Ossbauer Parameters Research Articles

Electronic-structure calculations, photoelectron spectra, optical spectra, and Mössbauer parameters for the pyritesMS2(M=Fe,Co,Ni,Cu,Zn)

Electronic-structure calculations on the basis of a self-consistent charge, linear combination of atomic orbitals band-structure method have been performed for the pyrites Fe${\mathrm{S}}_{2}$, Co${\mathrm{S}}_{2}$, Ni${\mathrm{S}}_{2}$, Cu${\mathrm{S}}_{2}$, and Zn${\mathrm{S}}_{2}$. Photoelectron spectra, optical spectra, and M\ossbauer parameters are evaluated and are found to compare well with experimental data. Molecular-orbital cluster calculations have been performed to derive local properties (M\ossbauer parameters) only, which are compared with band-structure and experimental results. Clusters which include ${\mathrm{S}}_{2}$ anion pairs, i.e., ${[M{({\mathrm{S}}_{2})}_{6}]}^{10\ensuremath{-}}$, yield reasonable results, while for the smaller clusters ${[M{\mathrm{S}}_{6}]}^{4\ensuremath{-}}$, even convergence could not be achieved. Our further investigation includes (i) the pressure dependence of $\ensuremath{\Delta}{E}_{Q}$ and $\ensuremath{\delta}$ in Fe${\mathrm{S}}_{2}$; (ii) the concentration dependence of $\ensuremath{\Delta}{E}_{Q}$ in the solid solutions ${\mathrm{Fe}}_{x}{\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{S}}_{2}$ ($x=0.01,0.25,0.5,0.75$); (iii) the sign of the nuclear-quadrupole coupling constant ${e}^{2}\mathrm{qQ}$, which was found to be negative except for Zn${\mathrm{S}}_{2}$; (iv) the various contributions to the electric-field gradient (EFG) tensor (in Fe${\mathrm{S}}_{2}$ the main contribution arises from the valence shell, and proceeding from Fe${\mathrm{S}}_{2}$ to Zn${\mathrm{S}}_{2}$ in the pyrite series, the valence contribution continuously decreases, and in the ${d}^{10}$ system Zn${\mathrm{S}}_{2}$ only a small and positive lattice EFG is left); (v) the interpretation of the independence of the M\ossbauer line intensity ratio from the single-crystal orientation with respect to the $\ensuremath{\gamma}$ beam on the basis of our calculated EFG tensor.

Radiation defects in ion-implanted silicon. I. Mössbauer spectroscopy ofSn119defect structures from implantations of radioactive antimony

Impurity-defect structures in silicon containing $^{119}\mathrm{Sn}$ have been studied by M\"ossbauer spectroscopy. The defects have been created by isotope-separator implantation of radioactive $^{119}\mathrm{Sb}^{+}$ ions at room temperature. Emission spectra of the 24-keV M\"ossbauer $\ensuremath{\gamma}$ radiation from the daughter $^{119}\mathrm{Sn}$ have been measured. The M\"ossbauer spectra are analyzed in terms of four independent lines. These lines are characterized by the measured isomer shifts, linewidths, quadrupole splittings, and Debye-Waller factors. From these parameters, different bonding configurations are deduced for the Sn impurity atoms. Thus, the lines are assigned to emitting Sn atoms as substitutional impurities and as atoms in more complex impurity-defect structures. The formation of the impurity-defect structures in the implantation process (and their annealing behavior) is controlled by properties of the antimony atoms, whereas the measured M\"ossbauer parameters reflect the response of the Sn daughter atoms in these structures. It is concluded that a large fraction of the Sn atoms occupy locally undisturbed, substitutional lattice sites [with an isomer shift of $\ensuremath{\delta}=1.83(6)$ mm/s and a Debye temperature of $\ensuremath{\Theta}=220(20)$ K]. A minor fraction is in interstitial lattice positions [$\ensuremath{\delta}=3.3(1)$ mm/s, $\ensuremath{\Theta}=240(30)$ K]. Two other lines [$\ensuremath{\delta}=2.6(1)$ mm/s, $\ensuremath{\Delta}{E}_{Q}\ensuremath{\approx}0.3$ mm/s, $\ensuremath{\Theta}=160(20)$ K, and $\ensuremath{\delta}\ensuremath{\approx}2.6$ mm/s, $\ensuremath{\Theta}=250(40)$ K] are attributed to vacancy associated Sn impurity atoms. Rough annealing experiments show annealing of the Sb impurity vacancy complex (with $\ensuremath{\Theta}=160$ K) between 700 and 900 K. The other impurity-vacancy structure ($\ensuremath{\Theta}=250$ K) is only formed after high-temperature treatment of the sample (\ensuremath{\gtrsim}1200 K). The interstitial fraction increases with annealing temperature.

