Calibration of the isomer shift of121Sb and119Sn by means of Mössbauer spectroscopy and molecular orbital calculation

The relationship between the isomer shifts in Mössbauer spectroscopy and the electron contact densities has been investigated for several antimony and tin compounds. Mössbauer spectra for¹²¹Sb in rapidly frozen solutions of antimony compounds were measured. The isomer shifts are compared with the valence electron densities at the antimony nucleus calculated with theab initio molecular orbital method. The relative difference of the nuclear charge radius ΔR/R could be obtained as ΔR/R =?(10.2±1.0)×10^?4 for the 37.15 keV M1 transition of¹²¹Sb. Further, some computations of the electron density at the tin nucleus for several tin compounds were performed. By comparing the valence electron contact densities with isomer shifts, which had been reported for several tin compounds in rare-gas matrix states, the value of ΔR/R for 23.87 keV M1 transition in¹¹⁹Sn was estimated to be (1.57±0.03)×10^?4.     

Relation of contact densities with Mössbauer isomer shifts for 35.46 keV M1 transition of125Te

The internal conversion electron and Mössbauer isomer shifts associated with the 35.46 keV M1 transition of¹²⁵Te were observed for different metal samples into which radioactive¹²⁵I as probe atoms were introduced by means of ion-implantation. From the correlation between the Mössbauer isomer shifts and intensity ratios of O shell to N_I shell conversion electrons, a relation between 5s-electron contact densityp _5s(0) in a.u. and isomer shift δ in mm/s was deduced to bep _5s(0)=106+30.3 δ±4.3[(δ?0.30)²+0.069]^1/2, where δ was measured relative to ZnTe in mm/s. The change of the nuclear charge radius in the 35.46 keV M1 transition of¹²⁵Te was found to be ΔR/R=(0.85±0.12)×10^?4 (corresponding to Δ<r ²>=(3.7±0.5)×10^?3 fm²) when a theoretical 4s-electron contact density of 928 a.u. was used.     

Investigation of the decay rate for71Ge in chemical compounds and calibration of the Mössbauer isomer shift of the 67 keV γ-transition of73Ge

Changes of the radioactive decay rate of⁷¹Ge (T _1/2=11.43 days) have been studied experimentally for⁷¹Ge in some compounds of bivalent and quadrivalent germanium. Relative changes Δλ/λ of the electron capture probability have been measured, and the chemical changes Δρ(0) of the electron density at the germanium nuclei have been determined from these measurements. Values for the Mössbauer isomer shift calibration constants for the 67 keV γ-transition of⁷³Ge have been estimated:C=δ/Δρ(0)=(0.061±0.020) mm·s^?1/a.u., Δ〈r ²〉=(21±7)·10^?3 fm², ΔR/R=(6.9±2.3)·10^?4.     

Estimation of the value of ΔR/R for121Sb nucleus by means of Mössbauer spectroscopy and molecular orbital calculation

The¹²¹Sb Mössbauer spectra of rapidly frozen solutions of antimony compounds were observed. The isomer shifts were compared with the electron density at the antimony nucleus calculated with the ab initio molecular orbital method. A value ΔR/R=?(11±2)×10^?4 was derived for the 37.15 keV M1 transition of¹²¹Sb.     

Mössbauer isomer shifts and quadrupole splittings in osmium compounds

From Mössbauer spectra of the 36.2keV and 69.5keV gamma rays of¹⁸⁹Os, isomer shifts and electric quadrupole interactions for a number of chemical compounds of osmium in the oxidation states +8, +6, +4, +3, and +2 were obtained. The results show that the dependence of the isomer shifts on the oxidation state of Os follows the general pattern established for other transition elements like Ru and Ir. A comparison of the isomer shift data with electron densities at the Os nuclei resulting from relativistic self-consistentfield calculations for free Os ions yieldsΔ 〈r²〉 [36 keV]=?2.0 · 10^?3 fm² andΔ 〈r²〉 [69 keV]=?0.13 · 10^?3 fm³ for the changes of the mean-square nuclear charge radius of¹⁸⁹Os. These values are in qualitative agreement with an interpretation of the low-energy level scheme of¹⁸⁹Os in terms of two rotational bands.     

