57Fe-Mössbauer study of the high-T c system GdBa2Cu3O7−x: local binding strength and oxygen coordination of the Fe sites

The GdBa₂(Cu_1?yFe_y)₃O_7?x system with y=0.005–0.01 and various oxygen concentrations was investigated in the temperature range 20 K to 300 K. The⁵⁷Fe-Mössbauer spectra of the orthorhombic phase were analyzed with four quadrupole doublets. Three of them are attributed to substitutional Cu(1) sites and one (with the smallest quadrupole splitting) to the substitutional Cu(2) site. From an analysis of the temperature dependence of the relative site intensities, different local Debye temperatures for the Cu(1) sites were derived. Measurements of texturized absorbers at various angles give evidence for a vibrational anisotropy of the Cu(1) sites. The Cu(1) site with the largest quadrupole splitting exhibits the lowest Debye temperature and the largest vibrational anisotropy.     

Binding Strength and Oxygen Coordination of the Fe Sites in High TC GdBa2 Cu3O7—δ

The GdBa₂(Cu_1－xFe_x)₃O_7—δ system with x=0.005－0.001 and various oxygen concentrations was investigated by ⁵⁷Fe-Mossbauer spectroscopy in temperature range 20K to 300K.The spectra of the orthorhombic phase were analyzed with four quadrupole doublets.Three of them were attributed to substitutional Cu(1) sites and one (with the smallest quadrupole splitting)to the substitutional Cu(2) site.From an analysis of temperature dependence of the relative site intensities,different local Debye temperature for the Cu(1) sites were derived.Highly texturized absorbers were investigated to study the polarization dependence of the four quadrupole doublets.Measurements at the magic angle gave evidence for a vibrational anisotropy (Goldanskii-Karyagineffect) of the Cu(1) sites.The Cu(1) site with the largest quadrupole splitting exhibited the lowest Debye temperature and the largest vibrational anisotropy.     

An Fe Mössbauer effect study of metastable Al86Fe14prepared by rapid solidification

Icosahedral Al₈₆Fe₁₄ has been prepared by rapid quenching from the melt. Mössbauer effect spectra are quadrupole split doublets. The temperature dependence of the isomer shift is consistent with the second order doppler shift and the quadrupole splitting decreases linearly with temperature.     

Mössbauer study on M1 and M2 sites of Fe2+ in natural hypersthene under dynamic crystal field potential approach

⁵⁷Fe Mössbauer spectra of hypersthene, a natural silicate mineral belonging to the orthopyroxene group, have been taken over the temperature range 77–292 K. At temperatures above 77 K, they show asymmetric quadrupole peaks. This asymmetry arises from the overlapping of two quadrupole doublets from Fe²⁺(3d⁶,⁵D) ions in two different sites (M1 and M2). The quadrupole splitting, isomer shift and their temperature dependence are appreciably different for Fe²⁺ ions in the two sites. The Fe²⁺ quadrupole splitting in the M1 site decreases linearly with temperature, which can be explained quite satisfactorily by using a very simple model of the orbit-lattice interaction.     

133Cs and deuteron NMR study of the superionic transition in CsDSO4

The temperature dependence of the ¹³³Cs and deuteron NMR spectra of a CsDSO₄ single crystal has been investigated between 20°C and 170°C. In the room temperature phase II' there are two physically non-equivalent Cs⁺ and deuteron sites per unit cell. On heating from room temperature to 110°C the doublet structure of the ¹³³Cs and deuteron lines collapses due to random Cs and deuteron exchange between the two non-equivalent sites. At the transition to the superionic phase III there is a further drastic decrease in the deuteron and a somewhat smaller decrease in the ¹³³Cs quadrupole coupling. The fact that the quadrupole coupling of these two nuclei remains non-zero in the superionic phase demonstrates that the Cs⁺ and deuteron translational diffusion involves well defined lattice sites and directions and is not completely “random” or “liquid”-like.     

