Chemical state of cobalt doped in and adsorbed on α-(Fe1-x Cr x )2O3 by emission Mössbauer spectroscopy

Mössbauer parameters (internal magnetic field, quadrupole splitting and isomer shift) of both α-(Fe_1-x Cr _x )₂O₃ (x=0, 0.2 and ≈1.0) doped and undoped with⁵⁷Co are in good agreement at room temperature and 77 K, except internal magnetic fields of both α-Cr₂O₃ doped with⁵⁷Co and enriched⁵⁷Fe. It is thus concluded that⁵⁷Fe produced from⁵⁷Co occupies the octahedral sites as Fe³⁺ with small distortion from cubic symmetry. Different internal magnetic field of doped α-Cr₂O₃ may be explained by the difference of canting of the spin against the [111] axis. By adding zinc ions, the adsorption of Co²⁺ ions on α-Fe₂O₃ particles in aqueous solution is decreased considerably in the pH range of 6.5–9.5 at 303 K, but the internal magnetic field of⁵⁷Co adsorbed on α-Fe₂O₃ does not change, although the internal magnetic fields of both samples with and without zinc ions are smaller than that of bulk α-Fe₂O₃ doped with⁵⁷Co. This suggests that densities of⁵⁷Co ions on the surface of α-Fe₂O₃ may be decreased by the addition of zinc ions and⁵⁷Co ions adsorbed on α-Fe₂O₃ are weakly bound on the substrate.     

Mössbauer effect of57Co in LiTaO3 — Effects of a variety of heat treatments

LiTaO₃ single crystals were doped with⁵⁷Co. Heat treatments of the crystals in the various atmospheres (Ar, Ar/H₂ etc.) very sensitively influenced the charge states of iron formed after the EC of⁵⁷Co. Fe²⁺ Fe³⁺ and metallic clusters are formed depending on the treatments in the various atmospheres. The observed effects were found reversible by changing the reducing and oxidising atmospheres.     

57Fe and57Co Mössbauer studies of highT c Y-Ba-Cu oxides

The⁵⁷Co emission Mössbauer spectra from YBa₂Cu₃O_6.92 (1-2-307) and YBa₂Cu₃O_6.00 (1-2-306) have been measured and compared with the⁵⁷Fe absorption spectra from YBa₂Cu_2.95Fe_0.05O_7?δ in order to clarify decisively the site assignments for the⁵⁷Fe quadrupole-split doublets in these compounds. Mössbauer spectra obtained from both specimens consist of four components whose hyperfine interaction parameters well agree with each other. It is shown that the Co and Fe atoms mainly substitute at Cu1 chain sites in 1-2-307, but in 1-2-306 the Co atoms occupy randomly the Cu2 plane sites and indicate magnetically-split sextet which converts to a paramagnetic doublet of S-state Fe³⁺ in 1-2-307 by a post-annealing in O₂ gas.     

Local states of iron in iron implanted hematite Fe2O3

Powder samples of⁵⁷Fe₂O₃ and⁵⁶Fe₂O₃ were implanted with⁵⁶Fe and⁵⁷Fe ions, respectively. By the use of Conversion Electron Mössbauer Spectroscopy it was possible to observe the local states of implanted ions (⁵⁷Fe in⁵⁶Fe₂O₃) or the states of iron atoms from the target which were displaced during implantation due to the ballistic processes (⁵⁶Fe in⁵⁷Fe₂O₃). The implanted and displaced iron atoms appear in three different states: (i) in regular substitutional positions of Fe₂O₃, (ii) as magnetite Fe₃O₄-type structures and (iii) paramagnetic FeO_1?x state. The observed fractions of each state agree rather well with the calculated values obtained from the local iron atom enrichment at the surface as well as from the analysis of the equilibrium phase diagram for the binary Fe?O system. However, in⁵⁷Fe implanted samples some enhancement of the FeO_1?x fraction was found in comparison with the⁵⁶Fe implanted hematite.     

