Asymmetry Parameter Eta Research Articles

A155Gd Mossbauer study of GdBa2Cu3O7-x: results concerning crystal structure and dynamics

Two samples of the compound GdBa2Cu3O7-x, one with x approximately 0 (Tc=93.5 K) and one with x>0.5 (Tc<1.8 K), have been studied by 155Gd Mossbauer spectroscopy in the temperature range 1.5 K<or=T<or=120 K. A non-zero value of the asymmetry parameter eta in both samples shows the point symmetry at Gd sites to be orthorhombic at low temperatures. The narrow linewidth implies a high degree of perfection of the authors' samples. Significant differences of the frequency spectra of Gd ions between the two samples are derived from the temperature dependence of the absorption cross sections and of the centre shifts.

Magnetoelastic coupling at Fe2+ions in antiferromagnetic CoCl2

The57Fe Mossbauer spectrum of a single crystal of (Fe0.002Co0.998)Cl2at 4.2K has been recorded and analysed in terms of the effective-field approximation to the hyperfine interactions. At this temperature, CoCl2(space group R3m) is antiferromagnetically ordered, with moments aligned in the a-b plane of the hexagonal unit cell. It was found that the Mossbauer spectrum, which indicated both magnetic dipole and electric quadrupole splitting, could not be adequately explained using a static crystal-field model (a 'rigid-crystal' model) for the wavefunctions of the Fe2+ion in a site of 3m (D3d) symmetry. In order to obtain a satisfactory interpretation of the data it was necessary to allow the orbit-lattice interaction to generate symmetry-breaking strains at the Fe2+site as a result of the transverse exchange interaction. The electric field gradient asymmetry parameter eta proved to be a sensitive indicator of the presence of such strains. The theory of calculating these strains is outlined. The magnitudes of the strains required to satisfy the Mossbauer data are consistent with the calculations, although estimates of the values of the orbit-lattice coupling parameters are very approximate. These results are analogous to the magnetoelastic effects produced in the uniaxial antiferromagnet FeCl2by a transverse applied field, and they indicate the possible observability of magnetoelastic effects in susceptibility and Mossbauer spectra of non-magnetic crystals of appropriate symmetry.

Sign determination of the57Fe quadrupole splitting in an amorphous Fe24Zr76alloy

57Fe Mossbauer spectra of an amorphous Fe24Zr76 alloy obtained both in the paramagnetic state and in applied magnetic fields show that the quadrupole splitting (QS) is almost exclusively positive. The spectra are consistent with a normally distributed QS of positive mean and a mean asymmetry parameter eta =0.67+or-0.07. The results are in contradiction with the predictions of dense random packing models and indicate the existence of strong short-range order.

Structural short-range order in amorphous rare-earth alloys from Mossbauer spectroscopy and EXAFS

By combining 151Eu Mossbauer spectroscopy (on the Eu2+ S-state ion) and EXAFS (extended X-ray absorption fine structure) measurements in a series of amorphous RE1-xXx (RE=rare earth, X='sp' metal) the authors have investigated the angular correlations and the radial distribution functions (RDF) of the local atomic arrangement restricted to the first neighbour shell. The electric field gradient (EFG) parameters, including the modulus of qz, its balance in sign and the marginal distribution of the asymmetry parameter eta , display strong dependences on the nature of the element X. These various distributions rule out a single description of short-range order and they are discussed by reference to models of structure. The shape of the first peak of the partial RDF centred on the major (RE) and on the minor (X) atomic components is asymmetric, with the extent of asymmetry depending upon the element X. No significant structural change is detected as a function of the rare-earth element, including rare earths with different valence (Eu2+, Gd3+). The degree of structural short-range order is evaluated consistently from the EFG distribution and from the asymmetry of the RDF as determined from EXAFS. The decrease of the short-range order in the sequence Au, Ga, Al ... is correlated with a decreasing enthalpy of formation of the corresponding crystalline compounds. On the other hand, no clear relationship can be established between short-range order in amorphous and crystalline phases. In the new amorphous metals Eu70Mg30, Eu70Zn30 and Eu75Cd25 which have been studied here, one observes similarly (from the EFG distribution) that the order is correlated with the enthalpy of formation.