Mössbauer study of incompletely alloyed nanocrystalline Fe100 − x Al x prepared by high energy ball milling

Mechanically alloyed Fe_100???x Al _x powders, with 20≤?x?≤90, have been studied by X-ray diffraction and room temperature ⁵⁷Fe Mössbauer spectroscopy. The milling time was chosen such that complete alloying does not take place. For a fixed milling time of 10 h, the rate of alloying was seen to increase exponentially with increase in Fe content. Mössbauer spectra of all the samples consist of a broad magnetic sextet and a quadrupole doublet. The isomer shifts and quadrupole splitting of the doublets are typical of Al-rich, Fe–Al alloys. The area under the quadrupole doublet is a maximum for x?=?66. Analysis of the Mössbauer spectra indicates the formation off- stoichiometric Fe₃Al phase for x?<?66, while the formation of Fe clusters is largely responsible for the magnetic hyperfine component in x?≥?66 compositions.     

Hyperfine interactions in amorphous Tm-Cu Alloys

Hyperfine interactions in sputtered amorphous Tm_xCu_1?x, with x=0.70, 0.50, and 0.33, have been measured over a temperature range from 1.7 K to room temperature using the Mössbauer effect in¹⁶⁹Tm. At low temperatures each spectrum shows a well resolved six-line hyperfine pattern indicative of magnetic order and a superimposed quadrupole doublet. The relative spectral weight of the magnetic component increases with increasing Tm concentration and decreases with increasing temperature. The magnetic component persists at unexpectedly high, temperatures with no decrease in overall splitting and no evidence of electronic relaxation. For all three concentrations, the magnetic splitting is the same as that of Tm metal at low temperatures, nearly the free-ion limit. The electric quadrupole shift of the magnetic component is the same for the three samples, but is significantly less than that of Tm metal (hexagonal) and much larger than that of crystalline TmCu (cubic). The non-magnetic doublets have quadrupole splittings that are very nearly the same for the three concentrations and show considerable asymmetry, a typical effect of electronic relaxation, which persists at temperatures above where the magnetic component disappears.     

Mössbauer effect in iron picolinate-dimer and monomer

A study of the quadrupole doublets of the dimer |Fe(pic)₂(OH)|₂ and of the monomer |Fe(pic)₂(OH₂)Cl|, which are asymmetric when observed at room temperature, has been made. For the dimer it has been proposed that this asymmetry was due mainly to effects of spin-spin relaxation between the two bridged iron atoms, antiferromagnetically coupled. Observation of the spectrum at a temperature of 4.2 K in the presence of an external magnetic field up to 18 KG, leads to identification of the sign of the electric field gradient and the relative direction of the fluctuating magnetic field. The observed small area asymmetry has been attributed the Karyagin-Goldanskii effect. For the monomer, the area asymmetry was attribtued to the Karyagin-Goldanskii effect, and the asymmetry of the line widths was explained in terms of a relaxation mechanism between the Kramers doublets.     

A study of the cobalt sites in cobalt and iron molybdate catalysts by emission Mössbauer spectroscopy

Molybdates samples containing 74MBq (2mCi) of cobalt 57 have been studied by emission Mössbauer spectroscopy. Spectra of⁵⁷CoMoO₄ exhibited the three usual Fe²⁺ doublets corresponding to α and β ferrous phases, and two Fe³⁺ doublets accounting for about one third of the absorption area. The Co²⁺ sites of β⁵⁷CoMoO₄ phase exhibit higher quadrupole splittings, i.e. are more dissymmetrical than Fe²⁺ sites of βFeMoO₄ phase. Cobalt and iron molybdates Fe_x ⁵⁷Co_1?xMoO₄ provided spectra where only one Fe³⁺ doublet appeared and occupied less than 7% of the spectral area. It is concluded that a part of cobalt was present in⁵⁷CoMoO₄ as Co³⁺.     

