Mossbauer Emission Research Articles

Ядерное квадрупольное взаимодействие и эффективные заряды атомов в кристаллах YBa2Cu3O7_x и La2_xSrxCuO4

Проведено сопоставление параметров эмиссионных мессбауэровских спектров на изотопах 57Co(57mFe), 67Cu(67Zn), 67Ga(67Zn), 155Eu(155Gd) и параметров ядерного магнитного резонанса на изотопах 17O, 137Ba с расчетными параметрами решеточного тензора градиента электрического поля для всех узлов кристаллических решеток высокотемпературных сверхпроводников на основе металлооксидов меди YBa2Cu3O7 и La2-xSrxCuO4. Согласие экспериментальных и расчетных параметров тензора градиента электрического поля может быть получено только для моделей, когда дырка, возникающая за счет нестехиометричности соединения YBa2Cu3O7 или аливалентного замещения La3+ на Sr2+ в соединениях La2-xSrxCuO4, находится преимущественно в подрешетке цепочечного (в YBa2Cu3O7) или плоскостного (в La2-xSrxCuO4) кислорода. Подтверждение предложенной модели пространственного распределения электронных дефектов в решетках La2-xSrxCuO4 (x = 0.1 - 1.0) было получено путем сравнения расчетных зависимостей отношения главных компонент тензоров градиента электрического поля в узлах лантана и меди с отношением постоянных квадрупольного взаимодействия в этих узлах для зондов 57mFe3+, 67Zn2+ и 155Ga3+. Наблюдаемые зависимости могут быть количественно объяснены, если дырка локализуется преимущественно в позициях атомов планарного кислорода. С использованием этих моделей определены коэффициенты Штернхеймера для зондов 67Zn2+, 137Ba2+ и 17O2-.

On the Structure of the Mössbauer Spectra of 119mSn Impurity Atoms in Lead Chalcogenides under Conditions of the Radioactive Equilibrium of 119mTe/119Sb Isotopes

The method of Mossbauer emission spectroscopy with 119mTe and 119Sb parent isotopes in the radioactive-equilibrium state is used to obtain information on the localization and valence state of 119mSn daughter atoms formed from the 119Sb and 119mTe parent atoms at cation and anion sites of the lattice of PbSe and PbTe lead chalcogenides.

Two-electron exchange between neutral and ionized negative-U tin centers in lead chalcogenides

Two-electron exchange between neutral and doubly ionized negative-U tin centers in PbS and PbSe has been studied by Mossbauer emission spectroscopy. The activation energy for this process in PbS is 0.11(2) eV, which is comparable to the depth of tin levels in the band gap of PbS, whereas the activation energy for this process in PbSe, 0.05(1) eV, is comparable to the correlation energy of negative-U donor tin centers in PbSe. The exchange occurs through simultaneous transfer of two electrons and involves valence band states.

Impurity tin atoms in glassy As x S1 − x and As x Se1 − x

119Sn Mossbauer emission spectroscopy data demonstrate that the impurity tin atoms resulting from the radioactive decay of the 119Sb isotope in the structure of glassy AsxS1 − x and AsxSe1 − x alloys reside on the arsenic site and act as two-electron amphoteric centers with a negative correlation energy: tetravalent six-coordinate tin acts as a singly ionized donor, and divalent three-coordinate tin, as a singly ionized acceptor.

Structure, thermal stability, and state of grain boundaries of copper subjected to high-pressure torsion at cryogenic temperatures

Evolution of the structure of tough-pitch copper upon severe deformation by high-pressure torsion at room temperature and in liquid nitrogen is studied. The thermal stabilities of the resulting structures are investigated. The state of grain boundaries is examined by means of Mossbauer emission spectroscopy. Factors that hamper the formation of a nanostructured state and the possibility of eliminating the effect of these factors are analyzed.

Effect of the physical state on the scavenging of quasi-free electrons by a solute

The reactivity of carbon tetrachloride toward quasi-free electrons generated during the decay of 57Co atoms in its alcohol solutions frozen at 80 K has been studied by Mossbauer emission spectroscopy. It has been found that an increase in CCl4 concentration to 10 mol/L slightly affects the yield of 57Fe2+ and 57Fe3+ ions as 57Co decay products. The inertness of CCl4 has been interpreted as a consequence of a change of the energy state of quasi-free track electrons results from freezing the alcohol solution. This effect inhibits the dissociative attachment reaction of scavenging of track electrons and increases the ground-state energy of quasi-free electrons (on passing to the frozen matrix), thereby fundamentally altering the energy balance of the electron scavenging reaction in comparison with the liquid medium and, hence, decreasing the rate constant of electron scavenging by CCl4.

Cation ordering in Sr(Fe1 − x Co x )0.9Ga0.1O2.5 with brownmillerite structure

Samples of solid solutions of the series Sr(Fe1 − x Co x )0.9Ga0.1O2.5 with brownmillerite structure are produced from appropriate amounts of oxides Fe, Co, Ga, and SrCO3 in different gaseous media and temperature conditions using solid-phase synthesis. Features of the distribution of cations over nonequivalent positions of the crystalline structure of the obtained samples are studied using the 57Fe Mossbauer absorption and emission spectroscopy.