Electric field gradient in copper alloys

The electric field gradient q and asymmetry parameter eta are calculated for non-magnetic dilute copper alloys. Both the valence and size-effect contributions are evaluated explicitly. The transition metal model potential theory is used to evaluate the contribution from the valence effect. The formalism due to Sagalyn and Alexander (1977) is adopted to evaluate the size-effect contribution. Only the size-effect parameter lambda is introduced to explain q and eta , whose calculated values are found to be in close agreement with the experimental values. The effective hardness of fourth row impurities is found to be larger than that of the fifth row impurities. The magnitudes of q and eta are found to be proportional to the effective charges on impurities if the evaluation of these charges includes both the size effect and bound state.

Phase transitions in phosphatidylcholine multibilayers.

The (2)H NMR spectrum of a multilamellar dispersion of 1-myristoyl-2-[14,14,14-(2)H(3)]myristoyl-sn-glycero-3-phosphocholine with 1 mol% cholesterol in excess water has been recorded at temperatures between -15 degrees C and 36 degrees C. Motionally averaged quadrupole coupling constants nu(Q) and motionally induced asymmetry parameters eta are obtained by spectral analysis. Values of these quantities indicate that, at temperatures below -4 degrees C, any rotational motion of the molecules about their molecular long axis is slow on the NMR time scale. At temperatures immediately above the pretransition these same parameters show that a fast-rotational motion is occurring about the molecular long axis. This rotational motion is hindered in that the molecules flip about a twofold symmetry axis. Between -4 degrees C and the pretransition, spectra appear as the superposition of two powder patterns, one corresponding to the pattern observed below -4 degrees C and the other to the pattern above the pretransition. The relative contribution of the latter increases with temperature until the pretransition is reached. These data have been interpreted in two ways: either the sample between -4 degrees C and the pretransition contains two populations of rapidly and slowly rotating molecules, or there is only a single population of molecules undergoing a 180 degrees flipping motion on the time scale of the NMR measurement. The latter interpretation is more consistent with other experimental findings. At the temperature of the main transition the hydrocarbon chains melt. In the absence of cholesterol, spectra are more complex in that the line shape is reproduced by the superposition of three spectral powder patterns between -4 degrees C and the pretransition and by the superposition of two spectral patterns above the pretransition. It is postulated that these two patterns observed above the pretransition are in direct correspondence to the two ripple structures observed by freeze-fracture electron microscopy in the absence of cholesterol.

Dynamics of amino acid side chains in membrane proteins by high field solid state deuterium nuclear magnetic resonance spectroscopy. Phenylalanine, tyrosine, and tryptophan.

We have obtained the first deuterium NMR spectra of individual types of aromatic amino acids in a defined membrane protein, bacteriorhodopsin, in the photosynthetic purple membrane of Halobacterium halobium R1. Isotopic labeling and high field (8.5 Tesla) operation permitted relatively rapid data acquisition at a variety of temperatures. At the temperature of growth (37 degrees C), we find that all 7 tryptophan residues are rigid on the time scale of the NMR experiment (approximately 10(-5) s), except for likely librational motions of approximately 10 degrees amplitude. By contrast, nearly all (9 +/- 2) of the 11 tyrosines and (13 +/- 2) 13 phenylalanines undergo rapid (greater than 10(5) s-1) 2-fold rotational flips about C gamma-C zeta, causing formation of line shapes dominated by effectively axially asymmetric (asymmetry parameter eta = 0.66) deuteron electric field gradient tensors. On cooling the phenylalanine- and tyrosine-labeled samples to approximately -30 degrees C, all such motions are frozen out, i.e. occur at rates less than 10(4) s-1, and axially symmetric (eta approximately 0.05) line shapes are observed. At T greater than 91 degrees C, phenylalanine-, tyrosine-, and tryptophan-labeled membrane spectra undergo dramatic narrowing to an isotropic line of approximately 4-9 kHz width. This transition is a reflection of the loss of tertiary structure in the membrane protein with resultant fast unrestricted motion of these aromatic side chains, and is only partly reversible. These results, in conjunction with those obtained using [gamma-2H6]valine-labeled bacteriorhodopsin (Kinsey, R. A., Kintanar, A., Tsai, M-D., Smith, R. L., Janes, N., and Oldfield, E. (1981) J. Biol. Chem., 256, 4146-4149) indicate the rather rigid nature of amino acid side chains in the H. halobium purple membrane, the principal fast lage amplitude motions being methyl group rotation and discontinuous benzene ring "flipping."