Lattice sum calculations and a Mössbauer study on electric field gradient in phlogopite

Lattice sum contributions have been calculated at the two octahedral sites in a phlogopite mica assuming a systematic distribution of octahedral cationic charges. This, unlike the case of a random distribution of charges, is able to reproduce broad features of quadrupole doublet spectra in Mössbauer experiments suggesting that the doublet assignments in terms of the two structural sites,M ₁ (trans) andM ₂(cis), are quite valid. Angle dependence of Mössbauer spectra has also been studied for a ferric-rich phlogopite mica sheet to determine the orientation of the EFG principal axis component and the sign of the quadrupole coupling constant. Quadrupole splitting values and the positive sign of coupling constants match the theoretical predictions quite well but the model fails to predict the observed EFG orientation.     

A Mössbauer study of octahedral site symmetries in systems Co x Mn3−x−y Fe y O4 and Ni x Mn3−x−y Fe y O4

A Mössbauer study of cation distribution in systems Co _x Mn_3?x?y Fe _y O₄ and Ni _x Mn_3?x?y Fe _y O₄ has been made. It has been found that in both systems all specimens withy<0.6 value exhibit quadrupole doublets corresponding to site symmetries Fe³⁺(I) and Fe³⁺(II) of octahedral site. As more and more cobalt or nickel is introduced into the matrix the intensity of the inner quadrupole doublet increases while on introducing iron that of the outer quadrupole doublet increases. After a certain concentration of iron the inner doublet starts becoming more intense. It is suggested that this arises possibly from the substitution of cations in the second co-ordination sphere of Fe³⁺(I) and Fe³⁺(II) sites. Fory>0.6 the Mössbauer spectra show relaxation effects.     

Mössbauer study of tin-doped 1-2-3 and 2-1-4 highT c superconductors

Samples of YBa₂(Cu_0.85Sn_0.15)₃O_7–y (1-2-3), La₂(Cu_0.95Sn_0.05)O4_4–x, and La_1.85Ba_0.15(Cu_0.95Sn_0.05)O_4–x (2-1-4), with different oxygen stoichiometry, have been studied by Mössbauer spectroscopy. These measurements reveal the existence of two inequivalent sites for Sn in the (1-2-3) compounds. The (1-2-3) spectra display two quadrupole doublets which we associate with Sn in Cu(1) and Cu(2) sites, respectively. La compounds show a single quadrupole doublet.     

Mössbauer studies of FexNbS2 (x = 0.25, 0.33 and 0.5)

Mössbauer studies of Fe_xNbS₂ (x = 0.25, 0.33 and 0.5) have been carried out for temperatures from 4.2K to about 715K. The iron exists in high spin divalent state for all the compositions. The temperature dependence of quadrupole splitting and center shift shows a reversible phase transition at about 600K and a possible disordering of Fe vacancies beyond this temperature for Fe_0.25NbS₂ and a reversible phase transition at 490K for Fe_0.5NbS₂. The hyperfine magnetic fields have been evaluated from the magnetically ordered spectra. The observed temperature dependent line intensities of the quadrupole doublet are attributable to the temperature dependence of the difference in the meansquare amplitude of vibrations parallel and perpendicular to the EFG axis.     

The anomalous temperature dependence of the Mössbauer linewidth of FeCO3 (siderite)

Both lines of the Mössbauer quadrupole doublet of FECO₃ reveal the same temperature dependence, thus their narrowing with increasing temperature cannot be explained by magnetic relaxation. The width of the σ line is markedly increased by oxidation even at about 570 K. To explain the anomalous temperature dependence of the linewidth a possible model, based on random transitions between two or more states of the iron nucleus surrounding producing different EFG, is proposed.     

A CEMS study of ac-axis oriented thin film of YBa2(Cu0.97 57Fe0.03)3O7

⁵⁷Fe Mössbauer spectra of a fullyc-axis oriented epitaxial thin film of YBa₂(Cu_0.97 ⁵⁷Fe_0.03)₃O₇, recorded by CEMS, are reported. Spectra taken at different angles between the normal of the film (i.e. thec-axis) and the gamma ray direction show that for two of the three observed quadrupole doublets the main component of the EFG lies in thea?b-plane, for the third doublet it is parallel to thec-axis. The EFG of all three doublets has approximately axial symmetry.     

