Mossbauer Isomer Shift Research Articles

Anomalous behaviors in high temperature superconductors: Lattice effect and d-electron delocalization effect of the normal state on the Bi, Tl, and Y-system superconductors

The dynamic studies of the hyperfine interactions and the lattice vibrational behavior are performed by means of temperature-dependent Mossbauer spectroscopy on the Bi-, Tl-, and Y-based superconductors, respectively. The temperature-dependent Mossbauer recoil-free factor f show that, above, but relatively close to, the transition temperature, there is an anomalous lattice effect which is related to the softening of phonon vibrational modes for Bi-2223, Tl-1223, and Y-123 superconductors. Moreover, for such three systems, the anomalous decrease of the Mossbauer isomer shift above Tc reveal the delocalization effect of d-electrons. Since the lattice effect is accompanied by d-electron delocalization effect at the almost same temperature region, it is believed that the structural instability involved in phonon softening is strongly interrelated to the change of the electronic states, and the phase separation behavior.

Low-Coordinate Homoleptic Iron(II) Thiolates Revisited

A reinterpretation of the Mossbauer and crystallographic data, together with some new Mossbauer results, is presented for low-coordinate iron(II) thiolates. It is shown that the coordination geometry about iron is best reassigned in some cases. Further, Mossbauer isomer shift and quadrupole splitting can distinguish the coordination geometry about iron in low-coordinate iron complexes, but not in the way previously proposed.

Electronic structure and hyperfine interactions of T, T* and T' phases of (La1-xGdx)2Cu1-yFeyO4

Electronic structures and hyperfine interactions associated with the Cu-O environment of T, T*, and T' phases of (La1-xGdx)2CuO4 were studied, using the density functional theory in an embedded cluster approach. Mossbauer isomer shifts, electric field gradients and contact hyperfine fields for Fe-substituted species were determined for comparison with experiments performed on Fe:(La, Gd)1.85Sr0.15CuO4. These data are used to describe the response of charge and spin densities around Cu sites to sixfold (T), fivefold (T*), and fourfold (T') oxygen coordination.

Molecular orbital calculation of Mössbauer parameters for tin compounds in low temperature matrices

Mossbauer parameters of tin compounds, Sn(CH3)nCl4−n (n=0, 1, 2, 3, 4), isolated in low temperature matrices are correlated with electronic properties at the tin nuclei obtained by molecular orbital calculations. The Mossbauer isomer shift and quadrupole splitting show good correlation with electron density and electric field gradient estimated by molecular orbital calculations, respectively. Structures of novel species (Sn(CH3)2CH2 and [Sn(CH3)2CH2]2) produced via photodissociation of Sn(CH3)4 in low temperature matrices were estimated by means of molecular orbital calculations as compared with Mossbauer parameters.

Molecular orbital calculation of Mössbauer parameters for Fe(CO)4 (C2H4) in low temperature matrix

Molecular orbital calculations have been performed to obtain the electron density and electric field gradient at the iron nucleus of tetracarbonylethene iron Fe(CO)4(C2H4) produced by UV-irradiation of pentacarbonyliron cocondensed homogeneously with ethene in a low temperature matrix, so as to estimate the Mossbauer parameters of the species. Mossbauer isomer shifts δ and electron densities at iron nuclei ρ (O) of Fe(CO)n (n=5,4,3,2) as well as Fe(CO)4(C2H4) are discussed: they have fairly good linear relationship to give −0.27 mm/s/aO−3. An isomer of Fe(CO)4(C2H4) produced via thermal reactions of Fe(CO)4 with ethene in a stratified matrix is discussed by comparing the calculated and observed Mossbauer parameters.

EFG studies of the AlCuFe quasicrystal and related structures

It is known that the structure and thermal stability of the AlCuFe icosahedral quasicrystalline (i-QC) phase is extremely sensitive to the exact composition. A stable, ideally quasiperiodic phase exists only along a line around Al62.0Cu25.5Fe12.5. The other i-QC samples show structural changes when annealed around 700 °C. The57Fe Mossbauer EFG properties and isomer shifts of these various i-QC phases and of the new rhombohedral approximant phase are presented, and compared with the properties of the Al site investigated by NMR. Certain new systematic trends will be presented.

Au-197 Mössbauer isomer shift in Au-Cu and Au-Ag alloys: The role of the Au 5d electrons

The Au-197 Mossbauer isomer shift (IS) for a series of Au-Cu and Au-Ag alloys is correlated with the change in the 5d electron population (5d hole count) at the Au site upon alloying. The Au 5d hole counts have been derived from the area under the whiteline of the Au L3,2 edge X-ray absorption near edge structures (XANES) of the alloys. It is found that there exists a linear corelation betweenIS and Au 5d hole counts, indicating that the redistribution of 5d electrons has a significant effect on the isomer shift. From theIS-5d hole correlation, the ratio of 5d loss to 6s gain (Δnd/Δns) is determined to be −0.62.