YIG formation in annealed iron implanted YAG

YAG and YIG crystals implanted respectively with 100 keV⁵⁷Fe²⁺ ions (1 × 10¹⁷ ions.cm^?2) and 50 keV²⁷Al ions (1.1 × 10¹⁷ ions.cm^?2) have been studied by conversion electron Mössbauer spectroscopy (CEMS) directly after implantation and after annealings in air at temperatures up to 1100°C. In both as-implanted samples iron is found mainly in three states: Fe²⁺, Fe³⁺ and small metallic precipitates. Annealing behaviour is divided into two stages: (i) up to 400°C the iron has become completely oxidized and (ii) between 400 and 850°C the epitaxial regrowth of the implanted layer takes place. During this process a part of iron ions are incorporated into octahedral and tetrahedral sites, thus making a Y₃ (Al Fe)₅ O₁₂ compound. The remaining iron part precipitates in the form of Fe₂O₃ particles.     

Site occupation in57Fe doped NdCo12B6: Mössbauer analysis

The analysis of room temperature and4.2 K Mössbauer spectra of NdCo₁₂B₆ doped with⁵⁷Fe is presented. A clear preference of the⁵⁷Fe atoms for the 18h sites, of increased number of Co near neighbour atoms compared with the 18g site, is obtained in good agreement with recent studies of related 1:12:6 compounds.     

57Fe Mössbauer study under high pressure; ε-Fe and Fe2O3

Using diamond anvil cell, the⁵⁷Fe Mössbauer spectra of pure iron foil and α-Fe₂O₃ powder under high pressure have been measured at room temperature.⁵⁷Fe Mössbauer spectra of α-Fe were measured from 15 GPa to 45 GPa. Isomer shift value decreased and the quadrupole splitting slightly increased as the pressure increased.⁵⁷Fe Mössbauer spectra of Fe₂O₃ under high pressure up to 72 GPa were observed. Above 52 GPa, the new lines appeared at the center portion of the spectrum corresponding to the new high pressure phase. The spectrum of new high pressure phase consisted of 6-line splitting and doublet, suggesting the existence of the two different kinds of iron states in it.     

Mossbauer spectroscopy on57Co implanted SiO2

Conclusions Different charge states of⁵⁷Fe are identified in the⁵⁷Co implanted SiO₂ matrix.It is interesting to see that the states appearing in solid oxygen [2 ] also appear in the SiO₂ matrix, although in neither of the matrices a preferential interpretation of the S₁-line being Fe^{+ 6} or Fe⁰, can be achieved.Further, implantation of⁵⁷Fe in SiO₂ could reveal whether the states appearing in the⁵⁷Co implanted SiO² are really after decay effects or part of a (⁵⁷Fe-SiO₂)-structure.The authors wish to thank Prof. P. Boolchand for the SiO₂ samples and L. Verwilst for the ion implantation.     

57Fe Mössbauer study of Li x Fe1-y Co y PO4 (y = 0, 0.1, 0.2) as cathode material for Li-ion batteries

Lithium iron phosphates LiFe_1-y Co _y PO₄ (y = 0, 0.1, 0.2) exposed to a charging process were studied by ⁵⁷Fe Mössbauer spectroscopy taking into account XRD and SEM data. Hyperfine parameters of the spectra were determined above and below the magnetic ordering temperature for all the samples. It was shown that the presence of Co impurity atoms in lithium phosphates gives no effect on the hyperfine interaction of ⁵⁷Fe²⁺ cations. However, Co atoms in the nearest cation environment of Fe atoms lead to a significant change of the hyperfine interactions of ⁵⁷Fe³⁺ cations. The Co impurity atoms distribution over the positions of the iron atoms in the structure is found not to be statistical,but correlated.     

