High-resolution measurement of NMR-ON for191PtFe

NMR-ON measurements on¹⁹¹PtFe were repeated to look for a reported quadrupole splitting effect: a sample annealed at 850°C and measured at 15.1(6) mK gives a spectrum with a single gaussian peak with a line width of 0.39(4) MHz, the narrowest ever observed. The spectrum obtained with another sample annealed at 650°C and measured at 8.3(4) mK resembles one with a quadrupole splitting, but the ratio of the second sub-resonance to the first is inconsisten with the estimation from a model which takes account of a quadrupole splitting and fast relaxation limit. The present study shows the quadrupole splitting ofv _Q (¹⁹¹PtFe)≤0.86 MHz.     

The DFT–DVM theoretical study of the differences of quadrupole splitting and the iron electronic structure for the rough heme models for α- and β-subunits in deoxyhemoglobin and for deoxymyoglobin

Quantum chemical calculations of the iron electron structure and ⁵⁷Fe quadrupole splitting were made by density functional theory and Xα discrete variation method for the rough heme models for α- and β-subunits in deoxyhemoglobin and for deoxymyoglobin accounting stereochemical differences of the active sites in native proteins. The calculations revealed differences of quadrupole splitting temperature dependences for three models indicating sensitivity of quadrupole splitting and Fe(II) electronic structure to small variations of iron stereochemistry.     

Correlations among hyperfine parameters in amorphous metal systems: Mössbauer linewidth asymmetries and fluctuation hyperfine correlation functions

The microscopic correlations among hyperfine parameters in amorphous metal systems appear in the Mössbauer linewidth asymmetries. The fluctuation hyperfine correlation functions can be derived using the formalism of Lines, Eibschutz and coworkers. This approach has been reviewed in detail in this paper. A quantitative estimate of the role of the anisotropic component of the hyperfine field toward linewidth asymmetries is presented. It is stressed that contrary to earlier claims, this approach does not give a correct measure of the variance of quadrupole splitting. It is not, therefore, possible to pass judgement on the validity of a linear correlation between quadrupole splitting and hyperfine field or quadrupole splitting and isomer shift. The experimental evidence against a linear correlation between the hyperfine field and the isomer shift, however, remains valid. A simple procedure for checking the validity of a linear correlation between the isomer shift and the magnitude of the quadrupole splitting is suggested. Critical comments on the method of analysis are given.     

Double resonance NMR-on study on oriented188Ir in iron

The quadrupole splitting of oriented¹⁸⁸Ir in iron was studied at 8.6 mK using a double resonance NMR-ON method. With r.f. power applied at the strongest resonance frequency, a second frequency was used to simultaneously investigate the second resonance component, where the splitting is caused by an electric quadrupole interaction. The electric hyperfine splitting frequency ν_Q=e ² qQ/h was measured to be 3.37(11) MHz. With the known electric field gradient of −0.283(6)×10¹⁷ V/cm² at Ir in iron, the spectroscopic quadrupole moment of¹⁸⁸Ir was deduced to be 0.492(26)b. The present results show that the double resonance method is a powerful tool in establishing the quadrupole splitting, if it leads to well-resolved NMR-ON resonance components.     

Ab initio study of the 57Fe quadrupole splitting in the heme models of α- and β-subunits in tetrameric deoxyhemoglobin

Ab initio Xα discrete variation method was used for calculation of quadrupole splitting for the rough heme models in α- and β-subunits of tetrameric deoxyhemoglobin accounting small stereochemical variations. The differences of theoretical values of quadrupole splitting for these heme models were obtained.     

Double nuclear magnetic resonance in the weak ferromagnet YCrO3

Applications of 2D-NMR spectroscopy to measurements of the quadrupole splitting in magnetic materials are discussed using the weak ferromagnet YCrO₃ as a model system. A simple qualitative dissussion and results from computersimulations are given for various experiments. In systems with distributed electric field gradients the method provides a unique way to measure hyperfine field, quadrupole splitting and correlations between these local parameters.     

Differences of the quadrupole splitting and iron electronic structure for the heme models for α and β subunits in deoxyhemoglobin and for deoxymyoglobin: Quantum-chemical calculations by the DFT-DVM method

The quantum-chemical calculation of the iron electronic structure and ⁵⁷Fe quadrupole splitting have been performed by the DFT-DVM method for rough heme models for α and β subunits in deoxyhemoglobin and for deoxymyoglobine, which take into account stereochemical differences of the active cites in native proteins. The calculations revealed differences in the temperature dependences of quadrupole splitting for the three models, indicating sensitivity of the quadrupole splitting and Fe(II) electronic structure to small stereochemical variations in the nearest iron environment. The theoretical results confirmed the possibility of approximating experimental Mössbauer spectra of tetrameric hemoglobins with allowance for the nonequivalence of the Fe(II) electronic structure in nonidentical subunits.     

