Mossbauer Emission Spectroscopy Research Articles

Using the 57m Fe3+ Mössbauer probe to determine the EFG tensor parameters at CuO cation sites

Mossbauer emission spectroscopy of the 57Co(57mFe) isotope has shown that the impurity iron atoms appearing at the CuO-lattice cation sites after the decay of 57Co2+ are donors and can become stabilized in two charge states, 57mFe3+ and 57mFe2+, and that the population ratio of these states depends on the Fermi level, whose position is governed by native CuO-lattice defects. A satisfactory agreement between the calculated and experimental values of the quadrupole splitting in Mossbauer spectra has been obtained for the 57mFe3+ centers. This permits one to consider the results obtained in the 57Co(57mFe) Mossbauer emission spectroscopy study of cuprates as reliable experimental data on the crystal-field EFG tensor parameters at copper sites.

Determining the position of antimony impurity atoms in PbS by 119Sb(119m Sn) emission Mössbauer spectroscopy

Emission Mossbauer spectroscopy based on the isotope 119Sb(119mSn) is used to show that the location of antimony impurity atoms in the PbS lattice depends on the conductivity type of the material: in n-type samples the antimony is localized primarily on the anion sublattice, while in p-type material it is primarily on the cation sublattice. It is noteworthy that when the 119mSn center appears in the anion PbS sublattice (i.e., as an antisite defect) after radioactive conversion of 119Sb, its charge state does not depend on the position of the Fermi level. When the 119Sb-center is in the cation sublattice of PbS, it acts like an electrically active substitutional impurity: in n-type samples the spectrum corresponds to the neutral state of a donor center (119mSn2+), while in p-type material it corresponds to the doubly ionized state of this center (119mSn4+).

Antistructural defects in PbTe-type semiconductors

Mossbauer emission spectroscopy on the isotope 119m Te(119m Sn) is used to identify tin impurity centers in PbTe and PbS lattices. Using the emission variant of Mossbauer spectroscopy makes it possible to stabilize the tin impurity atoms in the anion sublattice, i.e., to obtain antistructural defects. The charge state of the tin impurity atoms displaced from the anion sublattice is found to depend on the position of the Fermi level.

Charge states of atoms in HgBa2CuO4 and HgBa2CaCu2O6 lattices

Mossbauer emission spectroscopy on the 67Cu(67Zn) isotope has been used to determine the parameters of the electric-field gradient tensor at the copper sites of the HgBa2Ca n−1CunO2n+2 lattices (n=1,2), as well as to calculate these parameters in the point-charge approximation. An analysis of the results, combined with available NQR literature data for the 63Cu isotope, has shown that the best fit of calculated to experimental data can be reached by assuming that the holes due to the presence of defects in the material localize primarily in the sublattice of the oxygen lying in the copper plane.

Te-defect interaction in GaSb: donor–vacancy pair or DX-center?

We have studied 119Te donor atoms in GaSb incorporated by a recoil implantation technique by applying emission Mossbauer spectroscopy on 119Sn. Since 119Te decays via the intermediate 119Sb the thermal stability of the microscopic environment of the implanted Te atoms can be probed either in the Te state or, after transmutation, in the Sb state. It is found that part of the probes is situated in a strongly distorted configuration which cannot be annealed as long as the probes are Te. After transmutation to Sb the distorted state anneals at 405 K. From these results we conclude that the distorted configuration is implantation induced (very likely a probe-vacancy association) stabilized by the Te chemistry and not a DX center which should anneal with a much lower barrier.

Charge states of atoms in the lattices of the high-temperature superconductors Tl2Ba2Ca n−1CunO2n+4 and Bi2Sr2Ca n−1CunO2n+4

To determine the charges of atoms in the lattices of the compounds Tl2Ba2Can−1CunO2n+4 and Bi2Sr2Can−1CunO2n+4 (n=1,2,3), the parameters of the electric field gradient tensor at the copper sites of the indicated lattices were found by emission Mossbauer spectroscopy on the isotopes 61Cu(61Ni) and 67Cu(67Zn), and a calculation of these parameters was performed in the point-charge approximation. A comparison of the resulting values and the published data on 63Cu nuclear quadrupole resonance showed that agreement between the experimental and computed values of the parameters obtains for models in which the holes resulting from a reduction in the valence of some of the thallium (bismuth) atoms are localized predominantly at oxygen sites located in the same plane as the copper atoms (for the compounds Tl2Ba2Ca2Cu3O10 and Bi1.6Pb0.4Sr2Ca2Cu3O10—at oxygen sites in the same plane as the Cu(2) atoms).

Erbium impurity atoms in silicon

It is shown using 169Er(169Tm) Mossbauer emission spectroscopy that the photoluminescent centers in crystalline erbium-doped silicon are [Er-O] clusters and that the local symmetry of the Er3+ ions in these clusters is similar to that in Er2O3. The photoluminescent centers in amorphous hydrogenated erbium-doped silicon are clusters, whose local structure also corresponds to erbium oxide.

