Motion Of Oxygen Atoms Research Articles

Theory of harmonic dissipation in disordered solids

Mechanical spectroscopy, i.e. cyclic deformations at varying frequencies, is used theoretically and numerically to measure dissipation in model glasses. From a normal mode analysis, we show that in the high-frequency THz regime where dissipation is harmonic, the quality factor (or loss angle) can be expressed analytically. This expression is validated through non-equilibrium molecular dynamics simulations applied to a model of amorphous silica (SiO$_2$). Dissipation is shown to arise from non-affine relaxations triggered by the applied strain through the excitation of vibrational eigenmodes that act as damped harmonic oscillators. We discuss an asymmetry vector field, which encodes the information about the structural origin of dissipation measured by mechanical spectroscopy. In the particular case of silica, we find that the motion of oxygen atoms, which induce a deformation of Si-O-Si bonds is the main contributor to harmonic energy dissipation.

Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia

Inelastic neutron scattering measurements on monoclinic zirconia (ZrO_2) and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to 1373wK. Temperature-dependent phonon densities of states (DOS) are reported, as are Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Gruneisen parameters. These mode Gruneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhat more anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.

Changes of birefringence and fine structure tensor in KSc(MoO4)2 induced by ferroelastic phase transitions

Phase transitions in KSc(MoO4)2 doped with Cr3+ are studied by the electron paramagnetic resonance and birefringence techniques in the temperature range covering the three ferroelastic phases. The study is aimed at showing in what way a morphic birefringence in the ferroelastic phases in KSc(MoO4)2 may relate to the deformation of the coordination surroundings of Sc3+ ion seen in this crystal via changes of the fine structure tensor D. Based on the motion of oxygen atoms of the MoO4 groups constituting the environment of Sc3+ions, a new model of the crystal structure deformation on transition from the ferroelastic phase II to the phase III is built. This model accounts for the morphic birefringence and changes in the orientation of the optical indicatrix axes in KSc(MoO4)2.

Zero-Point Motion of Oxygen and Anomalous Isotope Effect in Sr2RuO4

Anomalous behavior of the isotope effect coefficient α of Sr 2 RuO 4 under impurity scattering is discussed on the basis of the d – p model. By taking into account the anharmonic zero-point motion of oxygen atoms, we obtain a new relation for impurity dependence on α, reproducing the behavior that α is negative in pure sample, but it becomes positive with small amount of impurities for a reasonable set of parameters.

Lattice Effects in High-Temperature Superconducting Cuprates Revealed by Thin Film Single Crystals

In contrast to conventional superconductivity where phonons lead to the formation of Cooper pairs, in high-temperature superconductivity (HTSC), the role of electron–phonon coupling has long been neglected. The in-plane Cu–O bonds in HTSC cuprates show unconventional broadening at low temperature as carriers are doped. Here, we focus on the high-quality polarized X-ray absorption spectroscopy (XAS) data for a model HTSC system, (La,Sr)2CuO4 (LSCO). Thin film single crystal samples were prepared by state-of-the-art MBE, precisely controlling compositions. High-quality data was obtained by use of a segmented X-ray detector. The in-plane Cu–O radial distribution function (RDF) in LSCO (x=0.15) shows broadening as temperature is lowered, which shows a sharp drop at the critical temperature, which is followed by a gradual increase (disorder). Comparing the data with resistivity, we find a remarkable coincidence between the sharpening and the onset of superconductivity. Since the sharpening of RDF is interpreted as correlated motion of oxygen atoms (phase coherence due to superconductivity), the results demonstrate that the superconducting state directly relates to the unconventional oxygen displacements in a bond stretching mode. The results will be discussed in relation to local models of distortion of the different nature (metallic vs. insulating), that is, strongly influenced by strain.

NMR and NQR study of atomic motion below 500 K inYBa2Cu3O7

Measurements of the intensity of the {sup 63}Cu NQR and quadrupolar-satellite NMR at elevated temperatures are reported for the planar Cu(2) sites in samples of nominal composition YBa{sub 2}Cu{sub 3}O{sub 7}. It is observed that spin-echo intensities decrease dramatically between room temperature and 500 K and that the onset of intensity loss occurs at lower temperatures in more defective samples. Measurements of the spin-echo decay rates reveal that the intensity loss is associated with loss of phase coherence exceeding that expected from spin-spin and spin-lattice relaxation. By means of a simple, illustrative model, these effects can be attributed to changes in the local atomic environment, presumed to be caused by motion of oxygen atoms in the Cu(1) layer. These motions occur on a time scale on the order of 10 {mu}s at 500 K and their probability is enhanced at lower temperatures by the presence of structural defects. {copyright} {ital 1997} {ital The American Physical Society}

Mössbauer study of oxygen-related lattice instabilities in oriented 1-2-3 HTSC

57Fe Mossbauer effect measurements as a function of temperature between 78 and 293 K in grain-aligned YBaCuO (Tc=89 K, ΔTc<2 K) with bothc∥γ andc⊥γ orientations revealed a sequence of anomalies in the recoilless fraction, center shift and linewidth of all57Fe quadrupole doublets in the host lattice. These anomalies are believed to arise due to antiferroelectric phase transitions driven by a nonlinear anharmonic vibrational motion of oxygen atoms in a breathing mode configuration. The results obtained are indicative of a possible coexistence of high-Tc superconductivity and antiferroelectricity in 1-2-3 ceramics.