57Fe Emission Mössbauer Study on Gd3Ga5O12 implanted with dilute57Mn

⁵⁷Fe emission Mössbauer spectroscopy has been applied to study the lattice location and properties of Fe in gadolinium gallium garnet Gd₃Ga₅O₁₂ (GGG) single crystals in the temperature interval 300 – 563 K within the extremely dilute (<10^?4 at.%) regime following the implantation of⁵⁷Mn (T¹_/2= 1.5 min.) at ISOLDE/CERN. These results are compared with earlier Mössbauer spectroscopy study of Fe-doped gadolinium gallium garnet Gd₃Ga₅O₁₂(GGG), with implantation fluences between 8×10¹⁵ and 6×10¹⁶ atoms cm^?2. Three Fe components are observed in the emission Mössbauer spectra: (i) high spin Fe²⁺ located at damage sites due to the implantation process, (ii) high spin Fe³⁺ at substitutional tetrahedral Ga sites, and (iii) interstitial Fe, probably due to the recoil imparted on the daughter^57?Fe nucleus in the β^? decay of⁵⁷Mn. In contrast to high fluence⁵⁷Fe implantation studies the Fe³⁺ ions are found to prefer the tetrahedral Ga site over the octahedral Ga site. No annealing stages are evident in the temperature range investigated. Despite the very low concentration, high-spin Fe³⁺ shows fast spin relaxation, presumably due to an indirect interaction between nearby gadolinium atoms.     

Mössbauer study of iron in sp-metals and semimetals

Bi, In, Pb, Sb, α- and β-Sn were implanted with⁵⁷Co and⁵⁷Fe. The results of the analysis suggested strongly hybridized bonds between the iron and the host atoms. An upturn of the isomer shift (δ) values relative to the δ values in hosts with completed d-shell was not justified. Charge states of Fe²⁺ or Fe⁺ also appeared. The possible role of the native surface oxide layers in the formation of these states was investigated.     

Possible cage motion of interstitial Fe in α-Al 2 O 3

In addition to spectral components due to Fe^2?+? and Fe^3?+?, a single line is observed in emission Mössbauer spectra following low fluence (<10¹⁵ cm^???2) implantation of ⁵⁷Fe*, ⁵⁷Mn and ⁵⁷Co in α-Al₂O₃. For the ⁵⁷Co and ⁵⁷Mn implantations, the intensity of the single line is found to depend on the emission angle relative to the crystal symmetry axis. This angular dependence can be explained by a non-isotropic f-factor and/or motion of the Fe ion between sites in an interstitial cage. It is argued that interstitial cage motion is a more likely explanation, as this can account for the lack of quadrupole splitting of the line.     

Emission Mössbauer spectroscopy study of fluence dependence of paramagnetic relaxation in Mn/Fe implanted ZnO

Emission Mössbauer Spectroscopy following the implantation of radioactive precursor isotope ⁵⁷Mn⁺ (T _1/2= 1.5 min) into ZnO single crystals at ISOLDE/CERN shows that a large fraction of ⁵⁷Fe atoms produced in the ⁵⁷Mn beta decay is created as paramagnetic Fe³⁺ with relatively long spin-lattice relaxation times. Here we report on ZnO pre-implanted with ⁵⁶Fe to fluences of 2×10¹³, 5×10 ¹³ and 8 × 10¹³ ions/cm² in order to investigate the dependence of the paramagnetic relaxation rate of Fe³⁺ on fluence. The spectra are dominated by magnetic features displaying paramagnetic relaxation effects. The extracted spin-lattice relaxation rates show a slight increase with increasing ion fluence at corresponding temperatures and the area fraction of Fe³⁺ at room temperature reaches a maximum contribution of 80(3)% in the studied fluence range.     

Mössbauer study of nano-TiO2 doped with Fe

A Mössbauer study of nano-TiO₂ doped with Fe is presented. The samples are prepared by sol-gel method, doping Fe by 5, 10 and 15 wt.%, respectively, which are measured with XRD, TEM and Raman spectra. Especially, Mössbauer spectra are emphasized in this study. The anatase phase is major in both doped and no-doped sample. The α-Fe₂O₃ phase is also in the doped samples. The grain size of doped sample is in 5–20 nm range, the major grains are about 13 nm. And the grain size of no-doped sample is about 8 nm. Studying Mössbauer spectra and Raman spectra, we concluded that in the doping process the Fe³⁺ ions entered anatase lattice and substituted Ti⁴⁺ ions. However, the amount of Fe ions in the site is limited to about 1.5 wt.%. It does not increase as the doping Fe increase. The more Fe doped, the more α-Fe₂O₃ formed. For comparing conveniently, it also can be described as (Ti_0.98Fe_0.02)O₂ by atomic percent.     