Möβbauereffekt am61Ni

With the Möβbauer technique the hyperfine splittings of the 67.4 keV transition of⁶¹Ni in compounds and alloys have been studied. A magnetic momentΜ _61,4=+ (0,477 ±0.031)Μ _n of the first excited state has been determined. From isomer shifts an order of magnitude estimate of the change in radius ofδ〈r ²〉/〈r ²〉=?6· 10^?4 can be inferred. The electric quadrupole interaction of the first excited level has been observed in Ni-J-boracite. In addition the hyperfine fields at the nickel sites were determined in the alloy series Ni _x Fe_1-x (0≦x≦1). In one case an effect of polarization in a high external field is measured. The fields in some rare earth (RE) intermetallic compounds of the form RENi₂ were deduced from linewidth to be less than 15 kOe.     

Isomer-shift analogue in neutron resonances

For the first time, the recently predicted chemical shift of neutron resonances, to be regarded as an analogue to the Mössbauer isomer shift, has been experimentally observed studying the 6.67 eV resonance of ²³⁸U. The experimental shifts were determined by a chi-square fitting technique from the time-of-flight transmission spectra of metallic uranium and four uranium compounds measured at the Dubna IBR-30 pulsed reactor. A computational method has been applied to estimate, and compensate for, the influence of the crystal-lattice vibrations on the experimental values thus obtained. The electron density differences at the nucleus have been calculated for the various sample pairs using available data on chemical X-ray shifts in uranium compounds, on Mössbauer isomer shifts in isovalent neptunium compounds and on free-ion electron densities. The resonance shift results lead to the conclusion that the mean-square charge radius of ²³⁸U diminishes by 1.7_?0.8^+1.2 fm² upon capturing the resonance neutron.     

Calibration of the isomer shift of the 14.4?keV transition?of? 57 Fe

With the aim of calibrating Mössbauer spectroscopic measurements, the electron contact density was calculated on the Fe nuclear position in various iron compounds. The full-potential augmented-plane waves plus local orbitals (APW+lo) method within the local-density approximation was used, considering different approximations for the exchange and correlation potential. By comparison with experimental ⁵⁷Fe Mössbauer data, the calibration constant ?? relating measured isomer shifts to the electron contact density was derived. An isomer shift calibration constant ???=???0.25₁ a.u.³ mm s^???1 has been obtained.     

Precise determination of hyperfine interactions and second-order doppler shift in <Superscript>149</Superscript>Sm Mössbauer transition

We have succeeded in precisely determining the hyperfine interactions, particularly the isomer shifts, in the ¹⁴⁹Sm Mössbauer transition. The difference in the nuclear radii between the ground and excited states is critical for the determination of isomer shifts but is relatively small in ¹⁴⁹Sm. Therefore, the precise determination by ¹⁴⁹Sm Mössbauer spectroscopy is difficult. The recent development of synchrotron-radiation-based Mössbauer spectroscopy allows the isomer shifts to be determined more precisely than previously with the help of wellcollimated synchrotron radiation. In particular, the time-window effect assists the precise determination of hyperfine interactions in the ¹⁴⁹Sm Mössbauer transition because this effect enables us to measure spectra with higher energy resolution than natural linewidth determined by the lifetime of the excited states. Meanwhile, highenergy-resolution measurements to determine center shifts by SR-based Mössbauer spectroscopy enable us to observe the second-order Doppler shift, which has not been discussed, particularly for heavy Mössbauer nuclei. We have discussed the precise determination of isomer shifts and the observation of the second-order Doppler shift using ¹⁴⁹Sm synchrotron-radiation-based Mössbauer spectroscopy.     

On the thermal expansion of nanoparticles

Magnetite nanoparticles with an average particle size of 7.5 nm were studied by Mössbauer spectroscopy in the 100–300 K temperature range. Using experimental data on the temperature variation of isomer shifts in the partial spectra of ⁵⁷Fe nuclei in the A and B crystal sublattices, the linear thermal expansion coefficient of the ultradisperse magnetite particles in the temperature range studied was estimated within the framework of the molecular orbital formalism: α=(1.2±0.2)×10^?4K^?1.     

