples are the same. This leads us to conclude, that the observed giant oxygen-isotope effect on the Curie temperature T C in the ferromagnetic manganites is intrinsic and is not caused by a difference in the oxygen contents of the two isotope samples.

Oxygen-isotope effect on crystal and magnetic structures of (La 1-y Pr y ) 0.7 Ca 0.3 MnO 3

The oxygen 16O/18O isotope effect on the crystalline and magnetic structures of (La1-yPry)0.7Ca0.3MnO3 has been studied by neutron powder diffraction in the temperature range from 10 to 293 K. The isotope-enriched samples with 16O and 18O have been found to be identical in crystalline and antiferromagnetic (AFM) structures down to the metallic ferromagnetic (FM) phase transition temperature (TFM) of the 16O sample. At low temperatures, both the crystalline and magnetic structures of the samples are different. The study of the samples with partial isotope substitution shows that both the FM-phase fraction and TFM strongly depend on 18O oxygen content, disappearing at about a 60% concentration of 18O. In comparison, the temperature of the AFM transition does not depend on isotope content.

Isotope effects on the pseudogap in high-temperature superconductors

Inelastic neutron scattering studies of the relaxation rate of crystal-field excitations have provided evidence for large oxygen and copper isotope effects on the pseudogap in high-temperature superconductors. Existing experiments in the slightly underdoped HoBa2Cu4O8 compound will be reviewed, as well as possible implications of such findings on the microscopic mechanism of high-temperature superconductivity. New relevant experiments performed in the La1-xSrxCuO4 family will also be presented.

Oxygen and copper isotope effects on the pseudogap in the high-temperature superconductorLa1.81Ho0.04Sr0.15CuO4studied by neutron crystal-field spectroscopy

The oxygen and copper isotope effects on the relaxation rate of crystal-field excitations in the optimally doped high-temperature superconductor ${\mathrm{La}}_{1.81}{\mathrm{Ho}}_{0.04}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4}$ have been investigated by means of inelastic neutron scattering. For the ${}^{16}\mathrm{O}$ compound there is clear evidence for the opening of an electronic pseudogap in the normal state at ${T}^{*}\ensuremath{\approx}60\mathrm{K},$ far above ${T}_{c}\ensuremath{\approx}32\mathrm{K}.$ Upon substituting ${}^{16}\mathrm{O}$ by ${}^{18}\mathrm{O},$ ${T}^{*}$ is shifted to about 70 K. On the other hand, no shift is found for the copper isotope substitution, i.e., ${T}^{*}{(}^{63}\mathrm{Cu})\ensuremath{\approx}{T}^{*}{(}^{65}\mathrm{Cu})\ensuremath{\approx}60\mathrm{K}.$ These results, together with the large oxygen and copper isotope effects on ${T}^{*}$ for ${\mathrm{HoBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8},$ give evidence that the pseudogap formation in the single-layer LSCO and bilayer YBCO type compounds is governed by an additional local mode in the latter.

Oxygen isotope effect of high-pressure synthesized (Cu,C)Ba 2Ca 3Cu 4O y

The oxygen isotope effect on T c in a four-CuO 2-layer cuprate of (Cu,C)Ba 2Ca 3Cu 4O y ( T c=117 K) has been investigated. The isotope-substituted samples of equivalent quality were successfully synthesized under high pressure directly from precursors which were prepared from the oxides obtained by heating metals of Ba, Ca and Cu in oxygen isotope ( 17O and 18O) atmosphere. We concluded that the oxygen isotope effect ( T c∼ M − α : M is the oxygen mass) is small but finite ( α=0.021±0.006) in this compound. This result agrees with the previously reported isotope effects in optimally doped cuprates of Y-123 and (Bi,Pb)-2223.