The NMR and electric-field gradients in σ phase vanadium alloys

Nuclear quadrupole effects in the 51V and 59Co NMR in sigma phase V-Co and V-Ni are studied for various alloying concentrations. Both 51V and 59Co spectra in V-Co are used to compare the quadrupole moments of the two nuclei and to reconfirm the ratio mod Q(59Co) mod / mod (51V) mod approximately=10. Local-environment effects are studied in quadrupole splittings of 51V resonance in both V-Co and V-Ni, and are compared with the calculated values taking into account only atoms on the coordinating polyhedron around the specific atomic site. The changes in the asymmetry parameter eta of the electric-field gradient at certain atomic sites sensitively reflect the local arrangement of atoms. The experimental values of e2qQ/h and eta for 51V are 2.6 MHz and about 0 for site II, and 2.2 MHz and about 1 for sites III and V.

Application of TDPAC techniques to diffusion models: diffusion mechanism of Ag in In

The general problem of anomalous diffusion is discussed in connection with the way in which the TDPAC (time differential perturbed angular correlation) technique may help determine the relative fraction of substitutional or interstitial positions of the diffusing atoms. The TDPAC technique referred to the Ag-In system is outlined. The present measurements give a fraction of 0.89+or-0.21 of Ag solutes in In substitutional positions. This suggests a dissociative mechanism for Ag diffusion in In. The value of 0.6+or-0.5 is obtained for the electric field gradient asymmetry parameter eta 2 of the assumed interstitial positions. This value shows a low symmetry for the interstitial sites, indicating that these positions might be split interstitials of Ag solutes in In.

The electric quadrupole interaction in ZrO2and HfO2

The electric field gradients in ZrO2 and HfO2 have been measured by applying the differential perturbed angular correlation method to the 133-482 keV cascade in 181Ta. The results for HfO2 have been compared with previous results. In general, a fairly good agreement has been found except for the electric field asymmetry parameter eta , in which the older result of Gerdau (1969) et al, rather than the more recent one of Gardner and Prestwich (1970) has been confirmed. The results for ZrO2 show the electric field gradient to be (13.3+or-0.7)*1017 V cm-2 which is slightly higher than the value of (12.8+or-0.6)*1017 V cm-2 obtained for HfO2. The corresponding values for the field asymmetry are 0.335+or-0.015 for ZrO2 and 0.348+or-0.016 for HfO2. Despite the small difference of only 4% between the corresponding means, the 't' distribution shows, with confidence of better than 99%, that the results for ZrO2 and HfO2 are different.

Magnetic Resonance Properties of Some Lunar Material

Paramagnetic resonance spectra of Apollo 11 fines and rocks were measured at 9 and 35 gigahertz and at 4 degrees , 80 degrees , and 300 degrees K. At both frequencies the material has an intense absorption at g = 2, with a line width of approximately 950 gauss. Fe ions with strong exchange interactions produce this resonance. A comparison of the resonance absorption due to Fe(3+) showed that the energy of the crystal field interaction was approximately 0.1 per centimeter. Mn(2+) was identified in several samples, and an absorption at g = 1.89 was tentatively attributed to Ti(3+). The nuclear magnetic resonance spectra of (27)Al had a distribution of asymmetry parameters eta ranging from 0.25 to 0.75 and had nuclear quadrupole coupling constants e(2)qQ/h of approximately 3 megahertz.