61Cu(61Ni) and 133Ba(133Cs) Mössbauer emission spectroscopy of Tl2Ba2Ca n−1CunO2n+4

The parameters of the electric-field gradient tensor at copper and barium sites in the Tl2Ba2Can−1CunO2n+4 (n=1,2,3) lattice have been determined by 61Cu(61Ni) and 133Ba(133Cs) Mossbauer emission spectroscopy, and calculated in the point-charge approximation. The calculated parameters can be reconciled with experiment if one assumes that the holes produced as the valence state of a part of thallium atoms is lowered are localized predominantly on the oxygen sublattice lying in the same plane with copper atoms [in the Cu(2) plane in the Tl2Ba2Ca2Cu3O10 lattice]. 133Ba(133Cs) Mossbauer emission spectroscopy data agree qualitatively with the proposed models of charge distribution in the Tl2Ba2Can−1CunO2n+4 lattices.

Ni-sulfide particles in NaY-zeolite for combined hydrodesulfurization and hydrocracking purposes

The influence of physisorbed water on the formation of nickel-sulfide species in a 57Co doped ion exchange type 57Co:NiNaY catalyst is studied by Mossbauer Emission Spectroscopy. The absence or presence of physisorbed water during sulfidation seems to have no influence on the local nickel environment of the formed Ni-sulfide species. This result is confirmed by EXAFS measurements. In spite of the resemblance found in MES and EXAFS a large difference is found for the initial HDS activities, which is explained by the different particle sizes (HREM) found at the outer surface of the zeolite.

IRRADIATION EFFECTS DETECTED BY MOSSBAUER SPECTROSCOPY IN IRON COMPLEXES

The nature and the extent of the Co-60 gamma radiolysis of several iron coordination compounds have been analysed by Mossbauer absorption spectroscopy. The comparison of the radiolytic yields with the after effects observed by Mossbauer emission spectroscopy in similar Co-57-doped compounds, supports the self-radiolysis model.

Embedded nanocrystallites prepared by ion implantation

Nanocrystallites embedded in substrates (granular materials) prepared by ion implantation have many unusual properties in comparison with bulk materials. Features are found such as high magnetic coercivity, size effect of the effective magnetic moment per atom, dependence of the interface magnetic hyperfine field on the Co concentration, nonlinear photoluminescence as well as charge transfer. We show that CEMS and emission Mossbauer spectroscopy are powerful tools for studying these granular systems.

Lamb Mössbauer factor of electronically excited molecular states measured by time-differential Mössbauer emission spectroscopy

The Lamb-Mossbauer Factor (LMF) of molecular crystals is expected to depend on the electronic molecular states by their different intramolecular vibrational frequencies. Revisiting Mossbauer spectra obtained by time differential Mossbauer emission spectroscopy of the low spin compound [57Co/Mn(bipy)3](PF6)2 (bipy= 2,2'-bipyridine) a ratio of 1.25 for the LMFs of the low spin ground state and of an excited high spin state decaying in the Mossbauer time window could be evaluated. The difference found is in line with the change of LMF observed for spin crossover compounds where the excited high spin state is populated by the so‐called LIESST effect. The initial population of the high spin state is close to 100%.

67Cu(67Zn) Mössbauer emission spectroscopy of Tl2Ba2CuO6 and Tl2Ba2CaCu2O8

The parameters of the crystal electric-field gradient at the copper sites of Tl2Ba2CuO6 and Tl2Ba2CaCu2O8 have been determined by Mossbauer emission spectroscopy using the 67Cu(67Zn) isotope, and calculated in the point-charge approximation. The results obtained are analyzed using the available 63Cu nuclear quadrupole resonance data. The experimental and calculated data can be brought in agreement if one assumes that the holes appearing after part of the thallium atoms lower their valency become localized primarily at the oxygen sites lying in the plane of the copper ions.

Mössbauer spectroscopy investigations and behaviour of impurity atoms in irradiated metallic targets

The properties of hot atomic microimpurities ( 57.58Co, 54Mn) in cyclotron metallic targets were investigated using the reaction 55Mn + α with emission Mossbauer spectroscopy employed to estimate the electronic state of the hot atoms. γ-Radiation self-absorption and sectioning methods were used to define the diffusion coefficients at 670–1200 K. The position of the 57Co atoms in the manganese lattice after irradiation and subsequent heating at the temperature of polymorphic α → β manganese (1000 K) transformations was established. Rapid diffusion of cobalt atoms at the polymorphic α → β manganese transformation was revealed. The radiochemical behaviour of 57,58Co, 54Mn after target dissolution was studied. The conclusion arrived at is that hot impurity atoms are a very effective tool in investigations of metallic cyclotron targets.