Tin U −-centers formed through nuclear transformations in vitreous arsenic sulfides and selenides

It has been shown using the Mossbauer emission spectroscopy for isotope 119Sn that impurity tin atoms formed after the radioactive decay of 119Sb atoms in vitreous arsenic sulfide and selenide are localized in arsenic sites and play the role of two-electron centers with negative correlation energy. The most of daughter 119mSn atoms formed after the radioactive decay of the 119mTe atoms in glasses are arranged in chalcogen sites; they are electrically inactive. Considerable recoil energy of daughter atoms in the case of decay of 119mTe leads to the appearance of 119mSn atoms shifted from chalcogen sites.

Mössbauer studies of two-electron centers with negative correlation energy in crystalline and amorphous semiconductors

The results of the study of donor U −-centers of tin and germanium in lead chalcogenides by Mossbauer emission spectroscopy are discussed. The published data regarding the identification of amphoteric U −-centers of tin in glassy binary arsenic and germanium chalcogenides using Mossbauer emission spectroscopy, and in multicomponent chalcogenide glasses using Mossbauer absorption spectroscopy are considered. Published data concerning the identification of two-atom U −-centers of copper in lattices of semimetal copper oxides by Mossbauer emission spectroscopy are analyzed. The published data on the detection of spatial inhomogeneity of the Bose-Einstein condensate in superconducting semiconductors and semimetal compounds, and on the existence of the correlation between the electron density in lattice sites and the superconducting transition temperature are presented. The principal possibility of using Mossbauer U −-centers as a tool for studying the Bose-Einstein condensation of electron pairs during the superconducting phase transition in semiconductors and semimetals is considered.

Aspartic acid interaction with cobalt(II) in dilute aqueous solution: A 57Co emission Mössbauer spectroscopic study

Emission (57Co) Mossbauer spectra of the aspartic acid—57CoCl2 system were measured at T = 80 K in frozen aqueous solution and in the form of a dried residue of this solution. The Mossbauer spectra, besides a weak contribution from after-effects, showed two Fe2 + /Co2 + components which were ascribed to octahedrally and tetrahedrally coordinated 57CoII microenvironments in the Asp–cobalt(II) complex. This dual coordination mode may be due to the involvement of the second terminal carboxylic group of aspartic acid in the coordination sphere of Co.

Charge states of atoms in ceramic superconductors HgBa2Ca n − 1Cu n O2n + 2, Tl2Ba2Ca n − 1Cu n O2n + 4, and Bi2Sr2Ca n − 1Cu n O2n + 4 (n = 1–3)

The parameters of the electric-field-gradient tensor for copper sites in the HgBa2Can − 1Cu n O2n + 2, Tl2Ba2Can − 1Cu n O2n + 4, and Bi2Sr2Can − 1Cu n O2n + 4 (n = 1–3) lattices have been determined using 67Cu(67Zn) Mossbauer emission spectroscopy and calculated in the framework of the point-charge approximation. The agreement between the experimental and calculated parameters has been achieved under the assumption that the holes formed as a result of the decrease in the oxidation state of a part of the mercury, thallium, or bismuth atoms are distributed over the oxygen sites in the Cu-O or adjacent planes. It has been demonstrated that the oxidation state of cations can be controlled in high-temperature superconducting ceramic materials.

Study of the grain-boundary structure in submicrocrystalline niobium after equal-channel angular pressing

The method of Mossbauer emission spectroscopy on 119mSn nuclei was used to study grain boundaries in submicrocrystalline Nb produced by the method of equal-channel angular pressing (ECAP) of poly-crystalline Nb (99.9%). ECAP included 16 passes at room temperature.

Time-space evolution of the cloud of ion-electron pairs formed by Auger electrons around Mossbauer ion 57Fe n+

It is shown that the emission of energetic Auger electrons during formation of the Mossbauer nucleus 57Fe leads not only to the formation of multicharged ion 57Fen+, but also to the formation of a cloud of several hundred (200–300) ion-electron pairs (H2O+, e−) around the Fe ion. This cloud is called an Auger blob. Its size (radius) is approximately ∼100 A. Fast radiation-chemical reactions in an Auger blob determine the experimentally observable ratio of yields of final chemically stable states (Fe3+ and Fe2+) of the Mossbauer ion. Knowledge of this ratio is important for an adequate interpretation of the results of Mossbauer emission spectroscopy. Although our assessments relate to iron nuclei in frozen aqueous solutions, they can be easily adjusted for media of other chemical composition.

Photostructural reconstructions of As-S and As-Se semiconductor glasses

The results of experimental investigation of photoinduced changes in optical properties of semiconductor glasses of the As-Se and As-S systems are presented. With using the data of the Mossbauer emission spectroscopy on the 129Te(129I) isotope, the observed changes in optical properties can be attributed to the reconstruction of the glass matrix under the effect of radiation.