99Ru,61Ni,57Fe,and119Sn Mössbauer studies of Heusler alloys containing ruthenium

⁹⁹Ru,⁶¹Ni,⁵⁷Fe and¹¹⁹Sn Mössbauer spectroscopic studies were made on ternary intermetallic compounds containing ruthenium, Ru_xY_3?xZ (Y=Fe, Ni; Z=Si, Sn). In the system of Ru_xFe_3?xSi, two different hyperfine magnetic fields were observed at the⁹⁹Ru nuclei (H _hf[Ru]) in the range ofx≤1.0 and the magnitude of eachH _hf[Ru] was found to decrease with an increase in the ruthenium concentrationx. Both the⁹⁹Ru and¹¹⁹Sn Mössbauer spectra of Ru₂FeSn could be analyzed with two sets of magnetically split lines. The⁶¹Ni Mössbauer spectra of Ru₂NiSn were obtained at 5 and 77 K.     

Änderung des quadratischen Kernladungsradius bei Rotationsanregung

Isomer shifts ofγ rays from rotational bands of deformed nuclei have been determined in Mössbauer effect measurements. The following changes of the mean square nuclear charge radii have been deduced:Δ〈r ²〉(¹⁵⁶Gd, 89keV)=(+2.6±0.8)·l0^?3fm²,Δ〈r ²〉(¹⁷⁰Yb, 84keV)=(+1.2±0.3)·10^?3fm²,Δ〈r ²〉(¹⁷¹Yb, 67keV)=(+1.1±0.2)·10^?3fm². The results are compared with theoretical calculations and additional experimental data, to study the effect of the Coriolis and centrifugal forces in rotational excitation on the motion of the nucleons.     

Electronic structure of βSn and Sb: Application to pressure dependent isomer shifts

The electronic structures of white tin and antimonh are calculated from the first principles LMTO method including all scalar-relativistic effects. The band structure of antimony is in excellent agreement with experimental information. The equilibrium lattice constants and bulk moduli are obtained and compared to experimental values. The volume dependences of the electron density at the nucleus are determined to provide, in conjunction with experimental results for the pressure variation of isomer shifts, independent calibrations of the ¹¹⁹Sn and ¹²¹Sb Mössbauer isomer transitions. These give the relative change of nuclear radii as ΔR/R= 1.53 · 10⁻⁴ and ΔR/R = -7.0 · 10⁻⁴ for ¹¹⁹Sn and ¹²¹Sb respectively.     

Mössbauer studies of europium and ytterbium hydrides

Mössbauer absorption spectra were obtained for the 21·6 ke V transition of ¹⁵¹Eu in EuH₂ at various temperatures and for the 84·3 keV transition of ¹⁷⁰Yb in YbH₂ at 4·1°K. The isomer shift of EuH₂ relative to Eu³⁺: Sm₂O₃ is ? 12·1 ± 0·3 mm. sec^?1, and the magnetic hyperfine field equals ? 305 ± 5 kOe at saturation. The Curie temperature is found to be 16·2 ± 0·05°K, and the critical parameters of the transition are D = 1·17 ± 0·02 and β = 0·35 ± 0·01. The magnetic field is perpendicular to the principal axis of the electric field gradient and the values of the quadrupole hyperfine interaction e²qQ₀(3 cos² θ ? 1)/8 is ? 28 ± 4 Mc . sec^?1. A large increase of the resonance area (21%) occurs at the transitio to the ferromagnetic state. The isomer shift of YbH₂ relative to Yb: TmAl₂ is ?0·11 ± 0·01 mm . sec^?1. The value of the quadrupole coupling constant e²qQ_c/4is ? 91·5 ± 2 Mc . sec^?1 and the asymmetry parameter of the electric field gradient equals 0·89 ± 0·05. The data for EuH₂ and YbH₂ is shown to be consistent with the hydridic model for the rare earth hydrides.     

Mössbauer isomer shift and hyperfine fields of ordered and disordered Fe3Al alloys

The Mössbauer technique was used to measure the hyperfine magnetic field and isomer shift of⁵⁷Fe nucleus in the ordered and disordered Fe₃Al alloys. The Mössbauer spectra were analyzed to yeld the hyperfine magnetic field distribution curves. A linear correlation has been revealed between the average hyperfine field and the average number of Al atoms in the first two nearest neighbour shells of⁵⁷Fe nucleus,N _Al, for both ordered and disordered alloys. The measured values of the mean isomer shift agree very well with the values expected from the Miedema-Van der Woude model.     