Charge distribution in the PrBa2Cu3O7 lattice

1 or yttrium! with the perovskite structure are superconductors with a transition temperature Tc;90 K ~Ref. 1!. An exception to the rule is PrBa2Cu3O7, and its anomalous behavior is connected either with Pr being tetravalent, or with partial mutual substitution of the Pr and Ba ions. Despite a large number of studies into this problem, it remains unsolved. Cu(Gd! Mossbauer emission spectroscopy data obtained by us for the R12xEuxBa2Cu3O7 solid solutions ~R 5Tm, Y, Dy, Gd, Eu, Sm, and Nd, with x,10) ~Ref. 3! and Gd Mossbauer absorption measurements made for GdBa2Cu3O7 ~Ref. 4! and for gadolinium impurity atoms in Pr sites of the PrBa2Cu3O7 orthorhombic lattice 5 permit one to decide between these two models. Mossbauer spectra of the above ceramics suggest that the rare-earth metal sites are occupied by Gd ions, and that at temperatures above the point where the rare-earth-metal sublattice undergoes antiferromagnetic ordering they represent quadrupole doublets with a splitting

Study of neutrino-recoil produced point defects in InSb by119Sn Mössbauer spectroscopy

We have utilized the electron-capture decay of119 Te to119Sb to produce isolated single Frenkel pairs in InSb. This effect is caused by the neutrino emission in the decay process which imparts a monoenergetic recoil of 12 eV to the119Sb atoms, thereby displacing about 20% of them to interstitial sites. Two distinct interstitial components can be observed. The process is traced by Mossbauer emission spectroscopy following the decay of119Sb to119Sn. The displacement thresholdE d is confined to 6 eV<E d <12 eV from auxiliary experiments employing119mTe isotopes.

A Mössbauer spectroscopic study of cobalt-iron molybdates

Fe x Co1−x MoO4 compounds prepared by coprecipitation were studied by XRD, electrical conductivity and mainly by absorption and emission Mossbauer spectroscopy. FeMoO4 and CoMoO4 samples were shown to contain Fe3+ and Co3+, respectively, in solid solution. Three kinds of Fe x Co1−x MoO4 solids can be described. Forx⩽0.16: one has a Β-Co(Fe2+, Fe3+)MoO4 solid solution. For 0.17⩽x⩽0.25: one has the same solid solution with its surface rich in Fe3+. Forx⩾0.26: one has the same solid solution with only bulk Fe3+, and ferric molybdate. Studies of reduction by hydrogen and of catalytic reaction of mechanical mixtures of CoMoO4 and ferric molybdate support these statements.

Fascinating electronic games in iron complexes

Coordination compounds of transition metal ions with open-shell electron configurations may exhibit dynamic electronic-structure phenomena, depending on the nature of the coordinating ligand sphere. The change of spin state with temperature («thermal spin-crossover»), light-induced electron transfer processes leading to long-lived metastable charge and spin states (e.g., «LIESST» effect), are some of the fascinating electronic games encountered in transition metal compounds, which are presently under extensive study by chemists and physicists. Mossbauer spectroscopy plays a dominant role in the investigation of such phenomena in iron compounds, as will be demonstrated in this paper. This work will focus on selected examples of «thermal spin-crossover» in iron(II) complexes and switching between different spin states by irradiation with light of different wavelength (LIESST effect), demonstrating that Mossbauer spectroscopy besides other physical techniques proves to be a highly elegant tool for following the spin state conversion and the concomitant changes of molecular and crystal structure properties. Finally, Mossbauer emission spectroscopy, both time integral and time differential, has been employed to generate and identify long-lived excited spin states by making use of the nuclear disintegration of 57Co as an intrinsic molecular light source (NIESST=nuclear decay-induced excited-spin-state trapping). Lifetime measurements by optical techniques on LIESST states and by time-differential Mossbauer coincidence spectroscopy on NIESST states prove that the relaxation pathways in the mechanisms for LIESST and NIESST are identical.

Lattice EFG tensors at the rare-earth metal sites in RBa2Cu3O7 and La2-xSrxCuO4

Parameters of the tensors of the electric field gradient (EFG) created by lattice ions at the rare-earth metal (REM) sites in RBa2Cu3O7 (R is a REM or Y) and La2-xSrxCuO4 (0<x<0.3) have been determined by means of 155Eu(155Gd) emission Mossbauer spectroscopy. The EFG tensors at the REM sites have been calculated in the point charge approximation. The experimental and calculated EFGs are shown to be in good agreement when holes are supposed to be mainly in sublattices of the chain and Cu-O plane oxygen for RBa2Cu3O7 and La2-xSrxCuO4, respectively.

61Cu(61Ni) emission Mossbauer study of hyperfine interactions in copper-based oxides

Simple (CuO2, CuO) and complex superconducting (YBa2Cu3O7-x, La2-xSrxCuO4) copper oxides have been investigated by 61Cu(61Ni) emission Mossbauer spectroscopy. A linear relationship between the quadrupole coupling constant for 61Ni and the calculated lattice electric field gradients is established, and the contribution of the Ni2+ valence electrons to the coupling constant is determined. Magnetic fields are observed at 61Ni nuclei in those cases when the copper sublattice shows magnetic ordering.