Effect of particle size and alloying with different metals on 57Fe Mössbauer spectra

Iron nanoparticles of various sizes have been synthesized using the chemical route which involves the preparation of iron bipyridine complexes in presence of different capping agents followed by thermal decomposition at 450°C in inert atmosphere. The bimetallic nanoalloys of Fe with Mg and Pd have also been prepared by following the same route. The resulting nanoparticles have been characterized by EDX-RF, XRD, AFM and ⁵⁷Fe Mössbauer spectroscopy. The appearance of quadrupole doublets in the Mössbauer spectra of Fe nanoparticles indicates the absence of magnetic interaction and variation in parameters is due to the varying particle size. The Mössbauer spectrum of Fe–Mg₂ bimetallic nanoalloy shows two doublets indicating the presence of superparamagnetism. The two doublets can be attributed to change in s-electron density of iron resulting from its two neighboring magnesium atoms. Fe–Pd nanoalloy Mössbauer spectrum is characterized by having a superparamagnetic doublet and a ferromagnetic sextet.     

Site analysis and calculation of the quadrupole splitting of Prussian Blue Mössbauer spectra

The insoluble Prussian Blue (IPB) structure, Fe\(^{\mathrm {3+}}_{~~~4}\)[Fe ^II(CN)₆]₃.14H₂O, is shown to have ten distinctly different ferric sites, as well as the single ferrous site. The statistical probability of these sites was determined and the point charge model used to calculate the relative quadrupole splitting values. Previous Mössbauer spectra of IPB have fitted the ferric site with one doublet. Our results show that there is a large difference between the quadrupole splittings of the different ferric sites and hence it is not a reasonable approximation to fit them using a single doublet.     

Mixed-valence states of biferrocene derivatives having long alkyl chains

Mixed-valence states of biferrocene derivatives having long alkyl chains such as ditetradecyl, dihexadecyl, and doctadecyl substituents were investigated. A little shortening in the inter-layer length by recrystallization from CH₂Cl₂ was observed in ditetradecyl and dihexadecyl derivatives, while no difference was observed in dioctadecyl derivative. Samples having a longer inter-layer length showed a temperature independent trapped-valence state. On the other hand, samples having a shorter inter-layer length showed a temperature dependence, in which two doublets observed at 80 K approach each other with an increase of temperature to converge into one doublet which is ascribed to an averaged-valence state. The results obtained in the present study prove the fact that the crystal structure affects the mixed-valence states.     

Mössbauer and FTIR characterization of a ferro-axinite

A ferro-axinite from metabasalts from the Trondheim region (Norway) has been studied by Mössbauer spectroscopy. Two well defined doublets attributed respectively to divalent and trivalent iron were found. On the basis of the values of the quadrupole splitting, the first doublet is attributed to the octahedral site of the axinite. The FTIR spectrum in the 1200--700 cm^-1 region is in agreement with the existence of the \nu₁ symmetric stretch of the Fe--O^[4] bond. On this basis the second doublet MS can be attributed to Fe³⁺ in the tetrahedral site.     

Systematic Mössbauer study of YBa2Cu3−x Fe x Oδ and PrBa2Cu3−x Fe x Oδ as a function of temperature

We study the similitudes and differences between YBa₂Cu_3?x Fe _x O_δ and PrBa₂Cu_3?x Fe _x O_δ systems, using Mössbauer spectroscopy. Both systems, withz=0.04, were studied at several temperatures. The spectra of PrBa₂Cu_3?x Fe _x O_δ showed four asymmetric quadrupole doublets with apparently different line widths, which were fitted with four symmetric superimposed quadrupole doublets with the same line width, but assuming a small (～4–6 kOe) residual magnetic field in the Fe sites, which are mainly the Cu(1) sites of the 1–2–3 structure. On the other hand, the PrBa₂Cu_3?x Fe _x O_δ spectra showed only three asymmetric quadrupole doublets which can be fitted with three superimposed symmetric doublets and a residual magnetic field. In this case, the Fe atoms also occupy the Cu(1) sites of the 1–2–3 structure. The temperature variations of the Mössbauer spectra and their parameters seem to indicate that, in the Pr-system, a structural change takes place between 12 K and 40 K.