Oxygen isotope effect of the plane-copper NQR frequency inYBa2Cu4O8

We report high-precision measurements of the temperature dependence of the plane-${}^{63}\mathrm{Cu}$ NQR line frequency ${\ensuremath{\nu}}_{Q}(\mathrm{Cu}2)$ and the linewidth in normal and superconducting ${}^{16}\mathrm{O}$ and ${}^{18}\mathrm{O}$-exchanged ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}.$ Whereas ${\ensuremath{\nu}}_{Q}(\mathrm{Cu}2)$ passes ${T}_{c}$ very smoothly without a discontinuity either in value or slope, the linewidth increases in the normal conducting phase down to ${T}_{c}$ and starts to decrease sharply in the superconducting phase to finally resume its high-temperature value of the normal phase. There is a well discernible oxygen isotope effect on the ${\ensuremath{\nu}}_{Q}(\mathrm{Cu}2)$ temperature dependencies. The temperature dependence of ${\ensuremath{\nu}}_{Q}(\mathrm{Cu}2)$ is described by an empirical expression consisting of two parts, one related to the thermal expansion of the lattice and the other due to charge redistribution during the formation of new electronic structures in the ${\mathrm{CuO}}_{2}$ planes. From the fit to the experimental data we determine for the conjectured formation of new electronic structures an energy scale $\ensuremath{\Delta}{(}^{16}\mathrm{O})=188.0(1.6) \mathrm{K}$ and $\ensuremath{\Delta}{(}^{18}\mathrm{O})=180.0(1.6) \mathrm{K}.$ This results in a partial oxygen isotope effect coefficient ${\ensuremath{\alpha}}_{{\ensuremath{\nu}}_{Q}}=0.42(11)$ which is larger than both the spin pseudogap coefficient ${\ensuremath{\alpha}}_{\mathrm{PG}}=0.061(8)$ and the ${T}_{c}$ coefficient ${\ensuremath{\alpha}}_{{T}_{c}}=0.056(12)$ [F. Raffa, T. Ohno, M. Mali, J. Roos, D. Brinkmann, K. Conder, and M. Eremin, Phys. Rev. Lett. $81,$ 5912 (1998)].

Influence of oxygen isotope exchange on the ground state of manganites

We report a study of oxygen isotope effects on low temperature specific heat, magnetization, and resistivity of La$_{1-x}$Ca$_{x}$MnO$_{3}$ and (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$. For the metallic compositions of La$_{1-x}$Ca$_{x}$MnO$_{3}$ and for charge-ordered La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ no change in low temperature specific heat has been detected with $^{16}$O -$^{18}$O exchange, while compounds of (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (0.4<y<0.6) show a significant change in low temperature properties. The low temperature specific heat indicates a presence of the charge-ordered phase even in compositions of (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ which are metallic at low temperatures. We suggest that the changes induced by the oxygen isotope exchange are caused by an increase of the charge-ordered phase in $^{18}$O samples.

On the Pseudogap State in Y- and La-Type High-Temperature Superconductors: Doping Dependence, Isotope Effects, and Electronic Properties Studied by Neutron Spectroscopy

The doping dependence as well as the oxygen and copper isotope effects on the pseudogap temperature T* were investigated by neutron spectroscopic experiments of the relaxation rate of crystal-field excitations in rare-earth based Y- and La-type high-temperature superconductors. We found three essential results from our truly bulk-sensitive experiments: (1) For all doping levels we had T* > Tc, even in the optimally doped and overdoped regimes. (2) In bilayer high-Tc compounds we observed huge oxygen and copper isotope effects on T*, which give evidence that the pseudogap formation is governed by lattice modes involving both the oxygen and copper ions. In single layer high-Tc compounds, on the other hand, no shift of T* was found for the copper isotope substitution. (3) Our results obtained for all doping levels support the unusual temperature dependence of the gap function in the pseudogap region reported for underdoped Bi-type compounds, i.e., a breakup of the Fermi surface into disconnected arcs in the temperature range Tc < T < T*.

Clues Obtained from the Oxygen Isotope Effect on NMR/NQR Parameters Observed in YBa 2 Cu 4 O 8

Nuclear magnetic and nuclear quadrupole resonance (NMR, NQR) techniques have a precision allowing one to determine rather small isotope effects. Well-defined oxygen stoichiometry and negligible oxygen diffusion makes YBa2Cu4O8 an ideal compound for studies of small isotope effects that require experimental results not hampered by reproducibility problems. We report on high-precision measurements of the temperature dependence of plane-63,65Cu NMR/NQR parameters such as Knight shift, spin–lattice relaxation rate R = 1/T 1, NQR line frequency νQ and NQR linewidth δνQ, as well as 89Y Knight shift, performed in normal and superconducting 16O and 18O exchanged YBa2Cu4O8.