A resonant γ-ray study of iron (III) bis and tris dithio chelates

The Mössbauer spectra of iron (III) bis and tris dithiocarbamates containing the piperidylradical have been investigated at various temperatures. The tris chelate shows a quadrupole splitting decreasing when the sample is warmed up which indicates the presence of a Boltzmann spin equilibrium. The bis chelate yields a temperature-independent quadrupole splitting due to the lattice contribution.Work sponsored in part by G.N.S.M. and in part by C.N.R.     

Nuclear quadrupole resonance in the gamma-resonance spectra of soft matter

It is shown that the harmonic librations (oscillations) of the principal axis of the electric field gradient tensor in “cages” of liquids, glasses, ferroliquids, and other “soft” systems qualitatively change the shape of the Mössbauer spectra of the quadrupole hyperfine structure. In addition to an effective decrease in the quadrupole coupling constant in the fast-libration limit, nuclear quadrupole resonance is predicted, which must be manifested in the Mössbauer spectra at the libration frequency that is approximately equal to the quadrupole splitting of spectral lines. By analogy with nuclear magnetic resonance, simple analytical expressions are derived, which describe resonance Mössbauer spectra in terms of the effective quadrupole coupling constant and the resonance splitting constant for the main lines. The observed features of the formation of quadrupole hyperfine structure spectra can be manifested in the Mössbauer spectra of soft matter and must be taken into account in analysis of experimental data.     

Mössbauer study on M1 and M2 sites of Fe2+ in natural hypersthene under dynamic crystal field potential approach

⁵⁷Fe Mössbauer spectra of hypersthene, a natural silicate mineral belonging to the orthopyroxene group, have been taken over the temperature range 77–292 K. At temperatures above 77 K, they show asymmetric quadrupole peaks. This asymmetry arises from the overlapping of two quadrupole doublets from Fe²⁺(3d⁶,⁵D) ions in two different sites (M1 and M2). The quadrupole splitting, isomer shift and their temperature dependence are appreciably different for Fe²⁺ ions in the two sites. The Fe²⁺ quadrupole splitting in the M1 site decreases linearly with temperature, which can be explained quite satisfactorily by using a very simple model of the orbit-lattice interaction.     

Splitting between quadrupole modes of dilute quantum gas in a two-dimensional anisotropic trap

We consider quadrupole excitations of quasi-two-dimensional interacting quantum gas in an anisotropic harmonic oscillator potential at zero temperature. Using the time-dependent variational approach, we calculate a few low-lying collective excitation frequencies of a two-dimensional anisotropic Bose gas. Within the energy weighted sum-rule approach, we derive a general dispersion relation of two quadrupole excitations of a two-dimensional deformed trapped quantum gas. This dispersion relation is valid for both statistics. We show that the quadrupole excitation frequencies obtained from both methods are exactly the same. Using this general dispersion relation, we also calculate the quadrupole frequencies of a two-dimensional unpolarized Fermi gas in an anisotropic trap. For both cases, we obtain analytic expressions for the quadrupole frequencies and the splitting between them for arbitrary value of trap deformation. This splitting decreases with increasing interaction strength for both statistics. For a two-dimensional anisotropic Fermi gas, the two quadrupole frequencies and the splitting between them become independent of the particle number within the Thomas-Fermi approach. Received 21 September 2001 and Received in final form 9 December 2001     

Quadrupole splittings and hyperfine fields in CdCr2x In2−2x S4 spin glass system

CdCr_2x In_2−2x S₄ spinels show spin glass and reentrant properties. The quadrupole splitting variations with composition and temperature are determined from paramagnetic spectra and interpreted from crystal field theory and partial quadrupole splitting treatment.     

Possibilities of M?ssbauer spectroscopy with a high velocity resolution in studying small variations in 57Fe hyperfine parameters of iron-containing proteins

The possibilities of M?ssbauer spectroscopy with a high velocity resolution in investigating small variations in the ⁵⁷Fe hyperfine parameters of iron-containing proteins are demonstrated. Differences of quadrupole splitting for human and rabbit oxyhemoglobins were observed. Variations of quadrupole splitting and isomer shift for human liver ferritin and its model Imferon were revealed. Small differences in these parameters were also found for iron-storage proteins in normal chicken liver and spleen tissues and in such tissues from chickens with lymphoid leukemia.     