Matrix-isolation study of FeCo molecules

Rare-gas matrix-isolation techniques have been used in conjunction with M\"ossbauer spectroscopy to study bimetallic molecules of FeCo. The electronic ground state for the heteronuclear diatomic molecule FeCo was determined from symmetry considerations as $^{6}\ensuremath{\Sigma}$. It was also determined that the coupling between the atoms was ferromagnetic. In addition the trimeric species ${\mathrm{Fe}}_{2}$Co and Fe${\mathrm{Co}}_{2}$ were identified and their M\"ossbauer parameters determined. Higher multimers of iron-cobalt were also studied and magnetic hyperfine splitting was observed with fields ranging between 276 and 345 kOe. This last value was obtained for 25 at.% metal in solid argon with a cobalt to iron ratio of one; at this concentration long-range order is already present.

Fe57Mössbauer study of chromium-doped magnetite,Fe3−xCrxO4(0≤x≤0.5) above the Verwey transition

$^{57}\mathrm{Fe}$ M\ossbauer measurements show that the ratio of the integrated intensities of the $^{57}\mathrm{Fe}$ $A$- and $B$-site M\ossbauer subspectra is in good agreement with those expected for the cation distribution $({\mathrm{Fe}}^{3+})({\mathrm{Fe}}_{1\ensuremath{-}x}^{3+}{\mathrm{Fe}}^{2+}{\mathrm{Cr}}_{x}){\mathrm{O}}_{4}$, $0\ensuremath{\le}x\ensuremath{\le}0.3$. The magnetic moments are also found to be in good agreement with this cation distribution. The compositional dependences of the M\ossbauer hyperfine parameters display no features in this compositional range that would result from a qualitative change in electronic structure. The observed line broadening and line shapes are readily accounted for by the local configurations of ions resulting from a random substitution of ${\mathrm{Cr}}^{3+}$ on the $B$ sites. Consequently, the conduction mechanism in these doped magnetites is similar to that in pure ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$. The marked changes in the spectra for $x\ensuremath{\approx}0.5$ are due primarily to the commencement of the occupation of the $A$ site by ${\mathrm{Fe}}^{2+}$ ions and the change of the M\ossbauer parameters between $x=0.3$ and $x=0.5$ are as expected if $A$-site ${\mathrm{Fe}}^{2+}$ ions are present.

Electronic and magnetic structure ofα-FeSO4

The study of the electronic and magnetic structure of $\ensuremath{\alpha}$-FeS${\mathrm{O}}_{4}$ is based on a model which consists of iron surrounded by six S${\mathrm{O}}_{4}$ tetrahedra forming a ${[\mathrm{F}\mathrm{e}{(\mathrm{S}{\mathrm{O}}_{4})}_{6}]}^{\ensuremath{-}10}$ cluster; its three-dimensional structure is based on x-ray diffraction data. For many-electron terms of ferrous high-spin compounds derived from semiempirical cluster calculations, we investigate spin-orbit coupling, electric field gradient (EFG), and susceptibility. In particular for the EFG we discuss valence, overlap, and lattice contributions. Within the paramagnetic region of $\ensuremath{\alpha}$-FeS${\mathrm{O}}_{4}$ we investigate the orientation of the EFG with respect to the crystallographic frame, the M\"ossbauer parameters $\ensuremath{\Delta}{E}_{Q}$ and $\ensuremath{\eta}$, and the susceptibility; we find our results in reasonable agreement with the recent experimental data of Wehner. Within the antiferromagnetic region at 4.2 K we study the influence of the molecular exchange field upon EFG, $\ensuremath{\Delta}{E}_{Q}$, and $\ensuremath{\eta}$, and we further estimate the orientation of the internal magnetic field (${H}_{\mathrm{int}}$) at the iron nucleus with respect to the crystallographic frame. Finally we derive an angle of 27.7\ifmmode^\circ\else\textdegree\fi{} between ${H}_{\mathrm{int}}$ and the main component of the EFG. Our low-temperature study of $\ensuremath{\alpha}$-FeS${\mathrm{O}}_{4}$ agrees with the recent experimental findings of Wehner, but it disagrees with those of Ono and Ito.

Mössbauer Study of Diffusion in Liquids: DispersedFe2+in Glycerol and Aqueous-Glycerol Solutions

M\"ossbauer parameters, diffusion constants, and coefficients of viscosity for ${\mathrm{Fe}}^{2+}$ ions, dissolved from Fe${\mathrm{Cl}}_{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} 4${\mathrm{H}}_{2}$O, in nominally dry glycerol, glycerol-5-wt% water, and glycerol-10-wt% water have been measured. The broadening of the M\"ossbauer line is observed to have a complicated temperature dependence, which we explain by assuming two mechanisms of diffusion. At low temperatures we postulate that diffusion is via a jumping process described by an Arrhenius relation, while at high temperatures, diffusion is via a gaslike motion obeying the Einstein-Stokes and Litovitz relations. A quantitative fit to the observed line broadening is excellent. A very rapid falloff in the quadrupole splitting and the logarithm of the recoil-free fraction is observed in the temperature region where diffusive broadening is measurable; this falloff is proportional to the observed broadening, with two distinct proportionality constants for the low- and high-temperature regions. These observations are explained by a phenomenological theory due to Jensen and by postulating that the average electric field gradient at the iron nucleus reduces in proportion with the iron diffusivity.