Metal substitution and atom displacement effects in PZT and PSZT

First principles electronic structure calculations in the framework of Local Density (LD) theory have been performed for the lead zirconate-lead titanate (PZT) solid solution near the experimentally interesting antiferroelectric-ferroelectric phase boundary. The Discrete Variational Xα Method (DV-Xα) with an embedded cluster scheme was used to investigate electronic structure, energy levels, charge distribution and chemical bonding in the paraelectric (PE) and antiferroelectric (AFE) regions of the PZT phase diagram. Isovalent metal substitution at the Zr site leads to measurable changes in the band gap and charge distribution. Mossbauer isomer shift parameters for tin substituted compound PSZT have been calculated.

Electron density redistribution on complexation in non-transition element complexes

Abstract Complexes of tin, antimony, silicon, germanium, iodine, boron and titanium halogenides with organic ligands were investigated using nuclear quadrupole resonance (NQR) and X-ray fluorescence spectroscopies, and Mossbauer effect and quantum-chemical calculations. A discussion of results which we obtained previously as well as literature data is included. The correlations between 35 Cl NQR and ClKα line shifts in SnCl 4 L 2 , SbCl 5 L and TiCl 4 L 2 complexes were obtained. The geometry of the acceptor was found to influence the value of effective charge on the chlorine atom ( q Cl ) in SnCl 4 L 2 complexes, but not to influence the chlorine atom in TiCl 4 L 2 complexes. Values of q Sn and q Sb were calculated from Mossbauer isomer shifts in the SnCl 4 L 2 and SbCl 5 L complexes. Electron-density transfer from the ligand to the vacant d orbital of the central atom on complexation was observed.

57Fe mössbauer studies of Bi2Sr4Fe3O12

Abstract 57Fe Mossbauer and magnetization measurements of an iron compound Bi2Sr4Fe3O12, isostructural with the high-Tc superconductor Bi2Sr2Ca2Cu3O10, are made. Fe ions are found to be in the trivalent state from the Mossbauer isomer shift and the paramagnetic Curie constant. The Mossbauer spectra show a hyperfine magnetic spitting below 60 K. On the other hand, the temperature dependence of the magnetization has no anomaly at this temperature but has a broad maximum at 40 K, suggesting the onset of magnetic order. The apparent disagreement of the magnetic transition of temperatures measured with the two methods can be ascribed to the difference of the characteristic times of the measurements. The slowing down of the fluctuation of Fe3+ magnetic moments into the ordered phase seems to occur rather gradually in this oxide.

Theoretical investigation of the electronic structure of the mixed-sandwich complex (.eta.5-cyclopentadienyl)(.eta.6-benzene)iron and its cation

SCF MS-Xa calculations have been carried out for the d6 [(Cp)Fe(Bz)]+ (1) and d7 (Cp)- Fe(Bz) (2) compounds (Cp = q5-cyclopentadienyl, Bz = +benzene), and the results are compared with those previously reported for metallocenes Fe(Cp)2 (3), Co(Cp)z (4), [Ni(Cp)2]+ (6), and d8 Ni(Cp)z (6) using the same technique. Analysis of the electronic structure of 1 and 2 shows that whereas Cp and Bz ligands play roughly the same role in digand to metal donation, they behave differently as far as 3d6 back-donation is concerned, the Bz system having a much more important role in this case. This is confirmed by spin-polarized (SP) calculations performed for 2 and 4, which show that the 3d6 orbitals exhibit a much larger interaction with the Bz ligand than with the Cp group. In addition, a considerable charge relaxation occurs at the metal upon ionization of 2, as the gross charges on the iron are roughly the same in 1 and 2. When using the MS-Xa results to calculate spectroscopic properties, a good agreement is found between theoretical and experimental results determined for both the electron field gradient (EFG) at the metal nucleus and Mossbauer isomer shift (IS) for compounds 1-3. Similarly, both MS-Xa ionization and electronic excitation energies calculated for 1 and 2 compare fairly well with photoelectron and UV-visible absorption data.

Correlation between Mössbauer isomer shifts and XPS binding energies

A series of low-spin pentacyanoferrates (‖) of the type Na3[Fe-(CN)5L] (L = phosphine or phosphite) have been characterized by57Fe Mossbauer spectroscopy and X-ray photoelectron spectroscopy. The Fe−P bond in Na3[Fe(CN)5L] is discussed in terms of a correlation between the isomer shift and the binding energy. It has been demonstrated that the isomer shift increases with the decreasing Fe2p3/2 binding energy. The Mossbauer isomer shift reflects the total electron density at the nuclear site, while the chemical shift in the binding energy is concerned with the electron density on the atom of interest. The increasing Fe-to-P back donation leads to an increase of the s-electron density at the iron nucleus, i.e. a decrease of the isomer shift. The increasing tendency in the Fe2p3/7 binding energy is interpreted by the increasing Fe-to-P back donation.