Microscopic Electron-Phonon Theory of Ferroelectricity in Perovskite Oxides

The energy of the electronic subsystem of oxides (BaTiO 3 , etc.) renormalized by dynamical electron-phonon hybridization of valence 2p oxygen states and conduction 3d Ti states (or Nb, etc. states) are calculated. The dependence of this energy on the electronic wave vector is also taken into account. The free energy is obtained. This free energy allows us to calculate the coefficients postulated in Landau-Ginzburg-Devonshire phenomenological expansion. The oxygen isotope effect of the phase transition temperature T c by substitution O 18 M O 16 in SrTiO 3 and KTaO 3 is calculated. In SrTiO 3 the dependence T c (x) of SrTi( 16 O 1 m x 18 O x ) 3 on concentration x is calculated in a very good agreement with the experiment. For SrTi 18 O 3 T c , 22.4 K. In KTaO 3 the predicted isotope effect does not lead to the phase transition. The isotope effect is caused by the quantum fluctuations and the electron-phonon nature of ferroelectricity.

Oxygen isotope effect inLa0.8Ca0.2MnO3+δand its dependence on oxygen stoichiometry

The oxygen isotope effect of the ferromagnetic transition temperature was investigated in various samples of the manganite ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_{3+\ensuremath{\delta}},$ as a function of sintering conditions. Sintering at 1200 \ifmmode^\circ\else\textdegree\fi{}C and above leads to oxygen-stoichiometric comparison samples with an isotope exponent of ${\ensuremath{\alpha}}_{\mathrm{O}}=0.41\ifmmode\pm\else\textpm\fi{}0.02,$ and transition temperatures ${T}_{C}(16)=189\ifmmode\pm\else\textpm\fi{}2 \mathrm{K}$ and ${T}_{C}(18)=180\ifmmode\pm\else\textpm\fi{}2 \mathrm{K}.$ Samples sintered at 1000 \ifmmode^\circ\else\textdegree\fi{}C, and below, always have oxygen excess. The oxygen excess leads to vacancies at the $A(\mathrm{L}\mathrm{a},\mathrm{C}\mathrm{a})$ and $B(\mathrm{Mn})$ sites, leading to a decrease in unit cell volume. The unit cell volume is identical for both isotopes, and this implies equal oxygen content for the isotopic comparison samples. The isotope exponent ${\ensuremath{\alpha}}_{\mathrm{O}}$ for oxygen excess samples is much larger than 0.4. Its value, as well as the transition temperatures ${T}_{C},$ depends on the ${\mathrm{Mn}}^{4+}$ concentration, as well as the defect structure induced by A and B sites vacancies.

Isotope Effects of Water Molecules on the Hydration of Alkali Metal and Halide Ions

Abstract Based on the results of hydrogen isotope fractionation in aqueous alkali halide solutions at 25 °C, reported in previous papers, the hydrogen isotope effect between water molecules hydrated to a cation or an anion and bulk water molecules is estimated with the assumption that the equilibrium constant of the hydrogen isotope exchange reaction between the hydration water of sodium ion and bulk water is unity. The orders of the hydrogen isotope effects of alkali metal cations and halide anions are found to be Cs+ &amp;gt; K+ &amp;gt; Na+ &amp;gt; Li+, and I− &amp;gt; Br− &amp;gt; Cl− for a constant hydration number, respectively, and the hydrogen isotope effect is mainly influenced in anions rather than in cations. The oxygen isotope effect for alkali and halide ions is evaluated from the literature. The hydrogen and oxygen isotopic effects for the cations and anions show the same trend, but the magnitude of the hydrogen isotope effect for anions is significantly larger than that of the oxygen isotope effect. The free energy of transfer of alkali metal and halide ions from H2O to HDO is calculated from the hydrogen isotope effect for the individual ions. The calculated values are compared with the free energy of transfer from H2O to D2O obtained by electromotive force measurements, and are compared with the free energy of transfer from H216O to H218O calculated from the oxygen isotope effect.