The influence of the conduction electrons on the EFG in amorphous intermetallic alloys

The quadrupole splitting in alloys is determined by the structural and electronic properties of the alloy. Only in cases where the density of states at the Fermi level is constant or known as a function of the concentration, relevant structural information can be obtained from the quadrupole splitting.     

Short range order in metallic glasses from quadrupole splitting distributions

Changes are observed in the short range order of several Fe based amorphous alloy systems through the quadrupole splitting distributions. The quadrupole splittings are determined directly in the ferromagnetic state by using the radio frequency collapse of the magnetic splitting.     

Features of dynamic crystal field effect in the Mössbauer spectra of human hemoglobins at zero field

Human normal hemoglobin (HbA), partial artificial denatured normal hemoglobins, and several abnormal hemoglobins were investigated by Mossbauer spectroscopy at zero field over a temperature range of 78–180k. The Mossbauer spectra of hemoglobins were approximated to two Lorentzian doublets by the least square method. A distinction of the quadrupole splitting and temperature dependence of quadrupole splitting, assoclated with different conformation of molecules, was detected. Furthermore the features of non—Lorentzian lineshape and temperature dependence of lineshape are discussed in correlation to the dynamic fluctuation of Fe (II) ion.     

145Nd NMR in Nd15Fe77B8H x

The effect of absorbed hydrogen on the¹⁴⁵Nd NMR in Nd₁₅Fe₇₇B₈ has been investigated at helium temperatures. Hydrogen absorption is accompanied by a drop of NMR frequencies and changes in the quadrupole splitting. The drop of NMR frequencies is related to the lattice expansion. The influence of the hydrogen on the spin reorientation can be responsible for the changes in the quadrupole splitting. This work was supported by Project CPBPU1, U4     

Mössbauer study of amorphous Fe2RE (RE=Ce,Er) alloys

Among amorphous Fe₂RE (RE=Er, Ce, Gd, La, Pr, Sm, Dy, Ho) alloys, Fe₂Ce exhibits a tendency toward short range order, while the other Fe₂RE compounds show clustering. However, we have almost no information about environments around Fe atoms. Using Mössbauer spectroscopy we have determined the quadrupole splitting distributionsP(QS) of two representative amorphous Fe₂RE (RE=Ce, Er) alloys, leading to local environments of Fe atoms. The analysis of the mixed magnetic dipole and quadrupole interactions in Fe₂Er shows two kinds of electrical field gradients (EFT) with the positive and negative signs in the sample, indicating a random packing of Fe atoms. Furthermore, the analyzed quadrupole splitting distributionP(QS) of Fe₂Er also supports random packing in this amorphous alloy. On the other hand, the amorphous Fe₂Ce alloy shows two kinds of distributions of quadrupole splitting; the major component indicating random packing and the minor component Ce-rich Ce-Fe clusters.     

First Mössbauer observation of the glass transition in an amorphous metal

We report the first Mössbauer spectra of a metallic glass obtained above the glass transition temperature (T _G ). The temperature at which both the Lamb-Mössbauer factor and the average quadrupole splitting decrease sharply is in agreement withT _G determined by thermal and mechanical measurements, but the width of the quadrupole splitting distribution starts to increase at a much lower temperature. These results are compared with data from similar measurements on more conventional glasses.     

An investigation of Fe oxidation states and site distributions in a Tibetan tektite

Room temperature ⁵⁷Fe Mössbauer effect measurements have been used to investigate the local Fe environment and the Fe³⁺/Fe²⁺ ratio in a Tibetan tektite. The spectrum has been analyzed in terms of two quadrupole splitting distributions; one corresponding to Fe²⁺ and one corresponding to Fe³⁺. The Fe²⁺ sites show a mean isomer shift of +1.011 mm/s relative to \alpha-Fe and a mean quadrupole splitting of 1.845 mm/s. The Fe²⁺ quadrupole splitting distribution showed a large peak for positive values, which has been identified with Fe²⁺ with local octahedral coordination, and a smaller peak for negative values, which has been identified with Fe²⁺ with local tetrahedral coordination. The Fe³⁺ sites showed an average isomer shift of +0.20 mm/s and a single peaked quadrupole splitting distribution with a mean near zero. The Fe³⁺/Fe²⁺ ratio, based on the relative areas of the spectra components was found to be 0.015. These results are discussed in terms of those obtained using fitting methods based on discrete sites and other distributions of parameters. Hyperfine parameters for the Tibetan tektite are compared with those obtained for tektites from other regions of the Australasian strewn field. All tektites studied show similar quadrupole distributions and similar mean hyperfine parameters. Fe³⁺/Fe²⁺ ratios ranged from 0.005 to 0.036. The relationship of the microstructure of the Tibetan tektite with that of tektites from various regions of the Australasian strewn field is discussed.