Lattice dynamical calculations of anisotropic temperature factors of atoms in quartz, and the structure of ?-quartz

Temperature factors for oxygen and silicon atoms in β-quartz were calculated on a Born-von Karman lattice dynamical model of an ordered structure. The calculated thermal ellipsoids were in excellent agreements with those of the previous structure refinements of the order model, for both magnitudes and orientations of the principal axes. The temperature factors are contributed mainly by the soft optic modes in Γ-M and the lowest-lying acoustic modes along Γ-A, which are also strongly temperature-dependent. The cusp-shaped temperature dependence of mean square displacements, 〈u2〉, of oxygen atom, observed previously around the α-β transition, are resulted from the softening of these modes. The temperature-dependent modes in Γ-A were also found to cause diffuse scattering extending along ±c* of the fourth hexagon of the hk0 reciprocal lattice plane. The negative expansion known in β-quartz were interpreted in terms of asymmetrical forces exerting on oxygen atoms in Si-O-Si bending modes. In β-quartz, librational motions of oxygen atoms around Si-Si lines with large amplitudes, whose center is just on the β-position of high symmetry, must be possible under the condition that bending tetrahedral O-Si-O angles is energetically more favourable than compressing or stretching Si-O bonds.

Internal friction due to oxygen relaxation in superconducting YBa2Cu3O7−δ above Tc

YBa2Cu3O7−δ has an internal friction peak near 210 °C measured at 1.18 Hz. The shift in the temperature of this peak with frequency as measured with different internal friction techniques indicates that the relaxation process has an average activation energy of 1.14 ± 0.05 eV and a pre-exponential factor, τ0 = 1.5 × 10−13s (log τ0 = −12.82 ± 0.23). The peak height decreases during annealing at above 375 °C in helium and increases when annealed in oxygen. The mechanism for the relaxation is believed to be the stress-induced motion of oxygen atoms in the Cu–O plane. Detailed analysis shows that the internal friction peak is determined by a spectrum of activation energies; this spectrum is a symmetrical single peak 0.12 eV wide at half-height. Models for the relaxation are considered and the resulting diffusivities are compared to tracer diffusion results.

Quadrupolar relaxation and hopping motion of oxygen in YBa2Cu3O7-x.

The quadrupole interaction at /sup 111/Cd probe nuclei in YBa/sub 2/Cu/sub 3/O/sub 7-//sub x/ has been studied over the temperature range 30--1100 K by the time-differential perturbed angular correlation technique. The highly asymmetric electric field gradient (EFG) with eta = 1 suggests Cu(1) site occupancy for the parent /sup 111/In atoms. The quadrupolar relaxation is attributed to a time-dependent component of the EFG due to the hopping motion of oxygen atoms. Activation energy for oxygen jumps is estimated to be 14 +- 4 meV on the basis of the variation of the relaxation parameter lambda with temperature.

NMR Study of the Dynamic Motion of Rare Spin 17O in Solids

NMR study of the dynamic motion of oxygen atoms in solids is not readily accessible because of the poor natural abundance (0.037%) of 17 O spins. We report that this study is possible by the measurement of T 1ρ in some solids thanks to two kinds of interactions (A) and (B): (A) modulation of the dipole-dipole interaction between 1 H and 17 O spins by rotational diffusion (reorientation) of ions containing oxygen atoms and (B) the cross relaxation between 1 H and 17 O spins. T 1ρ of 1 H has been measured in some solids, (NH 4 ) 2 S 2 O 8 , NH 4 HSO 4 , KHSO 4 and NH 3 CH 3 ClO 4 . Owing to the interaction (A), we may infer the detail for the reorientation of ions containing oxygen atoms, and determine the spin-lattice relaxation time and the quadrupole coupling constant e 2 q Q / h of 17 O owing to the interaction (B): the values of e 2 q Q / h are 9.6 MHz and 9.7 MHz in (NH 4 ) 2 S 2 O 8 and NH 3 CH 3 ClO 4 , respectively.

Spin-Lattice Relaxation of Proton in Ammonium Perchlorate: Effect of the Motion of Oxygen Atoms

Spin-lattice relaxation times T 1ρ of protons ( 1 H) in the rotating frame were determined in ammonium perchlorate NH 4 ClO 4 between 556 K and 125 K. The temperature dependence of T 1ρ shows two minimum values 1.5 sec at about 476 K and 0.94 sec at 238 K for an r f field H 1 =2.1 gauss. The origins of the minima T 1ρ are attributed to the motion of oxygen atoms 17 O of natural abundance (0.037%). Oxygen atoms affect T 1ρ of protons in different two ways. The minimum of T 1ρ at higher temperature is due to the cross relaxation between 1 H and 17 O spins, while that at low temperature due the the reorientation of ClO 3 -ions which modulates the dipolar interaction between 1 H and 17 O and also due to the cross relaxation between 1 H and 17 O. The value e 2 q Q / h =12.3 MHz was determined for 17 O in ClO 4 -ion.

A NMR study of the motion of oxygen atoms in solids

The authors report on the motion of oxygen atoms in some ammonium salts as studied by the measurement of the spin-lattice relaxation time T1 rho , of protons in the rotating frame. The temperature dependence of T1 rho for protons shows extremely large minimum values (T1 rho )min=1-3 sin NH4ClO4, (NH4)2-Cr2O7, (NH4)2S2O8 and (NH4)2Ce(NO3)6 at relatively high temperatures ( approximately=300K). T1 rho has also been measured for protons in partially deuterated ammonium perchlorate and ammonium dichromate. It is found that values of (T1 rho )min are independent of the concentration of the deuterons. The large minimum values, 1-3 s, or T1 rho are attributed to the reorientations of the anions that modulate the dipolar interaction between the protons in ammonium ions and the natural abundance of 17O(0.037%) in the anions. The activation energies for the reorientations of the anions ClO4, CrO3, NO3 and SO3 were also determined.