Influence of the coordination water on the after effects of the K-capture in57Co doped FeSO4×H2O (x=0,1,4,7)

Emission Mössbauer spectroscopy is used to investigate the physico-chemical state of⁵⁷Fe atoms generated by⁵⁷Co²⁺ clectron capture in⁵⁷Co: FeSO₄xH₂O (x=0,1,4,7). The spectra of all the hydrates show at least three main components, the normal bivalent state, Fe²⁺(N), an aliovalent state, Fe³⁺, and one (or several) anomalous ferrous species in which the coordination sphere has been perturbated by the self-radiolysis. No Fe³⁺ is observed in the anhydrous compound whereas the intensity of this component increases as a function of the hydration number, reaching a saturation value of 36% for x=4 and 7.     

Silicon and transition metal-silicides implanted with57Fe

Si(111) single crystals were implanted with⁵⁷Fe in a broad dose range in order to overlap the concentration range of bulk amorphous Fe _x Si_1−x samples. At high (≥10¹⁶ atoms/cm²) doses the measured hyperfine interaction values were found to be the same as in the bulk amorphous samples, suggesting the same Fe−Si bonding and a very similar structure for the two amorphous phases produced by different methods. A comparison of the isomer shift (δ) and quadrupole splitting (ΔE) values with the values of the stoichiometric crystalliine phases showed the same δ but different δE values indicating similar Fe−Si bonding but different atomic arrangement around the iron atom.     

Thermal evolution of high dose iron implanted sintered alumina

Sintered plates of alumina have been implanted at room temperature with 110 keV⁵⁷Fe⁺ at a dose of 1.2×10¹⁷ ions.cm^?2. The analysis of the Conversion Electron Mössbauer Spectrum indicated that implantation introduces iron in alumina in three charge state: Fe²⁺ (two components), Fe⁴⁺ and Fe⁰ (metallic clusters). The evolution of the iron depth distribution during annealings in oxiding or in neutral atmosphere has been followed using the Rutherford backscattering spectroscopy. Up to 800°C the profile as well as the charge states of iron evolve very slowly. A drastic change occurs' for annealing temperature around 1000°C. The total amount of iron is distributed among α-Fe₂O₃ and α-(Fe_1?x Al _x )₂O₃ precipitates. Some scanning electron micrographs have allowed to locate these precipitates. For highest temperature anneals, up to 1600°C, only substitutional iron remain.     

Reactions of 57Mn implanted into solid oxygen

The in-beam Mössbauer spectroscopy using a short-lived ⁵⁷Mn (T _1/2=1.5 min) beam was carried out to study the production of the exotic chemical species of ⁵⁷Fe atoms arising from ⁵⁷Mn implanted into solid oxygen. The obtained spectra can be analyzed by four components of doublets at least, which are assigned to be novel Fe species of FeO, Fe(O₂), (O₂)FeO₂, and Fe(O₂)₂, on the basis of ab initio molecular orbital calculations.     

Lattice dynamics of corundum crystals with vacancies in various charge states

The lattice dynamics of an α-Al₂O₃ crystal with vacancies in various charge states is simulated using the recursive method in the shell model. The frequencies of resonant vibrations induced by defects in various directions are calculated. Characteristic features in vibrational spectra of anion-nonstoichiometric α-Al₂O₃ crystals, mostly associated with changes in the effective interaction between vacancies and the nearest neighbor atoms, are analyzed and explained.     

Distribution of hyperfine fields of57Fe2+ in amorphous LiNbO3: Mössbauer study at low temperatures in high magnetic fields

Mössbauer spectra of amorphous LiNbO₃ doped with⁵⁷Fe²⁺ ions were measured atT≥4.2 K in external magnetic fields up toH=5.5 T. It was found that the oxygen octahedra around the high-spin Fe²⁺ ions are randomly distorted obeying no symmetry at all. The correlation of isomer shift and quadrupole splitting is probably due to T_2g distortions.