Influence of irradiation on the local environment of chalcogen atoms in vitreous films of the As-S, As-Se, and As-S-Se systems

The results of the experimental study of the photoinduced changes in the optical properties of vitreous films in the As-S, As-Se, and As-S-Se systems are reported. With the use of the data obtained from the 129Te(129I) Mossbauer emission spectra, the revealed changes in the optical properties are explained by the transformation of the glass structure under radiation.

The influence of amorphization on the local environment of atoms in arsenic chalcogenides

The influence of amorphization on the symmetry of the local environment of chalcogen atoms in the As2S3, As2Se3, and As2Te3 compounds is investigated using Mossbauer emission spectroscopy. In the 129Te(129I) Mossbauer emission spectra of crystalline As2Te3, three states of threefold-coordinated tellurium atoms appear to be indistinguishable. The transformation of the As2Te3 crystalline compound into the vitreous state results in the lowering of the local symmetry of threefold-coordinated states of the tellurium atoms and the formation of twofold-coordinated states of the tellurium atoms in chains of the type-As-Te-Te-As-. Two states of twofold-coordinated chalcogen (X) atoms in chains of the type-As-X-As-in the structure of the As2S3 and As2Se3 crystalline compounds manifest themselves in the form of a broadening of the Mossbauer spectra. The transformation of the As2S3 and As2Se3 crystalline compounds into the vitreous state is not accompanied by a change in the local symmetry of twofold-coordinated chalcogen atoms in chains of the type-As-X-As-; however, the structure of the glass in this case is characterized by the formation of twofold-coordinated states of sulfur and selenium atoms in chains of the type-As-X-X-As-. The fraction of twofold-coordinated chalcogen atoms increases upon changing over from As2 X 3to AsX.

Experimental determination of the spatial distribution of electron defects in La2 − x Sr x CuO4 and Nd2 − x Ce x CuO4 crystal lattices

The data of Mossbauer emission spectroscopy on 67Cu(67Zn) and 67Ga(67Zn) isotopes show that holes appearing as a result of the Sr2+ substitution for La3+ in the La2 − xSrxCuO4 crystal lattice are localized predominantly at oxygen atoms occurring in the same atomic plane as the copper atoms. In contrast, electrons appearing as a result of the Ce4+ substitution for Nd3+ in the Nd2 − xCexCuO4 crystal lattice are localized in the copper sublattice. These results are consistent with the model assuming that a mechanism responsible for the high-temperature superconductivity in La2 − xSrxCuO4 and Nd2 − xCexCuO4 crystal lattices is based on the interaction of electrons with two-site two-electron centers possessing negative correlation energies (negative-U centers).

Mössbauer study of matrix effects in formation of high oxidation states of iron and cobalt

To fix unstable states of pentavalent iron and cobalt, a matrix containing (according to the preparation conditions) potassium manganates (V) was simultaneously doped with iron and radioactive cobalt. The absorption and emission spectra of this matrix, with allowance for the reversal of the source and absorber functions, are similar, which is indicative of the identity of the oxidation states (IV, V, and VI) of iron and cobalt in it. Stabilization of 57Fe and 57Co in the β-MnO2 matrix is also investigated by Mossbauer absorption and emission spectroscopy.

Response of La0.8Sr0.2CoO3-δ to perturbations on the CoO3 sublattice

Emission and transmission Mossbauer studies of La0.8Sr0.2CoO3-δ perovskites doped with ∼0.02 stoichiometric units of oxygen vacancy or 2.5% iron corroborate the occurrence of electronic phase separation in these systems. The effect of the small perturbation of the CoO3 sublattice with either iron ions or oxygen vacancies on the bulk magnetization as well as on the Mossbauer spectra is in good agreement with the double exchange based cluster model. The magnetoresistance does not show any peak near the Curie temperature, but reaches -84% in a field of 7.5 T at T = 8 K. Below TC ≈ 180 K the Mossbauer spectra distinctly include the contribution from paramagnetic and ferromagnetic regions, providing direct evidence for phase separation. No contribution to the spectra from Fe4+ ions can be observed, which is an unambiguous evidence that at low concentration iron (either directly doped or formed from 57Co by nuclear decay) is accommodated in the cobaltate lattice as Fe3+ ion.

Defects in semiconductors—results from Mössbauer spectroscopy

Progress in the development of Mossbauer techniques with ion-implanted, radioactive precursors to a Mossbauer isotope is discussed. Results obtained for elemental group IV semiconductors and their alloys as well as for III–V and II–VI compound semiconductors are presented. Emphasis is put on Mossbauer emission spectroscopy with radioactive probe atoms where the recoil energy in the nuclear decay is sufficient to expel the daughter atoms from a (substitutional) lattice site. The interactions of such (interstitial) atoms have been studied for 119Sb →119Sn in III–V compounds and for 57Fe in silicon in particular. Finally, preliminary results, contributing to the question of the origin and nature of the magnetism in the Fe-doped, dilute magnetic semiconductor ZnO, are given.