Isomer shifts and chemical bonding in crystalline Sn(II) and Sn(IV) compounds

First principles self-consistent local-density calculations of the electronic structure of clusters representing Sn(II) (SnO, SnF2, SnS and SnSe) and Sn(IV) (SnO2 and SnF4) crystalline compounds were performed. Values of the electron density at the Sn nucleus were obtained and related to measured values of the Mossbauer isomer shifts reported in the literature. The nuclear parameter of 119Sn derived was Delta R/R=(1.58+or-0.14)*10-4. The chemical bonding in the solids was analysed and related to the electron densities obtained.

Contact charge density in metallic dysprosium

The contact conduction electron density in metallic dysprosium has been calculated following the APW formalism and taking into account the effects of correlation and spin-polarization. A comparison of this density with that obtained from the Mossbauer isomer shift measurement has been made. An estimate of Δ〈r 2〉, the change in mean square nuclear charge radius between the two levels involved in theγ transition, has been made and compared with those obtained by other workers.

Ferrisilicate analogs of ZSM-11

Crystalline ferrisilicate analogs of ZSM-11, free of ZSM-5 impurities, have been synthesized and characterized. The results obtained by various spectroscopic techniques (XRD, MAS n.m.r., e.s.r., framework i.r.), magnetic susceptibility, thermal analysis (d.t.a./t.g.), adsorption, ion exchange, and catalytic measurements provide convincing evidence for the presence of iron in the zeolite framework. The ferrisilicate FeZSM-11 was free of Al (SiO 2 /Al 2 O 3 > 4000). There was an increase in the lattice constants and volume on incorporation of Fe. The white color of the as-synthesized and calcined samples as well as the magnetic susceptibility values (μ = 5.5–5.8 BM) indicate that the Fe 3+ ions are present in a dispersed, magnetically dilute environment. The Mossbauer isomer shift value, δ = 0.23 at 297 K, and an e.s.r. signal at g = 4.3 suggest the presence of Fe3+ ions in a tetrahedral symmetry. The adsorption capacities exhibited by FeZSM-11 and AIZSM-11 were comparable. The significant catalytic activity of ferrisilicates in m-xylene and ethylbenzene conversions, due to Bronsted acid sites, indicates the presence of iron in framework positions. Though HFeZSM-11 is less active than is HAIZSM-11, the former is more resistant toward deactivation.

Mössbauer Spectroscopic Studies of the Hydroxofluorostannate(IV) Complexes

Mossbauer isomer shifts of 119Sn in a series of complexes K2Sn(OH)6-mFm observed at 78 K were −0.05, −0.05, −0.24, −0.27 and −0.40 mm s−1, respectively, for m=0, 2, 4, 5 and 6. These IS values were linearly related to both m and (the average Pauling electronegativity of the ligands): . The IS straight line was compatible within experimental error with the one reported by Parish and Row-botham for the hexahalogenostannate complexes SnX4Y22, namely, The revised IS straight line including both series of complexes could be expressed by the equation Quadrupole splittings observed in the complexes of m = 2,4 and 5 were 1.16, 0.80 and 0.73 mm s−1 respectively. They were linearly related to both m and .

Estimation of covalent effects in actinide compounds on the basis of Mössbauer isomer shifts

Covalent effects have been evaluated in actinide compounds. The correlation between 5f and 6d-neptunium, orbital populations in the compounds and the Mossbauer isomer shifts has been studied.

The Mössbauer isomer shift calibration problem — Revisited for the Mössbauer isotopes Sn to I

The great deal of work as it is continuously directed towards calibrating the Mossbauer Isomer Shifts of 5s–5p—atoms motivates a critical reconsideration of the corresponding block of data. A greater homogeneity is attained by imposing some methodical improvements particularly on those data which were inferred from measured nuclear decay rate variations. On the basis of these revised data we propose new “best” ΔR/R values for 119Sn,125Te and127,129I: 1.8 10−4, 8.0 10−5, −3.4 10−4 and 4.3 10−4, respectively.

Nuclear spectroscopic quantities under influences of covalency

On the basis of Mossbauer isomer shift data, nuclear quadrupole interactions and NMR1J(13C−119Sn) spin-spin coupling constants, the problem of the nonadditivity of the isomer shifts and unexpected quadrupole interactions in tin organic compounds SnX4−nRn (X-halide, R-org. lig.) with the degree of substitution n has been treated by means of pseudopotential HF SCF calculations supplemented by efficient methods for comprehending core reorganizations and nucleus near electronic charge distributions. The nonadditivity of the isomer shifts and the progression of the nuclear quadrupole interactions has been explained by the evident momentum separated electronic reorganization. The clearly proved relation between s electron densities and higher orbital imbalances brings the isomer shift in relation to the valence part of the electric field gradient (EFG). An interrelation between isomer shifts and NMR spin-spin coupling constants has been deduced.

Electronic structure of unconventional antimony chalcogenides: Theoretical calculations and121Sb Mössbauer spectroscopy

The electronic structure of five antimony compounds is investigated and the Sb Mossbauer isomer shift is calculated. Its value obtained from the full band structure calculation gives a quantitative account for the chemical notion of delocalization of the 5s(Sb) lone pair. A simple molecular description is also derived which relates the lone pair to the Sb local environment.