197Au Mössbauer study of Au/Ni artificial multilayers

¹⁹⁷Au Mössbauer measurements have been performed at 16 K on the Au/Ni artificial multilayers having three different thickness of the layers those are 10Å Au/10Å Ni, 30Å Au/30Å Ni and 53Å Au/53Å Ni on a 250Å pure Au buffer layer. Mössbauer spectra obtained can be decomposed into mainly two components. One is an unperturbed component having an identical isomer shift value to the bulk Au metal. The other is the component perturbed strongly by the Ni layer indicating a broad contribution at positive velocity side and its intensity depends on the thickness of the Au layer. The spectrum from 10Å Au/10Å Ni multilayer is an entirely perturbed one and its area ratio to the component rising from pure Au buffer layer indicates the large Debye-Waller-factor suggesting the supermodulus effect in this multilayer.     

The 4s electron densities of Fe impurity atoms in some transition and noble metals

From internal conversion experiments and the Mössbauer effect measurements, the 4s electron density and isomer shift for Fe impurity atoms in Cr, Ni, Cu, Pt and Au metals were studied. The M and N shell conversion electron spectra of the 14.4 keV transition of⁵⁷Fe were measured using a high resolution β-ray spectrometer and analyzed using a deconvolution methods. The Mössbauer effect measurements were performed by a standard spectrometer with a moving absorber. Most of the samples showed isomer shift values consistent with the published ones. It is concluded that the 4s electron density, \(\rho _{4_S} (0)\) , at the nucleus of⁵⁷Fe embedded is nearly equal in these host metals. The relation between \(\rho _{4_S} (0)\) and the isomer shift indicates that \(\rho _{3_S} (0)\) plays an important role in the variation of the isomer shift.     

Isomerieverschiebungen der 81 keV γ-Linie des133Cs

Isomer shifts of the 81.0 keVγ-transition in¹³³Cs have been measured by the Mössbauer technique for eight ionic Cs compounds, and for the intermetallic compound CsBi₂. The source was¹³³Ba in CaF₂. The shifts, relative to CsCl, range from ?1.4 · 10^?8 eV for CsNO₃ to +1.6 · 10^?8 eV for CsBi₂. The difference in electron density at the Cs nucleus between CsBi₂ and CsNO₃ is discussed using chemical bonding arguments. For the Cs halides, this difference is estimated from the overlap of outer electron shells in the lattices. A relative change of the mean square charge radiusδ〈r ²〉/〈r ²〉=+2 · 10^?4 is derived.     

Hyperfine interactions in intermetallic compounds between Gd and 3d transition metals

Measurements of hyperfine interactions at ¹⁵⁵Gd nuclei in metallic compounds between Gd and 3d transition metals and at ⁶¹Ni in GdNi compounds by Mössbauer spectroscopy are reported. The results are discussed in terms of various models proposed for the electronic structure of these compounds. The Gd isomer shifts with respect to metallic Gd are at variance with the model of a strong d electron transfer from rare earths to transition metal ions. From the observation of a linear relation between magnetic hyperfine fields at Gd and at Dy nuclei in corresponding compounds it is inferred that crystal-field induced variations of Dy moments are neglible and that the conduction electron polarization induced by 4f moments is directly related to that caused by 3d moments.     

Isomerieverschiebung im Eu151

The isomer shift of the 21.7 keVγ transition in Eu¹⁵¹ has been studied for various divalent and trivalent europium compounds using the Mössbauer technique. Theγ energy is lower by up to 1.1·10^?6 eV in divalent compounds than in trivalent compounds. Using data from atomic spectroscopy it is estimated that the electron density at the nucleus is larger by about 1.9·10²⁶cm^?3 for the configuration 4f ⁶ than for 4f ⁷ due to different shielding of thes ² shells. The difference of the mean square radii of the 21.7 keV state and the ground state is thenδ〈r²〉=+0.03 fm². The measured isomer shift between trivalent and metallic europium and the relatives-electron densities in the rare earth metals measured by the positron annihilation rates are used to establish a calibration scheme for isomer shifts in rare earth metals. This calibration scheme is used to deduce a changeδ〈r ²〉=+0.005₅ fm² between the 26 keV state and the ground state of Dy¹⁶¹.